Furnace ignitor working properly

Fix Your Furnace If It’s Not Blowing Warm Air 29


You wake up in the morning, or come home from work, and you can instantly tell something is wrong. The house is cold. You can hear the furnace running, and you can even feel air coming out of the registers. But it’s not hot air coming out, it’s cold! Well, it’s not COLD cold, it’s just room temperature. As if the furnace is just recirculating the air in your house without actually heating it.

Which is probably exactly what’s happening.

You did the right thing by running to the Internet to solve your problem. I’m glad it brought you here. 🙂

This same thing happened to me many years ago, before I got into doing my own home maintenance (and before blogs existed), back when I was still (over-)paying repair guys to come to my house to fix stuff (I’ve told this story before in this Home Maintenance Parts post). It was a dark and stormy night. No, seriously. It was one of the coldest winters I can remember in Seattle, and my furnace had stopped blowing warm air. The house was freezing, and my family was complaining, so I started phoning HVAC repair companies to schedule someone to fix it. But because of the cold snap, all the furnace repair guys were slammed. The soonest I could get anyone to the house was two days later… so we got by with sweaters, slippers, extra blankets, and running the gas fireplaces 24/7 for a couple of days.

When the repair guy finally showed up, he was in and out of my house in about 10 minutes. It took just as long to write up the bill as it did to actually fix the problem — a bill for $200 ($150 for the service call and $50 for the part). But my furnace was blowing hot air, and my family wasn’t complaining any more, so I didn’t complain either… at least not out loud.

But I was complaining a little on the inside. I’d watched the repair guy do his 10 minute job, and it didn’t look difficult at all. He’d simply swapped one tiny part, which turned out to be my furnace’s hot surface ignitor — a part you can get online for around $25.

Hot Surface Ignitors

A hot surface ignitor works much like the filament in a light bulb. It has two electrical wires that, when energized, heat the ignitor’s surface to the point that it glows orange. Your furnace then shoots natural gas close to the ignitor, igniting the gas which warms the air that your furnace’s fan blows through your house. You can see the ignitor glowing brightly on the right side of this photo:

Furnace ignitor working properly

Furnace ignitor working properly

Just like a light bulb, your hot surface ignitor will eventually “burn out” from normal use. One tiny crack is enough to stop the flow of electricity, and make your house cold. Here’s what my old ignitor looked like — notice the tiny crack in the middle:

A hot surface igniter works much like a light bulb, and can be replaced just as quickly.

A hot surface igniter works much like a light bulb, and can be replaced just as quickly.

Eight times out of ten, if your furnace is powering up, and everything else seems to be normal — except that it’s blowing cold air (well, unheated air) instead of hot air, you probably just need to replace your ignitor.

Replacing a Broken Hot Surface Ignitor

Luckily, swapping out a hot surface ignitor is easy. Turn off the power to your furnace (by hitting the switch, pulling the plug, or tripping the breaker). Depending on your furnace, you may or may not need a screwdriver to remove the main cover. Some units have more than one cover panel, so just keep removing panels until you find it. Or I suppose you could break down and actually read your owner’s manual… but where’s the fun in that? 🙂

Once you locate the hot surface igniter, take a very close look to see if you can find any cracks. A flashlight might come in handy if your furnace location isn’t well lit. If you do see a crack, you’ve discovered your problem. If you don’t see a crack, don’t waste money by replacing it anyway and hoping that’s the problem. Confirm that it’s fine by using a multi-meter to test for continuity through both wires attached to the ignitor. If you don’t get the “beep” confirming continuity, your ignitor needs to be replaced. If it does beep, then something else is wrong.

With an older furnace, you may have to use a screwdriver to disconnect the old ignitor’s wires from the furnace. But with a newer system, you can just unplug it. There will probably be at least one screw holding the ignitor in place. Once the old one is out, put the new one in its place, reconnect it, replace any covers on your system (many systems won’t power up with the covers removed), and turn your system back on.

If your thermostat is calling for heat, it shouldn’t take long for your furnace to click on, heat the ignitor, and start burning gas again.

Congratulations! You’re back on your way to a warm house. Don’t forget to tell your wife how difficult a job it was. 🙂

Finding the Right Ignitor

There are many different variations of hot surface ignitors, depending on the model of your furnace, though they all work in the same way. The fastest way to find the right one is to locate your furnace’s model number (it’s probably printed on a label inside one of the cover panels) and search online for your model number and the words “furnace ignitor” (or “furnace igniter” — both are acceptable spellings). When you have the part number, search for the number on Amazon and order two of them. Use the first one to replace the currently broken one, and store the second one somewhere close to your furnace — so it’s handy when (not if) it burns out again. All three of the furnaces at my house use part number LH33Zs004, which will work on most Bryant, Currier, and Payne furnaces. I duct taped the box holding my spare ignitor to the side of my furnace, so it’s really easy to find when I need it.

