Hot Water Tank Explosion

How To: Repair a Leaking Hot Water Tank Temperature Pressure Relief Valve 19

Hot Water Tank ExplosionIt’s 10PM. I get a text from our son (whose bedroom is in the basement) saying that he just got out of the shower and the water was barely warm, and now he’s hearing a “hissing” sound coming from the utility room across the hall from his bedroom… where the hot water tank lives. I rush downstairs, open the door to the utility room, and sure enough – water is all over the cement floor and there’s a hissing noise coming from the water heater. Yep. My hot water tank is leaking.

Hot water tanks generally leak for one of three reasons:

  1. A hose or fitting is loose. If this is the case, disconnecting it, re-applying teflon tape or pipe dope, and re-installing will generally fix it. Or maybe sometimes just tightening it alone will fix it.
  2. The water tank has rusted through (possibly because you never changed your anode or flushed it regularly). There’s no fix for this one — you’re in the market for a new hot water tank.
  3. The TPR (Temperature Pressure Relief) valve fails due to damage or wear, and no longer stays closed.

Issue #3 was the problem in my case. My TPR valve had failed in the “open” position, allowing water to gush out of the tank.

But before I explain how I repaired my TPR valve (spoiler alert: the only way to “repair” one is to replace it), let’s talk about what a TPR valve is, and why your hot water tank really, really needs one.

What’s a TPR Valve and Why Do You Need One?

The water inside your hot water tank is already under some amount of pressure, equal to your house’s regular water pressure, as cold water enters the tank. Generally, that’s between 40-70 PSI for an American residence. However, when that water heats up (and isn’t that the entire purpose for putting cold water in the tank in the first place?), the water molecules expand… creating additional pressure inside the tank. If you have a temperature pressure relief valve installed, that valve will open at some predetermined pressure (150 PSI in most cases) and/or a predermined temperature (210F in most cases), allowing pressurized hot water to escape out of the tank (and making room for cold water to flow in) until the pressure and/or temperature drops below the valve’s pre-set levels, at which point the valve closes.

This is what a water heater’s TPR looks like (this is actually my old one that failed and caused the leak):

A standard 150 PSI temperature pressure relief valve on a hot water tank

A standard 150 PSI temperature pressure relief valve on a hot water tank

If you don’t have a TPR valve installed on your tank, you need to have one installed… like right away. A hot water tank without a TPR valve (or with a faulty TPR valve that’s frozen shut) is literally an explosive missile waiting to be launched from your basement or garage. And yes, I literally know what the word “literally” means. Think I’m exaggerating? I’m literally not. Check out this video of the Mythbusters testing a 52 gallon water tank (which is smaller than the tank in my basement) with the TPR valve removed and the thermostat overridden so that it keeps heating the water until the pressure inside the tank reaches 300 PSI:

Now, before you go giving Adam and Jamie too much credit, watch a few minutes of this old-timey, grainy film from the Watts company, who actually invented the original “T&P” (temperature and pressure) valve in the 1900’s. They used this early marketing video to convincingly demonstrate to homeowners the importance of buying Watts’ new fangled valves — decades before the Mythbusters were even born (for the best explosions, watch the very beginning, then skip to the end):

No Hollywood effects were used in those videos. That’s what can happen inside your house if your TPR valve fails to open, folks. Minerals, salt, rust, and/or other corrosion can build up on the valve, causing it to freeze shut and possibly allow too much pressure to build up. Or, that same crud could build up on the plastic temperature probe that extends into the tank from the valve, which could insulate the probe and prevent it from opening if the water temperature gets too high (which also increases the internal pressure). That’s another reason to flush your hot water tank regularly — and I now also recommend testing your TPR valve at least monthly. Just grab the little test handle on the valve and lift it up and down a couple of times. That will manually compress the spring inside and open the valve (which will also break free any corrosion). If your valve is working, hot water (be careful!) should spurt out the end of the drain pipe that’s connected to the valve. But if only a trickle comes out, or no water at all, then your valve has failed in the “closed” position and should be replaced as soon as possible.

