No pressure from hot water expansion tank

Testing and Replacing a Hot Water Expansion Tank 147

I’ve been on a plumbing DIY kick lately, and after recently replacing a hot water recirculating pump that had been installed incorrectly (upside-down!) by a professional plumber, I’ve been reminded that it’s never a bad idea to periodically test things inside your own house — even if they were installed by a “professional.”

Now, I don’t want to bag on all professionals. Some, and probably most, do fine work. But paying someone to do something for you isn’t necessarily a guarantee that the job’s been done right, which is why I decided today to test the pressure on the potable hot water expansion tank at our Utah house. It wasn’t showing any signs of trouble. I just felt like testing it… and I discovered that it was dead. Any normal person would be bummed. Me? I was stoked — because that meant I got to perform a plumbing project I’d never attempted, and write a new blog post about it! 🙂

What’s a Hot Water Expansion Tank?

Expansion TankA hot water expansion tank is a small metal tank (usually 2-5 gallons) that’s installed just “downstream” of the cold water inlet valve that feeds your residential water heater. The expansion tank’s purpose is to protect your house’s hot water system from excessive pressure, which can cause damage to fixtures, your water heater, lines, and/or fittings.

Inside, the tank has a rubber diaphragm at around the mid-point of the tank; pressurized air goes below the diaphragm in the bottom half of the tank, and water flows in and out of the top half of the tank.

To visualize how an expansion tank works, imagine that the diaphragm is actually a rubber ball like you’d find on a playground. The rubber ball is partially filled with air, so that it completely fills up the tank. When the water in your system is “stronger” than the rubber ball, it pushes against it squeezes it smaller — allowing some of the water to force its way into the tank. But when the water pressure outside the tank returns to normal, the rubber ball is once again “stronger” than the water, and shoves the water back out of the of the tank.

In plumbing terms, the air pressure below the rubber diaphragm acts as a “cushion” against water hammer shock, and allows the tank to absorb excess water pressure caused by the expansion of heated water in the system. It’s far better to have pressurized water push against the “rubber ball” inside a reinforced tank, rather than push against your pipes and fixtures… or even the inside of your water heater. Expansion tanks are very cheap insurance against potentially dangerous and expensive damage to your plumbing. If you don’t have an expansion tank in your system, you really should get one.

Expansion tanks will normally wear out after anywhere from 5-10 years. To extend their service life as much as possible, it’s vitally important to match your expansion tank’s air pressure to your house’s water pressure.

Why Expansion Tank Pressure Is So Important

In order to for the shoving match between the water and the rubber diaphragm to work ideally, the air pressure below diaphragm the tank before it gets squeezed should be the same as the normal water pressure in your system. That air pressure is called the “pre-charge.”

Having the tank’s pre-charge set lower than the water pressure will allow expanding water to shove its way into the tank more often it should, which wears out the diaphragm prematurely. But having the tank’s pre-charge higher than the water pressure will prevent the expansion tank from allowing water into the tank, thereby rendering the tank pointless. It could also rupture the rubber diaphragm since it’s not designed to be inflated beyond 80 psi.

Most expansion tanks are set to a pre-charge of 20 psi when they leave the factory, while the majority of residential water systems have significantly higher water pressure than that. This means that whoever installed your tank was supposed to manually increase the  tank’s pre-charge to match the water pressure at the time of install.

But did they?

The only way to be sure is to test it yourself. Follow the simple test method below to make sure your installer set your expansion tank’s pre-charge properly, and verify that your tank is still operating as it should. If it’s not, you can also follow the instructions in this post to replace your expansion tank yourself. Don’t worry — it’s really easy.

 Tools You’ll Need

To test the water pressure in your house, I recommend using something like a Pasco 1431 or a Rainbird P2A water test gauge. It simply attaches to your system anywhere that a hose would connect and tells you the water pressure at the fixture. Because I didn’t want to wait for Amazon, I picked up the Pasco at a local plumbing supply store for $12. It’s worth having one of these in your toolbox for a number of projects.

To test the pre-charge of your expansion tank, all you need is a standard tire pressure gauge. If you don’t already have one in your car’s glove box, you really should. I used a standard dial type tire pressure gauge (about $9 from any local auto parts store). My tire gauge has a bleeder valve, which makes removing excess pressure really easy.

How to Check Your Water Pressure

First, find the pressure of the water going into your water heater by attaching the water pressure gauge to your water heater’s drain valve at the bottom of the tank (the same place you’d attach a hose when you flush your water heater). Open the drain valve all the way. You don’t need to shut off any other valves or do anything else. Take note of the water pressure. Mine at the Utah house is 60 psi:

Testing residential water pressure with simple and cheap gauge

Testing residential water pressure with simple and cheap gauge

How to Check Your Expansion Tank Pre-Charge

Before checking the pre-charge on your tank, try this “quick and dirty” test to see if there’s any water in the tank: knock a few times on the side of the tank with your knuckles. You should hear the hollow “tink tink tink” noise of a metal tank that’s mostly full of air, rather than the “thud thud thud” noise of a metal tank that’s full of water. If your tank sounds full of water, then it’s possible that:

  • the pre-charge is set too low, which is allowing the water pressure to “defeat” the diaphragm too easily and let too much water into the tank, or
  • the diaphragm has ruptured or otherwise failed, so there’s nothing to keep the water out.

If you think your expansion tank may be full of water, unscrew the plastic cover on the bottom of the tank that exposes the Schrader valve, then and briefly press in on the valve’s pin (like you would to let air out of a tire). If water comes out of the valve, then the diaphragm in your tank is ruptured, and the tank needs to be replaced. If only air comes out,  continue with the test procedures below, but don’t attach your tire pressure gauge just yet!

Before you can accurately check the pre-charge (the pressure inside the rubber diaphragm in your expansion tank), you first need to relieve the pressure that’s pushing against the diaphragm from your house’s water system. Imagine, for example, that you want to check the air pressure of a yoga ball. But if you test the pressure while still sitting on the ball, you’ll get a higher number than you would if you checked the pressure while standing next to the ball. So to get rid of the pressure from your water system “sitting” on your expansion tank’s rubber “yoga ball,” do the following:

1. Shut off the cold water valve that feeds your water heater (it should be just “upstream” of your expansion tank). With these lever types, the valve is open when it’s parallel to the line, and closed when its perpendicular. This photo shows it closed:

Cold water valve turned off

Cold water valve turned off

2. Turn on a hot water fixture that’s closest to your water heater, such as a hot faucet. Keep it open throughout this procedure.

3. Turn the knob on your water heater to the PILOT position, to prevent the burner from firing:

Gas valve set to PILOT

Gas valve set to PILOT

4. If you have a hot water recirculating pump, turn it off and close the valve on the return line:

Turn off the recirculating pump and close the return valve

Turn off the recirculating pump and close the return valve

5. Connect a drain hose to the drain valve of your water heater (the same valve where you tested the water pressure), and open the valve all the way. Do a slow count to 10, then close the valve. That should remove enough water from the system to relieve any water pressure that’s pressing against the rubber diaphragm inside the expansion tank.

Partially drain your water heater to relieve water pressure

Partially drain your water heater to relieve water pressure

With the water pressure relieved, you can now get an accurate reading from your tank. Expose the Schrader valve (same valve you’d find on a bike or car tire) on the bottom of the expansion tank (you may have to unscrew a plastic cap) and firmly press your tire pressure gauge against the valve to get a reading. If the resulting pressure is higher than your house’s water pressure, then take some air pressure out of the expansion tank a little at a time by pressing in on the pin inside the valve (exactly as you would when lowering the pressure in your car’s tires). If the expansion tank’s pressure is lower than your house’s water pressure, you’ll need to fill the tank to the matching pressure, using the same type of air compressor you’d use to re-inflate your car’s tires.