If This Doesn’t Fix It

Of course, a broken ignitor isn’t the only thing that can go wrong with your furnace. It just seems to be the most common (in my experience).

The next thing I’d check is the fuse on your control board. Depending on the age of your system, it could be an older glass fuse, or a newer spade type. If the fuse is blown, simply replacing it might solve your problem. But if you replace the fuse and it blows again, then something more serious is causing that fuse to blow, and it’s probably time to call the repair guy.

If the fuse isn’t blown, then take a look at your control board to see if there’s a blinking LED light. When there’s trouble, your furnace will blink this light in a certain sequence to give you an idea of what’s wrong. Usually, the key for the blinking codes will be on a sticker on one of the cover panels. If you can’t find the key, do a web search for your furnace’s make and model to find out what the light is trying to tell you. It could be something easy you can fix without needing to call the repairman.

The ignitor won’t work if your furnace isn’t powering up at all. In that case, the next likely culprit could be the 24V transformer. It’s possible, but rare, for these to simply fail on their own. Usually, something else has caused the transformer to fry (usually a problem with the common side of the transformer being improperly grounded to the metal chassis — look for a screw that has pierced your wiring). Use a multi-meter to test for power on your furnace’s control board (touch one probe to the 24V terminal, and the other to the COM). You should see 24V when the unit is powered up (be aware that some units have a safety switch that kills the power when the panel is removed — you’ll have to tape this switch in the down position when testing). If you don’t, then the problem might be the transformer. You can buy a new one for around $50. But again, it’s likely that the fried transformer is a symptom of a bigger problem, not the problem itself. If you replace the transformer and it fries again, check how it’s grounded.

These tips cover the most common issues, but of course… you might just be one of the unlucky ones to experience an uncommon problem. In those cases, you may just have to bite the bullet and hire a professional. But watch him like a hawk, learn, and then please come back here and comment on the fix so others can benefit.

You Can Do It!

It’s easy to be the hero and restore warmth to your house by simply replacing the hot surface ignitor in your furnace when it stops blowing warm air. Just be grateful we no longer live in a time where fixing your furnace looks like this. 🙂

As always, I welcome your comments, questions, and feedback below.

  • psiu

    Flame sensor rod is another fun one. Few minutes of cleaning with Emory cloth and back in business. Symptom will be main burner ignition followed very shortly by main burner shut down. Good tip on the ignitor. Think I might go look for this info now before the middle of January…

    • Be careful with the emory cloth. Some rods are coated to help them from deterioration. If there is a need to repair the flame sensor, you might as well order a new one to have. Next time you have a flame-out, just replace it.

  • There are several other things, high limit switches (which you can test by jumpering them), pressure switches, etc. Often if you look at the main board to the furnace it will be flashing a code series that will give you a good indication of where to start.

  • Steven

    I’ve gone through and check all these and none are broken. Not wanting to pay for a service call, and since you said 9 times out 10 it’s this, I just went ahead and replaced them all. I still don’t have hot air, help!

    • Steven: Any LED lights flashing on your control board that could help you diagnose? Is the unit powering up and blowing cold air? Or just not powering up at all?

  • Brittany

    We’ve been having issues with our gas furnace for over a month. At times it works perfectly and other times it only blows cold air out of the vents. While we can still get heat intermittently we’re noticing it “breaking” and blowing cold air more and more – including several times a day. We’ve had 4 different HVAC techs from 3 different companies out to try and fix it. They’ve replaced the heat exchanger, gas valve, rollout switch, and flame sensor. No luck. The last tech said we needed to get our chimney relined because our older chimney wasn’t removing the heat properly and causing the furnace to shut down for safety reasons. The most common status code we’ve received is a code 34 – Ignition-Proving Fault. We had our chimney relined – and still no solution. We’re desperate for a solution and are curious what could be causing the apparently uncommon issue.
    Our furnace is a 2008 Carrier model 58CVA090. Would appreciate any help!

    • Hey Brittany, I’m not an HVAC technician, so I’d be surprised if I could help you given the others having been out there. I also want to caution you to understand this before trying some of the things I mention as furnaces can be very dangerous.

      The ignition fault seems to indicate something in the chain is not hitting a checksum upon call for heat. I had the same problem and read the manual to understand exactly what is happening, most furnaces go something like this. Call for heat from thermostat, kick on blower fan. The pressure switch then checks for positive pressure in the manifold, if there is none, no call for gas. This could be as simple as the pressure switch is bad or the rubber line up to it is cracked. Online references show how to check that.