Luckily, my TPR valve had failed in the “open” position, allowing semi-heated water to gush out of the tank just as fast as new cold water flowed in. That meant I was wasting money by wasting water (that’s never good), but at least I wasn’t staging a Saturn V rocket in my basement (which is one of the few things worse than wasting water and money).

And even though the copper drain pipe attached to the valve was directing the majority of the escaping water into a drain in the floor, a decent amount of water was still dripping and spraying out of the hole in the valve where the test handle inserts. That spray was causing the hissing noise my son heard… and the watery mess on the floor. This TPR valve had originally been installed with the tank in 1998, and after 15 years of pressure and heat, it had finally given up.

Troubleshooting My Failed TPR Valve

With the water still leaking, the first thing I did was try lifting the TPR valve’s test handle a few times, hoping that maybe I’d break free some minor corrosion inside the valve and allow it to close again. No such luck — the handle flopped back and forth easily with no spring tension, so I knew it was toast. To stop the leaking, I turned off the water inlet valve that feeds cold water into the tank. That valve should be above your tank, attached to the water line that comes out of the wall and into your tank. If you don’t know where this is on your hot water tank, go find it now… before you really need to know where it is (it’s also never a bad idea to know where your whole-house water shutoff is, too).

Next, I turned the tank’s thermostat all the way down (mine is a gas-powered tank, but had it been electric, I would have just shut it off completely). Then I put some towels on the floor to mop up the water, and went upstairs to bed. It was 10PM, remember? The hardware stores in my area were already closed, and I didn’t have any spare TPR valves lying around, so there really was nothing else to do. I still wanted a hot shower in the morning, and I know other family members would, too — so I used my phone to look up what time the local hardware store opened (7:30AM), and set my alarm for 6:30AM, thinking that would give me plenty of time to remove the old valve and arrive at the hardware store when it opened.

Removing the Old TPR Valve

The following morning, I woke up at 6:30AM, grabbed my tools, and went to work. The first order of business was to drain the tank to a level below where the TPR valve was connected, so the tank wouldn’t leak when the valve was removed. I grabbed the hose I use when flushing my tank, connected it to the tank’s drain valve, and routed the other end of the hose to the drain, and opened the drain valve. I let the tank drain for a few minutes, until I was sure it was below the TPR valve location, then closed the drain valve.

Next, I focused my attention on removing the old TPR valve. Because a plumbing sub-contractor for the original home builder had installed it, they’d used soldered copper tubing to build a drain pipe that connected to the end of the TPR valve, went down a few feet to the floor, bent 90 degrees to the right, headed over a few feet toward the drain, bent 45 degrees to the left to go around a doorway, went another few feet to the drain, and then bent 90 degrees down into the drain hole. It would be impossible to rotate the valve with all that tubing attached, so I grabbed my hacksaw and cut through the tubing about 10″ down from the valve:

Cutting away the old drain pipe tubing

Cutting away the old drain pipe tubing

Once all but that 10″ section drain pipe was removed, I grabbed a large pipe wrench and tried to unscrew valve. Nothing happened. I tried harder…. nothing. I hung my entire body weight on the wrench, but the valve still wouldn’t budge. 15 years of corrosion had that valve stuck in there tight. So I went into my workshop and grabbed the best friend of every DIY plumber:

Working with corroded plumbing fittings? Then you needs some of this!

Working with corroded plumbing fittings? Then you needs some of this!

I sprayed some Freeze-Off onto the TPR valve’s threads, gave it a few minutes to penetrate, and then tried again with my pipe wrench. Still no movement. So I decided to ditch the DIY plumber’s best friend and upgrade to the overall handyman’s best friend: a hammer!

Since I’d left 10″ of copper drain pipe on the valve, I had a sufficiently large surface area on which to strike the hammer, thereby providing a high amount of leveraged impact force on the seized threads.

In other words, I smacked the #&$% out the copper pipe just below the valve three or four times, and the valve started to rotate. I then used my pipe wrench to finish removing it. If you look closely at this next photo, you can see the dent in the drain pipe where the hammer hit it. Just remember: any time you can find an excuse to use a hammer while plumbing, your awesome points get multiplied.