If your pre-charge was set properly or only needed a minor pressure adjustment, then HOORAY! You’re good to go.

But, if you get no pressure reading from your tank (which is what happened to me today), then that probably means the diaphragm in your tank has ruptured or otherwise failed, meaning your expansion tank needs to be replaced. Here was my zero pressure reading today:

No pressure from hot water expansion tank

No pressure from hot water expansion tank

Replacing a Potable Hot Water Expansion Tank

If you’ve determined your expansion tank is bad, or it’s old enough that you just want to replace it anyway, installing a new one is really easy — especially if you’ve already followed the above steps to test the pre-charge (meaning you’ve already relieved pressure from the line and closed all the appropriate valves).

Generally, you should replace the old expansion tank with a new one of the same size. Going bigger is fine — going smaller is not. Most water heaters that are 50 gallons or smaller should use a 2 gallon expansion tank, such as an Amtrol ST-5 or a Watts PLT-5. But if you have two water heaters hooked together, or a water heater that’s larger than 50 gallons, go for a 4.4 gallon expansion tank like the Amtrol ST-12 or Watts PLT-12. Since I have two 40 gallon water heaters that are connected in series (for a total of 80 gallons in the zone), I picked up a Watts PLT-12 at a local plumbing supply store for $50, which is about what they sell for on Amazon. You can also find them at Home Depot, Lowes, or your local hardware store. The original installing plumber had actually used the smaller tank size in this house, so I figured this was the right time to correct that mistake and go one size bigger. I’m not sure the smaller size contributed to the failure of the tank, but the price difference between the two sizes is minor, so go for the larger one if you can.

After bringing the new tank home, I first made sure all the proper valves were closed and pressure relieved from the system, following all the above steps required to test the expansion tank pressure — including turning the gas valves to PILOT and partially draining the tank.

Next, I removed one side of the support strap that was helping to keep the old tank in place (it was screwed into the ductwork overhead). Your unit may not have a support strap (it only needs one if the tank is installed horizontally).

Remove old support strap

Remove old support strap

Third, I placed a towel on top of the water heater, underneath where the expansion tank connected to the cold water line. Since the failed tank is likely full of water, be ready for it to spill.

Fourth, unscrew the tank from the threaded fitting in the line. You may be able to get enough leverage on the tank to unscrew it without any tools, but channel locks will come in handy if you can’t. Here’s the fitting with the tank removed:

Fitting where the expansion tank connects

Fitting where the expansion tank connects

Here’s a shot of my old Amtrol tank (blue) next to my new Watts tank (white). Both manufacturers make quality tanks right here in the USA. I simply picked the Watts tank because it happened to be in stock.

Old Amtrol tank next to the new Watts tank

Old Amtrol tank next to the new Watts tank

As mentioned earlier, all new expansion tanks come from the factory with some amount of pressure in the diaphragm , meaning the diaphragm will be inflated enough so that reaches the opening of the tank. On a functioning tank with pressure in the diaphragm, you should be able to stick your finger through the top and touch it.

However, when I inspected my old tank, I couldn’t feel the rubber diaphragm through the opening. That means it had indeed failed, as I suspected. I stuck my phone’s camera into the hole to get a peek at the failed diaphragm deep inside the tank:

Failed diaphragm inside an expansion tank

Failed diaphragm inside an expansion tank

Even though the new tank already had some amount of pre-charge from the factory, it’s still important to set the correct pre-charge pressure in the new tank before installing it and allowing water pressure to push against the rubber diaphragm. Again, even if your yoga ball was partially inflated when you bought it, you still wouldn’t try to inflate it all the way while you’re sitting on it, would you?

Any type of air compressor that you’d use to fill your car’s tires will work to set the proper pressure in your expansion tank. I used a next-door neighbor’s compressor (it was easiest to take my tank to him, rather than bring his compressor to me). First, I tested the initial pressure of the new tank, and it was exactly 20 psi, as stated on the box:

Watts expansion tank initial pressure was exactly 20 psi

Watts expansion tank initial pressure was exactly 20 psi

It didn’t take long to add some air and take the pressure to 60 psi:

Adding air to the expansion tank with a compressor

Adding air to the expansion tank with a compressor

Expansion tank pressure set to match system water pressure

Expansion tank pressure set to match system water pressure

Before installing the new tank, I added some Teflon tape to the male threads:

Add Teflon tape to male threads to ensure a good seal

Add Teflon tape to male threads to ensure a good seal

Then I installed the new tank by slowly twisting it into place, and tightened it down with channel locks:

Tightening the new tank into place

Tightening the new tank into place

Finally, I tested for leaks by slowly turning the cold water inlet valve back on:

Cold water inlet valve opened to test for leaks

Cold water inlet valve opened to test for leaks

No leaks! I secured the new tank with the existing metal strap that had been supporting the old tank. If the tank is installed vertically (either above or below a line), you don’t need any additional support. However, if installed horizontally, you’ll need support straps to help hold the weight of the tank when it’s partially full of water.

Securing the new tank

Securing the new tank

After making sure the tank was securely in place, I turned my gas valve back to ON, turned on my recirculating pump and opened the return line valve.

Now, my system is once again protected against potential damage from thermal expansion and water hammer. I only spent $50 on the new tank, $9 on the water pressure gauge (which I’ll reuse often), and used a tire gauge I already had. The entire process, including the testing, took less than 30 minutes. I promise it’s something you can do, even if you’re not particularly handy.

But this important piece of safety equipment in your house can’t work properly if it’s not set properly, and if it’s not tested regularly. I was surprised today when I discovered mine had failed, and glad that the fix was inexpensive and straightforward. I’ve now added “Check expansion tank pressure” to my home maintenance schedule and plan to do it every six months (it’s easy to do when you’re flushing your tank and checking your anode), and suggest you do the same.

As always, I welcome your comments and questions below!

Special thanks to BJ Plumbing Supply in Orem, UT for their expert advice on tackling this project, and for always providing the parts I need at a good price.



  • Mother

    Please come and play testing plumber at our place…please, please, please

  • Sam

    I had an expansion tank installed with my new water heater, with-in 2 months my water bill trippled. My water meter was continously fluctuating, after several visits by the plummer it was determined the expansion tank was causing the problem. The plummer removed the tank and my water bill went to normal. Could this problem have been caused by a faulty Expansioon tank? Have you ever heard of a situation like this?

    • Unless the “problem” was a water leak, I don’t see how an expansion tank could cause a bill to increase at all. When an expansion tank fails, the bladder ruptures and the tank simply fills with water, but that doesn’t create any additional demand or water use. I’d be curious to know exactly what your plumber claimed the problem was. Now, if you had a pressure tank (which is different than an expansion tank) that was increasing the water pressure to the fixtures in your house, then yes – that will increase water usage because more water is flowing through your fixtures when they are open. Although that’s not normally a “faulty” pressure tank, but simply a need to adjust the pressure down so that you balance pressure with usage.

      • Eoghan

        If the expansion tank was installed incorrectly and the bladder had no useful affect on curtailing the expansion, then the safety valve may have been continuously opening and you would have been loosing hot water. But even that would not have been a large amount of water, and only would have happened as the system was getting up to temperature each time.