Disclaimer: My plumber buddies tell me that you never want to use a hammer while plumbing, and that doing so could actually crack the steel fittings in the tank (I got lucky and that didn’t happen to me, but YMMV). They state that you should instead use leverage to your advantage, such as using hollow pipe as a “cheater bar” to increase the force on the TPR valve. I’ve also been told that briefly heating the threads with a torch can help break it loose.

Remember, kids: if at first you don't succeed, go grab a hammer.

Remember, kids: if at first you don’t succeed, go grab a hammer.

Once the faulty TPR valve was removed, I could see 15 years of gunky buildup attached to the plastic probe inside:

Old, sad, worn out TPR valves look like this.

Old, sad, worn out TPR valves look like this.

Finding the Correct Replacement TPR Valve

Before heading to the hardware store with the old valve, I measured the approximate lengths of the old drain pipe sections and wrote down which fittings I’d need to route a new pipe to the drain (two 90s and one 45). Even with the extra time I wasted trying to get the old valve removed, I still made it to the hardware store before 8AM!

The metal tag on the old TPR valve told me everything I needed to know about finding a suitable replacement:

A TPR valve's tag tells you all the important details you'll need to replace it

A TPR valve’s tag tells you all the important details you’ll need to replace it

On the bottom, next to “ASME” (which stands for American Society of Mechanical Engineers), it says “3/4.” Next to the “PSI” it says “150” (sorry – it’s kinda hard to read in this shot). Finally, next to the degrees Farenheit abbreviation, it says “210.” So I knew I needed a 3/4″ threaded TPR valve whose spring tension is set to open at 15o PSI and whose wax-filled temperature probe is set to expand (and press open the valve) at 210F. Waiting for me in the plumbing section was a Watts brand (the original!) TPR valve with those exact specs for $14. That’s actually slightly less than what you’d spend for the same valve on Amazon.

In addition to the replacement TPR valve, I also picked up two 10′ sticks of 1/2″ Schedule 40 PVC pipe, along with all the necessary joints to build a new drain pipe. I also may have picked up a bottle of micro-brew root beer from the cooler near the “impulse buy gauntlet” through which all shoppers at Johnson’s Hardware in Maple Valley must pass before they are allowed to reach the checkstands.

Installing the New Valve

Back home, installing the new valve was easy. Here it is ready to be installed:

Oooooh! Is that a new TPR valve? Shiny!

Oooooh! Is that a new TPR valve? Shiny!

Of course, it’s important to wrap some teflon tape on the male threads before installing it in the tank:

Teflon tape is your friend.

Teflon tape is your friend.

Once the new valve was inserted and hand-tight, I snugged it down with my pipe wrench (although I didn’t get to use my hammer this time — sad face), and then made sure it was rotated properly so that the outlet side was pointing straight down. Here’s the new valve all comfy in its new home:

Shiny new TPR valve installed in my hot water tank!

Shiny new TPR valve installed in my hot water tank!

Comparing the amount of text on both sides of the old warning tag vs. the new warning tag, it seems lawyers have found a lot more to say over the past 15 years! 🙂

Before going any further, I turned on the cold water supply valve, allowing the tank to begin refilling. I also turned the thermostat back up to its normal heating position. I briefly lifted the TPR valve’s test lever, and heard air gush out. That’s perfect, since the water level in the tank was still below the valve’s location, but the pressure from the incoming water was forcing the air out of the open valve. After the tank finished filling, nothing leaked out of the valve. So far, so good!

Building a New Drain Pipe

As the tank heated up, I built a new drain pipe of out PVC. I decided to use 1/2″ Schedule 40 tubing because it’s way cheaper than copper, way easier to cut and join without any soldering, and still rated for the appropriate temperature and pressure — especially since the pipe would never actually be pressurized, due to the drain end being open. Keep in mind, however, that this is not “up to code,” since PVC is only rated for exterior use. My plumber friends tell me that you should actually use CVPC (chlorinated PVC) for interior work. PVC is rated for water up to 140F, while CPVC is rated up to 180F. You could also just purchase something like a Watts 100DT TPR valve drain that hand-threads to your valve.

To install my home-made drain, I first put some teflon tape on the threads of a 3/4″ x 1/2″ adapter and screwed it into the bottom of the TPR valve. Next, I cut the necessary lengths of PVC, connected all the couplers, and routed the drain pipe to the floor drain.