  • madeline


    my husband replaced the expansion tank and when we turn on the heat for the house, there is this noise that sound like a microwave is running. Is that normal? Will this noise go away.

    Thank you

    • I bet the tank is vibrating against your water heater. If you hold it in place, does the noise stop? If so, you can try securing it with a metal strap!

  • Mark

    I’m replacing a 40 gallon water heater with a 50 gallon. There is no expansion tank currently. I’ve bought the Watts plt-12 (4.5 gallon) expansion tank. Is this too big? Should I use the 2 gallon model? Thanks!

    • Hi, Mark. The bigger expansion tank that you got will work fine! Go ahead and install it. Let me know how it goes!

  • KenB

    Thanks, best blog on expansion tanks. However, I don’t agree with your “yoga ball theory” that sitting on it will yield a higher pressure. If that were true then filling my car tires with 35 psi, before I put them on my car would yield a much higher pressure – but I know that’s not true.

    • Hi, Ken. Thanks for your comment! You’re forgetting about sidewalls on your car tires, which are rigid and help support downward pressure and keep the shape of the tire intact, and therefore the volume of the air cavity in the tire more constant. A yoga ball has no sidewalls. Nor does a balloon, which is why you can easily pop a balloon by sitting on it (decreasing the volume and increasing the pressure to the point of rupture). Google “Boyle’s Law” for a discussion of how volume changes affect the pressure of gasses (like air). Or, you can test emperically. Your pressure gauge when testing your expansion tank will read higher when your house’s water pressure is squeezing the air bladder in the tank vs. if you close the feed valve and relieve the water pressure briefly via the TPR valve or the drain. A proper pre-charge pressure will make this change more noticeable. If set too high, it would prevent water from entering the tank. Give it a shot and see!

  • Elly

    Hi Steve,
    My husband replaced our old expansion tank to a new one 2 days ago due to leaking. The water was hot but got warm after about 10 minutes right after he replaced the tank. However since yesterday there was no hot water at all. When he shut off the expansion tank and run the hot water, we get hot water but when he turned it back on we get cold water. Also the sound of the circulator pump was loud when he turned on the tank . We couldn’t figure out what’s the problem. Do you know what happened? Please help, thanks!

    • Probably air in the system. Bleed it out by opening all your hot faucets one at a time until they flow without sputter. That should do the trick!

      • Sam

        Steve, the plumber who installed and removed my exspansion told me the water pressure on the bladder in the tank was expanding and deflating causing the water pressure in the line fluctuate with forward and reverse pressure to cause the water meter to spin forward and backward. The meter at the main turnoff valve was in fact spinning forward and backwards causing the meter to record a constant use of water. Once removed the meter immediately registered the correct use of water. Regardless of what the purpose of installing a pressure tank. I believe it’s a ploy to make money for the water company thereby making money for the city. We have been using water heaters for 70+ years and never had a problem. This is my belief.

        • Hi, Sam! Thanks for your comment. Your problem can be easily fixed with a check valve. I’ve got a swing check valve on my 1 inch line just “upstream” from the main water shutoff into the house, which prevents water from flowing back toward the street (and the meter). A check valve costs less than $5, and can be installed in less than 15 minutes. If that solution didn’t occur to your plumber in about 2.4 seconds, then you need to find yourself a better plumber. 🙂

          The only way your expansion tank would be able to generate enough pressure to push water back up your main and out to the meter is if your pre-charge was set WAY above your “normal” pressure, and/or your “normal” pressure from the street is extremely low.

          Again, I think your plumber is steering your wrong.

        • Richard M

          Sam, it helps if you have a full understanding of the physics behind hydraulic systems. If anything can send water back toward the meter, then the meter does not have a check valve and you do NOT have a closed system. This is the type of system used for most of your 70 years and water pressure in your home could never exceed the supply pressure. Check valves are code today because you don’t want your neighbor’s fault irrigation or sewage system backing up into the water mains. Has nothing to do with making money. That’s paranoid and silly.
          If you don’t have a check valve… then you don’t need an expansion tank. THAT was your problem… you installed a device into a system that did not have the parameters necessary for it to function properly and the result was unintended consequences. The bladder was “breathing” because of normal fluctuations in water pressure and the inertia of the water column filling the bladder. This would not have happened if the bladder was properly pre-charged to the highest water pressure measured in a 24 hour period with a memory gauge.
          So, instead of heading right to the gremlins or corrupt government conspiracy answers, it’s best to know what you’re doing and absolve yourself of all blame.

      • Elly

        Hi Steve, thanks for your prompt reply. I really appreciate it. I opened the hot water faucets one at a time this morning, but the water was still cold. My husband checked the pressure of the tank and it matched our water pressure (20 psi). He thinks the problem could be the circulator pump. Because if we shut off the pump, we get hot water but if we turn on the pump we get cold water. We never had this problem before until the old expansion tank leaked and we changed the tank. Please let me know your opinion on this.
        Thanks so much for your help Steve!

        • First, 20psi is extremely low for residential water pressure. If you’re on a well, you should look into increasing your pump pressure. If on city water, call and complain. You should be above 40psi, ideally at around 60psi. At that low pressure, it’s possible your recirc pump is pushing harder than your base water pressure! Check to make sure that the recirc pump is pumping in the right direction (pulling from the house and pushing toward the tank). You should have a check valve between the recirc pump and the tank, to ensure water only flows the correct way. Make sure you don’t have any check valves on the cold side between the water tank and the expansion tank, but you can (and probably should) have one on the cold water feed right as it comes out of the wall. Order of components from the wall to the tank on the cold side should be: check valve, shut-off valve, expansion tank, hot water tank. Also, are you checking your expansion tank pressure with the water shut off and pressure drained from the hot tank?

  • All of a sudden tonight my potable water expansion tank is rattling/vibrating every two minutes then it stops after about ten seconds. What is going on and how do I make it stop?

    • Have you shut off the cold feed to the water heater, relieved the pressure, and checked the pre-charge pressure on the tank?

      • Shelley

        No and not sure I know how. Anyway to just disengage it until I get a plumber here?

        • Yes – this article explains how to shut off the water coming into your tank and check the pressure. Follow those steps to see if your tank has possibly failed.

  • Kevin


  • Greg

    Greetings. My house is 16 years old. I had the original gas water heater tank replaced about one year ago. It is 75 gal. due to a spa bath tub. I did not have the original expansion tank replaced because the plumber stated it still worked (by releasing air from the stem). I don’t know if they messed with the pressure when they replaced the WH. One year later, periodically there are vibrations coming from the pipes just above the expansion tank. This has occurred in the middle of the night when no water has been run for several hours. It lasts for about ten minutes then goes away. What to you think? Many thanks in advance.

    • Do you have a pressure gauge on the system? I’d be interested to see if those noises correlate to increased pressure. If so, then I’d guess your pressure is set wrong in the tank. And the pressure was probably never set right by the plumber… they almost never are. 🙂

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  • Josh

    Thank you great explanation. I am replacing my water heater and had a few questions about the expansion tank which you answered.

  • Stewart

    Steve: thank you for the brilliant details and information. In your photo titled “Partially drain your water heater to relieve water pressure” above, there is a white hose end which seems to lead to an in ground drain. What is that? I ask because my water heater in the recently purchased house has a solid pipe with an open end to the floor, but the drain is 3 ft away so the previous owner had a small bucket under the exposed pipe end which occasionally releases water into the bucket which I then check and drain as needed. I thought perhaps the expansion tank was shot but the knuckle test clearly seems to indicate that the upper portion is filled with water and the lower half of the tank has air (though the expansion tank sits vertically above the water heater with the pipe connecting from the bottom. Thank you for your time and kind assistance.