New drain pipe attached to TPR valve

New drain pipe attached to TPR valve

Here’s how the rest of the drain pipe looked when completed (you can also see the old copper drain pipe on the floor, as well as the drain hose I used):

New PVC drain pipe installed

New PVC drain pipe installed

Note that the drain pipe always slopes downward, all the way to the floor drain. This is important to make sure water never gets trapped inside.

I considered using PVC glue on the pipe fittings, but because the fittings required a decent amount of pressure to push onto the pipe, I decided I didn’t need it. As stated earlier, since the end of the drain pipe is open, it will never have to withstand anywhere near the type of pressure for which it’s rated. If a fitting ever came loose, the worst case would be some water on the cement floor of the utility room that evaporates or eventually flows to the drain. And because this new drain pipe is easy to take apart, I can now easily remove and inspect my TPR valve… which, after watching those videos, I now plan to do on a much more regular basis!

After finishing the drain pipe, I tested the TPR valve once again, and heard water flow down the pipe and into the drain. None of the PVC fittings budged. Success!

I cleaned up my tools and went upstairs for a celebratory hot shower. Hot water feels so much more satisfying when you’ve personally done something with tools to provide it for your family, rather than just turn a knob. 🙂

What I Learned and Will Do Differently

So what have I learned from this DIY project?

First, I learned that a properly functioning TPR valve is an extremely important safety feature in your home. I’ve added a monthly valve test to my home maintenance checklist.

Second, I learned that things in your house will always break at the most inconvenient time, and when you can’t just go grab a new part. If I’d had an extra TPR valve in the house, I could have restored hot water within 10 minutes (including hammering the old valve). I still would have had to hit the hardware store the following day for drain pipe supplies, but the new valve would have held just fine overnight. I now keep a spare TPR valve sitting on top of the hot water tank — the same way I keep a spare furnace ignitor on top of the furnace in the same utility room.

Third, I learned that 15 years is probably a bit too long to let a TPR valve sit in your water heater. If you’re flushing your hot water tank and replacing the sacrificial anode as needed, your hot water tank can easily outlive the service life of a TPR valve. 15 years was clearly way too long for this particular valve. Spending $14 and 10-15 minutes of my time every few years is cheap insurance against a leak (at best) or a ticking time bomb sitting below my family (at worst), so I’ll be scheduling my TPR valve for replacement every 6 years (I keep a Google Docs spreadsheet with household maintenance records and schedules — you should, too).

Finally, I reconfirmed something I always knew about DIY projects: whacking things loose with a hammer feels awesome.

Seriously. Go check your hot water tank’s TPR valve. Right now. Be careful, as pressurized hot water will come out. Or at least I hope it does. If it doesn’t, then you could have a ticking time bomb in your basement. If you’re even the least bit handy, follow the steps in this post to replace your Temperature Pressure Relief valve quickly, safely, and easily. Or, you can hire a professional plumber to come to your place and do it for you. Either way, you should test your TPR valve regularly, and replace it at the first sign of trouble. If you do a Google Image Search for “hot water tank explosion,” you’ll be amazed by some of the photos. Please don’t let photos of your home end up in those search results. Feel free to share this post with your friends and family so they can learn about and check their TPR valve, too.

As always, I welcome your comments, questions, and feedback in the comments below!

  • The TPR’s are designed to fail in the open position (which is why they leak instead of go boom)… usually.

    Oh, and the Watts safety film looks like it was the inspiration for Monty Python’s equally-informative “How not to be seen” sketch. 🙂

    • You’re right, Paul. If it’s gonna fail, better to fail open than closed. 🙂

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  • Junaid

    Hi, thanks for the write up! We have lived in our current house for about 10 years and did not know that we are supposed to check the relief valve every year so it was a surprise when we found a huge puddle in our basement.

    Sure enough the old valve had a ton of mineral build up on it, and we replaced it immediately.

    Thank you!

    • That’s great. I’m glad you guys caught it, and glad you were able to replace it yourselves. Nice!