  • Rob

    Wow! I work in industrial maintenance. I at times write “How to” on the job. This is by far one of the most comprehensive DIY explanations I have seen on the internet, ever. It is how I try to make any set of instructions I put out. I am very familiar with thermal expansion and the tanks used to address the issue. Clear and informative with lots of pictures…….great.

    • james

      I have a similar background as Rob. I agree with his evaluation of this blog.

      • Thanks, James. That kind of feedback makes it all worthwhile. 🙂

    • Thanks, Rob! That’s very nice of you to say! 🙂

  • bill

    if my expansion tank is bad,and I check my water pressure at the water heater like you’ve shown,wont that pressure reading include existing”thermal expansion”? I would think that is not the reading I would want to set my new tank pressure too. What am I missing?

    • If it’s bad, the tank will actually leak water from the valve (assume the valve is pointed down when you test it… and ideally, it should be). Once you’ve determined the tank isn’t leaking water (meaning it’s almost certainly good), just follow the steps in this article to test: shut off the flow into the tank, relieve some pressure from the tank with a partial drain, and then test the pre-charge pressure on the tank! 🙂

  • Kiley Ferons

    We just replaced the expansion tank, but when we went to test the water pressure from the drain valve on the water tank, no water is coming out. We have water running in the house and everything else seems fine, but not sure how to test the water pressure if nothing is coming out of the drain valve on the water tank.

    • Hi, Kiley. You can test your water pressure from any faucet attached to your house (including an outdoor hose-bib) with that pressure gauge. However, as you probably suspect, you may have something blocking your drain valve on your water heater.

      If you put a bucket under the water heater drain and open the drain valve, nothing is coming out?

      • Kiley Ferons

        The relief valve when flipped on does release water. But the drain faucet does not release more than a hot trickle.

        • Then yes – something is clogging your drain valve. If it hasn’t been flushed in a while (see my post on flushing water heaters), sediment and gunk can do that.

          On a day when you have the time to do so, try clearing the blockage by: 1) turn the gas to pilot, 2) attach a drain hose to your drain valve, 3) shut off the cold water inlet valve, 4) open a hot water faucet somewhere on the system to prevent vacuum, 5) turn off your recirc pump (if you have one), 6) open the drain valve (even if it’s just a trickle), 7) open the relief valve all the way until the water in the tank drops to the point where it doesn’t drain from it any more, then close the relief valve, 8) wait a while until the tank drains — if you can wait a few hours, that’s great, 9) open the cold water inlet valve in short bursts to see if you can swish water around in the bottom and clear the blockage.

          Another idea is that if you can turn the gas valve to pilot and wait long enough for the water in the tank to cool (overnight is preferable), attach a hose to the drain valve, open the valve, and blow in the other end of the hose (a short hose works best). You might be able to blow the sediment back into the tank, where it will disperse and drain easier. But be VERY careful with this method. Make sure the water has cooled off, of you’ll burn yourself! 🙂

  • Bret

    I recently replace the water pressure tank on my rural water system in the house, (the old ones bladder had failed). Now the pressure is more consant however i have noticed that when the hot water is on it wil get to a certain point the water flow will stop completly, wait a few seconds and then it starts again with now problems. I’m guessing this has something to do with the start stop pressures on the swtich, but not understanding completely what to next.


  • Kevin

    This was an awesome guide! Thanks for taking the time to share your knowledge! I have an eighty gallon tank with an expansion tank size of 2.1 gallons. Should this be replaced with a larger tank? The house is new and still under warranty. Any help appreciated. Thanks again.

    • When it comes time to replace your tank, I’d go ahead and put the bigger one in. Probably no need to swap it out right away.

  • Steve, Thanks for posting this, it has GREAT information!

    We just bought a new house, and in the morning or after work, when I first turn on a water faucet I get a big rush of water for a fraction of a second. I also noticed that the water heater’s pressure relief valve is dripping most of the time. This lead me to get a pressure gauge, and do some testing. I found that when we come back from work the pressure is 150PSI!! As soon as we turn on a faucet for even a few seconds, the pressure drops back down to about 45-50PSI. This is a clear indication that we have a check valve and the expansion tank has failed.

    This has actually resulted in our water heater dying and needing to be replaced, since it isn’t supposed to be sitting at 150PSI all day, so this very simple check could save people HUNDREDS in repairs if they don’t!!

  • Theresa

    Thank you very much for the clear details! We are getting ready to buy a new water heater, a 40 gallon gas water heater (not sure what to buy yet!) The plumber says, we do not need an expansion tank. I thought all water heaters do need an expansion tank. He said there was something that my previous installation had that should suffice in place of an expansion tank! Not sure what that might be! Do you know what he is referring to? Thanks! Theresa

    • Hmm… I’d be very interested to know what he says is taking the place of the expansion tank. It’s very cheap insurance against major plumbing damage, and I can’t think of anything off-hand that can truly taking its place.

      • AMS

        A water hammer arrestor can take its place theoretically, but they’re normally used near the fitting that’s causing water hammer, thus they’re really local solutions and not a housewide solution. An expansion tank is (or at least should be) the best housewide solution.

  • Theresa

    Additionally, there was no expansion tank when our previous one was installed about 10-12 years ago, a direct vent A.O.Smith. Thanks again!

  • Robert D


    Thank you for this comprehensive page! I have some questions for you: the TnP pipe on my hot water heater has been dripping since we’ve turned it on a couple of weeks ago. Additionally, our gas boiler is also dripping from the backflow pipe when both are running. The two lines where the leaks are occurring are connected directly and indirectly to one another. The cold lead into the hot water tank feels warm to the touch. The boiler has an expansion tank but the 40 gallon hot water heater does not. I think it could be one of two things. Either the backflow valve on the boiler needs to be replaced (what several people have told me) and/or there needs to be an additional expansion tank ( what I think as a layperson). The water pressure is within the expected range. I feel like installing an expansion tank would stop both leaks. Should each system (boiler/hot water) have their own expansion tank or would one work for both systems? I haven’t tested the pressure in the line expansion tank but i think it might be fine. Any thoughts on your end?

    • Hi, Robert. Backflow valves (or check valves, as I call them) do eventually go bad and should be inspected and replaced as needed. I’d start there with the boiler.

      Next, if your TnP valve is a few years old, it might just be time to replace it, as well.

      As for the cold water inlet on your hot water tank being warm to the touch, I had that same problem. I fixed it by putting a check valve on the cold water side of my water heater, but you need to make sure it’s installed “uptstream” of your expansion tank. If you also run a vacuum breaker valve in your setup (like I do), the check valve needs to be “downstream” of the vacuum breaker. Check this blog post (and pay particular attention to the last photo) to see how I have mine set up:

      If your current expansion tank is installed so that no check valves exist in the plumbing between the water heater and the boiler, then one large expansion tank SHOULD be able to help absorb any temporary over-pressure issues on both.

      But if you have any check valves that could prevent any flow between the boiler and the tank, your safest bet is to throw a separate expansion tank on the water heater. They are cheap insurance. 🙂

  • DHD

    If I follow your directions to check expansion tank pre charge and I see that it’s on the low side can I pump it up while it is still attached or must I remove it, charge it and then reinstall?

    • As long as you have the valve that lets water into the main hot water tank shut OFF (no there’s no water pressure hitting the expansion tank from the “upstream” side) and the main hot water tank is partially drained (so there’s no water pressure hitting the expansion tank from the “downstream” side), then you don’t need to remove the expansion tank to set the pre-charge pressure properly. There must be NO external pressure on the tank’s bladder from water pressure anywhere in the system while you set the pre-charge pressure.

      • DHD

        Followed your great directions for replacing the expansion tank, except I cannot get the old tank off. It’s been on for 12 years. I can get a good grip on it, easy access but it won’t budge. I’m afraid of damaging the pipe, etc. Any tips. Seems simple enough.

        • Try applying some heat to the thread location. Heat gun is preferable, but light torch if you have to.

          You might also try some type of anti-seize blasting spray (check your hardware store).

          If you’re lucky, the old tank might actually have a nut-shaped collar near the base of the threads, and you can use a wrench. Just be careful to hold onto any pipes that connect to it, so you don’t pull anything else loose.

          • DHD

            Thanks for the quick response. It does have a nut collar on it and I am able to get a good grip on it, it just won’t budge. I have not tried anti-seize. I did spray it last night with WD 40. Is anti seize that much different?

          • It’s slightly different, but I think your best bet is heat. If you have a butane torch, hit it for a second or two and see if that makes the metal expand enough to let go.

  • William

    Hi Steve.
    Thanks so much for sharing your knowledge. I was able to replace my pressure tank but still have rumbling and rattling sounds from the pipes. I cannot figure out were they are coming from. I tried draining but was not successful. Please advice me.

    • Is it water hammer type rattling? You might want to check into a water hammer arrester. I installed two on my washing machine, and that fixed the last of the “popping” noises for me!

  • Hi there. I moved into my house in Aug 2014. First time having an expansion tank. It’s very loud. Sounds like a choo choo train. I don’t think it’s right. I posted a video on my fb profile. Has it failed? Sometimes it knocks really loud. Thank you.

    • Hi, Karen. I watched your video, and what strikes me is how rhythmic the sound is. You said in the caption of the video that it’s while your washing machine is running. Does it ever do this when the washing machine isn’t running? There’s a good chance it has failed. If you shut off that water valve and check the pressure of the tank with a tire pressure gauge, what does it read?

      • My husband replaced the tank ( first time dealing with an expansion tank). I came home from work and he was visibly shaken up, after almost losing his head when the new expansion tank nearly went through the ceiling… I asked him about it, and he said it was because he didn’t have any faucets open when he was putting it on, so the pressure built up. … I guess that’s what happened. Well, it’s working, but our pressure is low in the faucets. He says the pressure in the faucet outside is about 50-60 psi. So, I don’t know what to do to make it better. Any suggestions for me, to relay on to him? I’m afraid he might have lost some pressure in the tank also, because of the mishap.

        • Hi, Karen. Yikes! Yes, it’s VERY important when dealing with any pressurized system (water, air, propane, natural gas, etc.) to make sure the system is depressurized before attempting repairs. But I would have loved a photo of the look on hubby’s face when the tank lifted off. 🙂 Glad he’s OK!

          If he’s certain the pressure on the “downstream” side of the tank is 50-60 PSI, but the faucets are still not flowing fast enough, have him check to see if he can remove the flow restrictors on the faucets. If you remove them, you’ll use more water, but the flow rate will be higher.

          As for whether he created an issue with the tank, that’s easy to check (he probably didn’t). Just follow the steps in this post for testing the tank. If it’s holding pre-charge pressure, it’s fine. 🙂

  • Paul Revis

    Steve, thank you for this tutorial. It was a big help when my tank failed this morning.

  • Kathy Dwyer

    I have a home in Big Sky mt, So learning how to do the things I can saves me tons in trip charges. Anyway, instructions were perfect. The only problem is I am having a hard time removing the failed tank. Any tricks to compensate my puny arm strength?

    • Kathy Dwyer

      I GOT IT OFF! turn the tank clockwise to loosen. Thanks.

  • Im based in the UK but i imagine the expansion tanks are the same. I have and Air Source heat pump system installed and in the last two years the expansion tanks ruptured twice. the only sign its happens is the leaking through the tundish valve. do you have any ideas why it keeps happening?

    our systems 60psi as well.

  • Flint McCullar

    Great post, easy to follow instructions, but I still have a problem. My pressure regulator failed and I had pressure above 100psi the pipes vibrated and howled. at the same time we replaced the hot water heater and they added a new expansion tank. A few days later I replaced the pressure regulator. Pipes still vibrating and howling, tried every pressure between 25 and 85 on the water pressure. I read your article and thought that must be it, so I set my regulator to 50psi and checked my expansion tank then set it to 50psi after removing it. I strapped the horizontal mounted exp. tank to the rafters and finished up by trying to bleed the air out my lines. There was alot in there, especially the hot waterside. But after all this, I still have vibrating pipes and howling. It vibrates cold or hot when the faucet is turned on, but not when no water is running. Sometimes you can open the tap 1/10th the way open and get a trickle of water with out the hammer sound.

    I called the water company out they verified that I do have a backflow prevention valve and my street pressure from the main is 110psi.

    I am about to swap out my regulator again, add hammer arrestors to all of my appliance fill valves. then I am out of ideas, and I fear calling a plumber, as they wanted $700 to swap out a $50 pressure regulator that took me as a layperson less than an hour to do.

    Any Ideas?

    • Hi, Flint. You’ve got 110 PSI water at the street? Wow.. that’s WAY too high. The first thing you should do is get a 70 PSI regulator and install it right where the water comes into your house. I bet that solves a heap of problems for you. 🙂

      • Flint McCullar

        Steve, I have a pressure regulator set at 50 PSI just downstream of my cut off inside the house. Do you mean I need a 2nd Pressure reducing valve upstream of that set at 70 PSI?

        After replacing my pressure reducing valve and setting the pre-charge on the expansion tank to match at 50psi, I bled the air from the lines and thought I was done. Wrong, I still get a vibration, though not as bad, loud, or as long and not every time I turn on the water. But we are still plagued with the sound intermittently. So when say the washer is filling (which has hammer arrestors on the shut off) and the pipes start to vibrate, I have found that if I open up a faucet it will reduce or eliminate the noise. I assume this is relieving some of the pressure difference. I have replaced one bad toilet fill valve, and am about to replace the others for good measure. Then I am going to replace faucet valves or faucets to eliminate any bad valves.

        please advise,

        • Hi, Flint. Ah – I didn’t realize you already had a pressure regulator in place. You don’t need a second one. But have you tested with a pressure gauge to make sure that the water pressure on the downstream side of regulator (basically anywhere in your system — easiest place to check is the drain valve on your water heater) is right at 50 PSI? I’d check that first to make sure it’s working properly, and then make sure your pre-charge on your expansion tank is set correctly to the actual pressure (make sure you check and set the pre-charge pressure with the cold water near the water heater shut off, and the pressure relieved via an open hot water faucet).

          I’d also check to see if there are any air chambers in your system that may have become clogged (especially if the water hammer is a recent thing).



  • Dennis

    Steve, Thank you for the detailed information. Our hot water tank failed, we were able to confirm our expansion tank was not the cause. Very good information that I will use in the future. Thank you!