  • Artis

    Hey Steve, is the pump I see there to boost your pressure? If so, How do you activate the power to it so it is not always running? I have thought of a pump, as my city pressure is not so good and the shower heads don’t put out like I would like!

    Thanks, Artis

    • Hi, Artis. No, that white tank you can see is actually an expansion tank, and I’ve also written a blog post about how to check and replace them (click here to read it). If you don’t have an expansion tank in your house, you should strongly consider it. It’s a great way to add longevity to all your fixtures and fittings.

      I actually do have a pressure tank in the house, but it’s currently bypassed. We used to be on a community well, which had variable pressure, so I used the tank to compensate. We were switched over to city water a few years ago, but the pressure is only 40 psi, and I’d prefer it between 60-70 psi, so maybe I’ll test it, fire it back up, and write a blog post about it. As for how they work, they have a compressor that pumps up the tank to a certain psi, and then shuts off. As you use the water, the psi drops, and the compressor kicks on again. If you’re not using water, it won’t run all the time.

  • Robert Brewer

    Thanks to share this! I didn’t know how to check the relief valve and this post really helped!

  • Allen Harper

    Yeah it really helped! Thanks for the information about relief valve. I use to get my home inspections done from ‘The Shock Doctors’ at Orillia. They are very good at their service. So I don’t care that much about my home safety. It is just them who cares it all.

  • Joe

    Great post and your instructions will make the job easy for anyone wishing to do it themselves. LOVED that Mythbusters episode but mind you I love all their episodes 🙂 Cheers, Joe

  • Nick

    The discharge line should not be reduced (3/4 x 1/2). You need to replace that 1/2″ pvc with 3/4″. While you are at it you might as well just make it CPVC like you had noted but at a bare minimum it needs to be 3/4″ throughout. It should clearly state this somewhere on that yellow tag. From Watts ( The discharge line must be the same size as the valve outlet, and must pitch downward from the valve.

    • Hi, Nick. Thanks for commenting. I didn’t see it in the link you shared, but I did find another page where Watts says a discharge pipe should “Not be smaller than the diameter of the outlet of the valve served and shall discharge full size to the air gap.” That said, I still scratch my head and wonder why it’s a such bad idea to reduce the discharge line down to 1/2″ Schedule 40. I can see where it might possibly be an issue if I were to try and reduce it down to 1/4″ or smaller (and so it’s probably easiest for the code to just say “Use the same size!”), but this discharge line is a very short pipe, is completely open on the bottom, and therefore will never be pressurized. When I manually test the valve, the amount of water pressure that comes out with the TPR fully open is nowhere near enough to stretch the limits of 1/2″ Schedule 40. But code is code, and since anyone can easily grab a Watts 100-DT 3/4″ discharge pipe from Home Depot for under $4, that’s probably the cheapest and easiest way to go. 🙂

      • Grace Duling

        when I search the Watts 100-DT 3/4″ google. It directs me to this website:
        And then under the “where to buy”. It doens’t list any home depot OR Lowes. Is there something else I should be looking for?

        • That’s the drain tube, and not what you want. You want the actual TPR valve itself. Depending on what your existing TPR valve looks like (how long the shaft is), you want one of these two: or

          • Grace Duling

            correct. My set up is a bit different than yours with the drain pipe. I have a copper pipe that is attached to the TPR valve and comes down about 4 inches alongside the tank. So you had to cut your copper pipe since it went around a corner. I’m assuming my copper pipe unscrews and I can reuse the same one. (I hope). I live in a townhome so my water heater is in the small shed out back. (not in the house per se).

          • Yes, you can re-use the copper pipe, but because the TPR outlet pipe is never pressurized (the drain end just opens to the drain), you can replace any part of it (or all of it) with PVC or CPVC. Much easier to work with (you can cut and glue it).

  • beth

    Our Pressure relief valve was leaking so we replaced it. However, it’s still leaking. We replaced the expansion tank because that was old and that still didn’t solve the problem. Any ideas of what else could be the problem? The hot water heater isn’t even a year old.

    • Is it leaking from the base of the pressure relief valve, or from the top (where it’s designed to relieve pressure?) If leaking from the base, just use more Teflon tape and tighten more!

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