  • Brian

    My wife and I recently purchased a house. It’s close to 50 years old. It has an oil burner with forced hot water through baseboard radiators for heat. As with any new environment you have to get used to certain noises and quirks. Some of the noises coming from the pipes are nothing I’ve ever heard before. Some of the different noises range from water gushing through the pipe, scrapping noises, wall knocking, and sometimes a snaking a pipe type sound. The hot water also comes out extremely hot. After some research I checked the pressure on the burner and it said 30 psi. I wonder whether the aqua stat settings are set too high resulting in the higher pressure and extremely hot water? I also am wondering if the expansion tank is bad as well. Both the top half and bottom half are hot to the touch and both halfs sound the same. Advice for homeowner newbies?

    • My first advice is to be very careful when dealing with hot and pressurized water! Then I’d just start going through and learning about each system, and test along the way (checking the expansion tanks is a great place to start). The stat settings being too high will indeed create higher pressure, so I’d also experiment with turning that down and see how that affects things. In general, just fix one problem, then move on to the next. Don’t be afraid to make mistakes (just always take safety seriously), and you’ll be DIY homeowner pros before you know it. Good luck! 🙂

  • Mike Bermingham

    HI, I have underfloor heating. The pressure relief vessel gauge is set at about 1 bar. (the red arrow) and the black is usually just above 0.Few people know much about the system of UF heating. I think the tank bladder has failed. It is about 10 yards away from the boiler and never gets warm, I think this is correct? I have to top up the system with the little knurled knob close to the the vessel every day, up to the level of the red arrow mentioned above. I think this indicates a leak but before I fix the leak I want to replace the expansion vessel with a REFLEX 11 litre with a precharge pressure on the box of 1.5 bar. I think this is a different matter to what you are talking about in general in your fine article above? The heating system has a working temperature of about 48 on flow and about 45 on the return and apart from the above problems it works very well. It has an oilfired boiler in an outside boilerhouse.ANY help or guidance you can give me would be greatly appreciated,
    Mike age 71

  • sb33334


    I had contractors put a 2d floor on the house and a second zone for my hydronic water boiler. When first installed, I was getting lots of water out of the pressure relief valve. The plumber “took a little air out of the explansion tank” (very little, I watched him) and the PRV leaking stopped for a short while. (Long enough for the plumber to make his escape). Anyhow, eventually the leak returned in short order and is getting worse.

    Do you think the design of the 2d zone is the issue, or the relief tank? Where the tank is located is after the zone valve for the original, 1st zone. When that zone is closed, I don’t see how the 2d zone would have access to the tank- and I noticed that the PRV leak happens mostly (or exclusively) when the 2d zone is open/running andf the 1st is closed. Hope this makes sense. I have purchased and plan to replace the relief tank anyway but wanted to ask your opinion.

  • Chris

    Just curious if an expansion tank is necessary on a well system with a pressure tank?
    Thanks very much
    (awesome article!)

    • Hi, Chris. No, that’s not needed… since the pressure tank actually has its own bladder inside.

      • Chris

        I kind of figured, just wanted an experienced insight.
        Thanks very much.

  • tom

    Hi Steve, I just installed a new Watts prv and I notice pressure creep and the pressure relief valve dumps. Do I need a thermal expansion tank to solve the problem? If so can I install it anywhere in the water line. I have a crawl space that would be a perfect location. Tom

    • Hi, Tom. It’s not a guaranteed fix, but it could help. The closer to the water heater the better, but anywhere on the line is better than nowhere. 🙂 Just make sure you can get to it to check it periodically. They usually fail within 5 years.

  • sb33334

    Hi Steve- No clue on my question, above?

  • Bogdan

    Hi Steve. In preparation to replacing a broken hot water expansion tank (your first test gave a “thud, thud” sound and then I found that water was gushing through the Schrader valve when pressed) I checked water pressure in my pipes. The reading was 80 to 85 psi (municipal water in Ohio). Is it acceptable or should I do something to lower it?

    • Hi, Bogdan. That’s pretty high for city water. I recommend putting in a 75 psi regulator just downstream of your main shut off. Once you get higher than 75 psi, residential plumbing systems can get unhappy. Come back and let me know how the tank replacement went!

      • Bogdan

        Thanks Steve. Your comments are precious. I was about to replace the expansion tank (following your advice I bought the Watts PLT 12, one size bigger then the old one for my 75 gal water heater) but now I am lost. Can I install a pressure regulator myself? Can I buy at Home Depot? I may call a plumber, unless… you will convince me that I can do this myself :-). This would have to be as easy as replacing the expansion tank. Best regards.

        • Hi, Bodgan. It’s more involved than simply swapping out an expansion tank, but assuming you have a 3/4″ line coming into your house, you’ll just need this:

          You can get that part at Home Depot, too (about $60). If you have a different size of pipe (like 1″) then get that size instead, of course.

          To install, just to shut off your main water, then find a location as close to the shutoff valve as is practical (and before it splits off anywhere else or goes into any fixtures or appliances). Measure the distance between the two SharkBite ends on the regulator, then cut a piece out of your pipe that is 2″ SHORTER than that length, because about 1″ of your pipe will slide into each end of the regulator (or maybe the regulator will have instructions as to what size you need to cut).

          Once it’s installed, you can turn the water back on and then turn the top screw to adjust anywhere between 10-70 psi (the default of the regulator is 45 psi).

          It’s totally something you can do yourself. 🙂

          • Bogdan

            Steve, OK then. I am going to do this. Incoming line is copper, 7/8 inch outside diameter (does it go for 1″?). Tomorrow back to Home Depot for a pressure regulator. I’ll keep you posted on the progress. My wife though is getting concerned :-). Thanks for your fast and very detailed advice.

          • If you’re certain it’s 7/8″ instead of 1″ then get the 7/8″. I found a YouTube video that shows how to install a standard regulator (with female threads) using two SharkBite fittings, which will essentially be the same as getting a SharkBite brand regulator like I suggested originally, so I’d go with whatever is cheapest. 🙂 Lemme know how it goes!

  • Bogdan

    Steve, Mission accomplished! I installed the Watts DET 12 (Home Depot – $63) Water Heater Expansion Tank and Watts Water Pressure Reducing Valve LF25AUB-Z3 ($110 in a local plumbing supply store. Add to this a water pressure gauge ($10) and tube cutter (another $10). Since it is my first major plumbing project one must add to the expense side also lot of emotional tension and uncertainty. However, now I feel great. I am a retired chemist and I feel that plumbing could be my second vocation. Steve, thanks for your advice and the words of encouragement.

    • Wow – that’s great to hear, Bogdan! I know how you feel. I’m a computer geek, and only added plumbing to my DIY skill set a few years ago… but for some reason I really enjoy it. Yes, up-front investment in the right tools will pay off in the long run. I’m sure you’ll find plenty of uses for that tube cutter… much to your wife’s chagrin. 😉

      Now the real question: did reducing the pressure and installing a new tank fix your water hammer issue?

      • Bogdan

        Yes. Shutting water does not cause any audible sound (water hammer). By the way, I forgot to mention that I found out that 7/8″ outside diameter of copper pipe is the plumbers 3/4″. Also, a warning: while unscrewing the malfunctioning water expansion tank one has to be prepared for its heavy weight. When a 2 gal tank is filled with water it may surprise you how heavy it feels, especially when handling in quite uncomfortable position. Once again thanks.
        P.S. My wife showed a markable restrain during the process! :-).

        • Fantastic! Yes, 3/4″ is the most common copper pipe size for residential plumbing. 1/2″ is a close second, particularly on stub-outs to toilet valves (they usually “tee” off the 3/4″ line to a 1/2″ line).

          But now your wife thinks you’re a hero. Hope that paid off for you. 😉

  • Todd

    Steve, We just bought a house built in 1998 and immediately replaced the original hot water heater due to a gas leak. After we showered we hearing “hammering”
    in the walls. When we firs turned on some of the faucets in the house they did spurt out some air and then water. We have not opened all the faucets yet. I checked the diapraghm which is installed upside down (the valve is at the bottom facing the floor) as per your instructions and water comes out of the valve immediately. Could this be the cause of the hammering?

    • Hi, Todd. The tank itself isn’t technically causing the water hammer, but the fact that the tank has failed does prevent it from fixing it. I’d replace the tank. It will probably fix or greatly reduce it.

  • Todd

    I just posted about the diapraghm leaking water. I went back downstairs and put a water gauge on the new hot water tank and it reads 100. I was told by Lowes it shouldn’t be higher than 70 or 80.
    What do you think? Todd

    • Hi, Todd. You can’t properly test the tank pressure when it’s heated, as that doesn’t give you a true reading of the incoming water pressure (the heat expands the water, so it’s normal for it to be higher than 70-80). You should test the cold water pressure somewhere in the house. If it’s over 70, then I’d look into installing a pressure reducing valve close to the main shutoff valve.

      • Todd Greenberg

        Thanks Steve to your response on both questions. I will let you know the results.

  • Jay

    Hi steve
    I had an expansion tank added to my water heater last month. I live with a water holding tank. Since the expansion tank was added my water boaster pump now tanks longer to start and run several minutes after I turn the water off and it is getting worse. Any idea?

    • That’s VERY strange. I can’t think of why that would cause it, unless the pre-charge pressure in your expansion tank is way off.

  • Chad

    Im replacing my expansion tank soon. Do I need to shut off my main water line to do this or can I simply turn off the cold water valve that feeds the heater? Thanks, chad

    • Hi, Chad. As long as the tank is “downstream” of your cold water shutoff to the tank (which it should be), that’s all you’ll need to close. Good luck!

  • chad

    Hi Steve,
    The water pressure outside the house is 55psi whereas the pressure of the water heater is 60psi.
    How much air pressure should I pump into the expansion tank 55 or 60 psi?
    Thanks again,

    • Hi, Chad. I’d go with the 55 psi pressure. That extra 5 psi is probably due to heated water increasing the pressure.

  • James

    In preparation for replacing my HW heater and installing an expansion tank (didn’t have one before), I ended up on your blog while researching how expansion tanks work. Fortunately, during my plumber’s visit I remembered your caution that the pre-charge must be adjusted to match the system pressure before installing. When it became obvious that my plumber was going to skip this step, I asked him about it. He said that it was pre-set at the factory and was fine as is. Thanks to your instructions, I was able to successfully challenge my “master” plumber who agreed to increase the pressure. You made me knowledgeable enough to avoid being left with a 40 psi expansion tank on a 70 psi system. Thank you, thank you!!!

    • Nice job, James! The “master” becomes the “student.” 😉 Of course he was partly correct (as you know because it’s clear you read the post) that it was “pre-set” at the factory… just preset to the wrong pressure for your house. Getting feedback like yours is the main reason I write DIY posts — you made my day. Here’s to many years of happiness with your new water heater. BTW – I also recommend this post on water heater flushing to help you get maximum life out of your new purchase!

  • Cole

    Hi Steve. Great article. I have a question. My water pressure is over 100psig. It appears there was once an expansion tank as the connection is there; however, only a plug exists now. The house was built in 1964, but I just moved in and know no history. Is it possible my regulator is bad allowing full pressure through the valve or can it simply be wide open? Would there be a reason to remove an expansion tank to maybe have pressure within the house greater than 80psig (most regulators go to 75psig max)? Theoretically, there could be enough pressure drop in the 3 story house (and enough branches) to require more pressure but greater than 75/80psig seems high thinking of it in feet (175ft or so). I’m looking to add an expansion tank, but I don’t want to go through the hassle only to find out water trickles out of my shower head on the top level. Your expertise would be appreciated.

    • Hi, Cole. 100 PSI is STRONG, and will likely cause issues. If you do have a regulator, I’d bet it’s gone bad and/or is stuck open. Most experts suggest 75 PSI maximum. And yes, if a previous owner also had pressure that high, they probably kept rupturing expansion tank bladders and likely just yanked it out of the system. 🙂 You’re wise to look at the cause (likely a failed regulator) vs. the symptom (high pressure). I’d start with swapping out your regulator. It should be close to the main shutoff at your house. Come back and lemme know what you find!

      • Cole

        Thanks Steve. I tried adjusting the regulator up to four turns counterclockwise to reduce the pressure. The pressure never went down so likely a failed regulator. One thing I noticed though is if I connect my gauge to the hot water tank it will initially read above 100 psig or so; however, if I turn the sink on (directly adjacent to the heater) the pressure drops to nearly 40 psig which is not desired if someone is taking a shower. My assumption is that the expansion tank would keep this pressure up to a desired level (say 75) with a new regulator and tank installed. Agree?

        • The expansion tank isn’t good at keeping sustained pressure, so it probably wasn’t that. If you’re testing pressure at the tank when the water is hot, it will probably read higher pressure because the heat causes expansion (and pressure).

  • Linda

    I have turned on my irrigation system and for the first time I have loud hammering when I initially turn it on and when it changes stations. The pipes in the basement around the expansion tank shake and there is the loud hammering. The house is 4 years old and I have never had this ptoblem.

    The irrigation company checked it out and said it is not the system itself. I was then told to have the backflow tested which I did and it is fine. I did have the water heater release valve replaced due to leakage and then they had to come back and do something to the pressure due to it still leaking. I do not have tgrvshsky pipes and loud hammering except when the irrigation starts up. I have already spent over $350 to have these services and I feel I am getting the run around from the plumber. What can you suggest? Great blog about expansion tank replacement.

    • Hi, Linda. I’d recommend you check out the Rachio Iro WiFi sprinkler controller (I’m currently writing a review of it). In addition to a bunch of money-saving features to reduce water use, it also has a setting that fights water hammer on start-up by slowly opening the valves.

      • Josh L.

        I love my Rachio. This is my second summer with it and have enjoyed it. Their version 2.0 of the software will be out soon in both iOS and Android. Way better than the current version.

    • Linda Domina

      Hi Steve

      Thank you for your reply. I have never had this problem in the past 3 years. My neighbors with the same irrigation system and company don’t have the sudden hsmmering. It seems funny that I have this after the water heater and something done with my water pressure due the continued leaking from the water heater after the relief valve was installed. Also the expansion tank has some water in it when I move it.

      • If there’s water in the “top” half (that comes out of the threaded hole), that’s fine. If water comes out of the Schrader valve on the “bottom” that means your tank has failed.

        Just make sure your incoming pressure to the house is not too high (no higher than 75 psi) and that the pre-charge on your expansion tank is set to match the existing pressure.

  • somnath

    what is pressure consider for hot water expansion tank calculation lower pressure and high pressure?

    • You shouldn’t have to calculate anything. Measure the actual pressure coming in, and set your pre-charge at that level. The tank will still operate within a range of that pre-charge, but will operate best if you have the pre-charge set right. If your incoming water pressure exceeds 70 PSI, you should put a pressure limiting valve right where it enters the building, prior to the expansion tank. Residential plumbing and fixtures shouldn’t be operated above 70 PSI.

  • Clive

    I have been having all kinds of leaks within the last few weeks, the hot water outlet connection started leakng on my tank after a year and could not resolve it with Teflon, putty of flex line, I think I eventually got it with a combination of things, a week later I got a brown ceiling tile and noticed a hot water shut off connection leaking and dripping on to the tile. After fixing that my P/T valve is triggering and spilling out water ,,, this cant all be a coincidence. do you think an expansion tank would help wit this situation. ? I think the hot water is heating up and busting out of some connections. I don’t have the water set to super hot , just “hot” on the dial guage on the heater. Thanks

    • Hi, Clive. Not sure if it will fix it, but it might… and it certainly won’t hurt! Do you monitor your water pressure? Has there been an increase in your incoming pressure recently?

  • Michael

    Steve – what a helpful web site. A question about my thermal expansion tank. The existing tank (which I think has failed – I plan on following your testing instructions) is mounted with the water side down and the air side on the top. That is different from your illustrations. Is that acceptable or is there a reason NOT to leave it that way?

    • Hi, Michael. That’s no problem at all. It’s pressurized, so it doesn’t matter which way it’s facing. As long as it’s secure, you’re good. 🙂

  • It only “needs” an expansion tank if the local code requires it. That said, even in locations that don’t require it. I always install one. It’s cheap insurance for a problem that could get VERY VERY bad if things go wrong.

  • Hi, Brian. Sounds like a very interesting situation. First, I wonder if your system pressure is truly 160 psi. That’s REALLY high. Do you have the ability to test the pressure on a cold faucet somewhere? Only cold water comes into your house, and that will be a true test of the “baseline” pressure of your system.

    The heat in the tank is probably contributing to the high pressure readings, as heat increases pressure. We also need to be open to the possibility that your gauge is reading improperly.

    Having one expansion tank for both water heaters is fine. And yes, if you plan to swap out the tank, may as well put the bigger one in there. You’ll want to set the pre-charge pressure to the true baseline pressure at your house (again – measure the cold pressure for a better accuracy).

    As for what is making the pressure fluctuate, my sense is that it’s only fluctuating like that at the heater, where the heat itself is increasing the pressure in the gauge. I’d be curious about the water temp, too. If it’s crazy high, it will create crazy high pressure.

    Lemme know what happens!

  • PJ

    Hi, great tutorial Steve. I recently had a new 40 gallon water heater replaced with an expansion tank installed. Now when I turn on my hot water, pressure comes out like it used to and within about 5 seconds the pressure goes down and stays down. This is happening in all my faucet and showers

    • Hi, PJ. That could mean that the pre-charge is set wrong. Did the installer set it properly, or just install it without thinking about it (like most do)? You can follow the steps in this article to verify and set the proper pressure yourself, if you have the proper gauges.

  • Grace Duling

    I read your entire post and have been reviewing posts below. My expansion tank was replaced 2 years ago. The T&P valve was not replaced at that time. the Gas Water heater is approx 10 years old. A slow leak is occurring from the pressure valve pipe. Of which I have a bucket under it now and I still have hot water to the house. How do I determine if it’s the expansion tank is needing replaced again, it’s the pressure valve or it’s time to replace the whole unit?
    Thank you

  • Ivan

    This is great info and thanks for the information. I just had a plumber come over and tell me that both my expansion tank was probably ruptured and that my pressure regulator valve was broken. He showed me the pressure against and outside hose and it did indeed show 100 psi.. Can a failed expansion tank affect what the pressure regulator reading? Also, the plumber also indicated that I could NOT adjust the pressure regulator which doesn’t seem true. Any insight?

    • Hi, Ivan. No, a broken pressure tank won’t be affecting the pressure coming into the house. Also, the plumber is right about the pressure regulator: it’s factory set for 70psi, and should remain there. That 100psi needs to be reduced to 70psi right as it comes in, or it could cause damage to your pipes and fixtures. You can replace the tank on your own, but maybe use the plumber to replace the whole-house regulator.

  • Jerry

    Can you instal a tank with a flexible water heater line?

    • As long as the line and connections are rated for residential pressurized hot water (like PEX tubing), then yes.

  • Wow! 90PSI is some strong shower pressure! Nice job figuring that out and fixing.

  • It’s correct to have it on the cold side, and generally speaking, the closer to the tank, the better. However, 25 feet away should still work fine.

  • No problem at all. An oversized tank will still work fine!

  • Hi, Cindy. If there’s water coming out of the Schrader valve in your expansion tank, then the bladder is ruptured and it needs to be replaced. And to answer your other question, there’s no need to change the pre-charge pressure in your tank as along as your house’s water pressure doesn’t change… which it shouldn’t! 🙂

  • Jean Koz Claud

    My heating system doesn’t have a separate hot water tank. It’s an all in one system. How would I check pressure otherwise? And what else would I shut off other than the water line? Also, are all expansion tanks the same for every unit? Mine hangs and is full of water. (Installed by a reputable company, btw) it’s as large as your white one, can the water that’s in that tank come out through that bottom valve?


    • Hi, Jean. If water comes out of the expansion tank’s Schrader valve, the bladder inside the tank has ruptured and it needs to be replaced. Hopefully, whoever installed your heating system also installed a pressure gauge. If not, and your expansion tank is leaking water, whoever you hire to replace it can install a gauge and set the pre-charge for you.

      • Jean Koz Claud

        I know the tank is no good and needs to be replaced. I read your article and understood it. But, your article speaks of a hot water tank and mine runs through the furnace. Just want to know what I asked.
        Why does the hot water need to be running while I replace this?

        Unit isn’t running, I have no hot water, now.


        • Hot water does not need to be running when you replace it. It needs to be running when you set and/or test the pre-charge so that the pressure in the tank is accurate.

  • Glad to hear it! It’s a great feeling to attack a DIY project you’ve never done before, and emerge victorious! 🙂

  • Yes, the T&P valve is bad. You can call a plumber if you want, but it’s VERY easy and I know you can do it by reading this other article where I walk you through it:

    Or if you have a handy friend, even if they’ve never done it before, they can easily follow my steps.

  • Chris Porosky

    Fantastic write up Steve! It took seconds to test my tank and have water come out the air valve. This was after a “professional” needlessly replaced the tnp valve and wanted to put in a prv next! I’m attaching a chart pic from the new tank which shows that the water pressure (mine is 80 psi) along with the heater size determines size of the expansion tank. Another “professional” who installed this heater put in the smaller det-5 tank and probably didn’t set the precharge properly (hence its premature failure):

    • Nice job! I’m assuming you replaced the tank yourself with the proper pre-charge this time?

      • Chris Porosky

        Yes, sure did with your help. We have a 4 year old 50 gallon GE water heater. I’m also gonna check the anode per your other post.

  • Chris Porosky

    My t&p valve started leaking after the expansion tank failed and filled with water. It would only leak when the water was heating up and trying to expand.

    • Then it was doing its job: opening when it was under pressure!

      Those are easy to replace, too. I always keep a spare on top of my water heater. They seem to fail about 10 minutes after the hardware store closes. 😉

  • Della Taylor

    Great article! I am a mechanical engineer who is always looking for practical how-tos for home fixes. This is about the easiest to read one that I have found, and the theoretical stuff (which is about all that I really understand) was very well explained. Thanks!

  • Dan Fonnesbeck

    Thank you very much for the post! Extremely helpful. Much appreciated.

  • Mom

    Could a bad exspasion cause pies to have a high pitch humming?