      If the pressure switch responds correctly, the furnace then checks the limit switches, of which there are a couple. They basically check that temperature isn’t too high, if it is, no call for gas. These can be checked by “jumpering” past them just temporarily. If it works while jumpering, your limit switch is bad. If all those are correct, then call for gas and heat the ignitor. Your particular furnace may have more or less.

      You said it was intermittent, which to me sounds like either your temps are too high when it doesn’t turn on, or a switch is sticking and starting to go bad. Has the furnace just run recently when the ignitors don’t light?

      One last thing that can help you is to get the control board model from the furnace itself and look at troubleshooting options. The trouble code you have is a good start, it can also help you narrow down the other things I mentioned as they may have their own trouble codes.

  • my two cents.

    If you have any kind of a flue damper , there is a switch inside them that can go bad. Even if the damper opens and closes, the switch inside can be faulty. This will lead to the gas valve not opening.

  • Rick Hardesty

    I shut mine off for about 10 min. when I turn it back on it blows warm air for a short time, usually not long enough to bring it to the temp setting. It then shuts off and immidietly kicks back on blowing cool air. Would this be the same thing?

    • Hi, Rick. If it’s blowing warm air at all, then your ignitor is fine. I’d check your flame sensor and your pressure switches.

      • Rick

        I will do that. Thank you very much!!!!

  • Nice! My furnace pumps little heat and I found out it was the filter too dirty. Thanks for the inspiration!

  • Tessa Billington-Bush

    I have a rather odd question for you. This morning, I had the pleasure of waking up to a house that was so cold, I could see my breath. I turned up the heat and it came on, but seemed to be blowing cool air. It came on every few minutes, then promptly went back off, each time, lowering the temperature of our home. Eventually, I fired up our gas fireplace to take the edge of the living room at least. Then the heat started working! Was this coincidence, did the boost of heat stir something in the thermostat, or what? I have an HVAC guy on the way and don’t want to cancel in case it’s just a fluke. Is there a reason for this apparent fix?

    • Hi, Tessa. I think it was probably coincidence — I can’t think of a way that the warmer air from the fireplace could “kick start” a furnace that’s being naughty. Probably a good idea for the HVAC guy to find out why your furnace wasn’t working. If it hadn’t started working, I’d have recommended that you check out my article on troubleshooting furnaces… but if it wasn’t working but now turns on and stays on, then probably best to have an expert check it out. 🙂 Good luck!

  • Jessica

    My furnace is blowing cold air i have noticed for the last 2 days. Last year i cleaned off the sensor as it kept turning off and on for short periods of time. Since it is only blowing cold air i replaced the ignitor yet still the big blower goes on and still blows cold air and i do not see the ignitor glowing to start the flame. I replaced the batteries in the thermostat also just in case it could have been that. Any suggestions? I have a small premie baby in the house with below zero temps lastnight using electric heaters to keep the room warm. Currently unemployed and not enough money to call a HVAC tech.
    Any help with suggestions are greatly appreciated.
    Thank you

    • Hi, Jessica. If the ignitor isn’t glowing, do you have an electrical tester to make sure the new ignitor is good? Test both sides of of the leads for continuity. If that checks out, when it’s running and trying to heat, see if the ignitor connector is getting any voltage. It’s rare, but there might be something wrong with the wires going to the ignitor.

  • Hi Steve, our gas furnace sometimes blows out warm air at full force and our house will reach 70 degrees within an hour. Other times it blows out warm (not cold) air, but it barely comes out with any force at all and takes all day for it to even reach the 68 degree mark. Two days ago, the electricity went out. We live in an older home and our breakers kick off frequently. So I went and flipped all breakers on and off. We then realized that the whole block was out of electricity for about an hour. When it came back on, our furnace was working at full force again. Stayed that way for two days continually. So nice in this colder weather. Now today we are back at it barely flowing again. Reset the breaker, but still comes out at low force. Any suggestions. We sure are stumped.

  • Bryan

    The furnace is about 5 years old. The furnace will kick on and the igniter will light for a few seconds then kick off

  • Mandy

    For the last week we have noticed that our house drops down to 62-63 degrees at night (and the thermostat is set at 69). During the morning we hear the furnace kick on and warm up and when we come home from work it is at 69 where we have it set but then at night it drops again. We replaced the thermostat to see if that was the issue but the same thing happened. What would make it cool down so much at night but be fine during the day?

  • Gene

    Programmable thermostat set to night time economy settimg?

  • Bob

    I had a Bryant gas furnace installed 5 months ago and it looks to be upside down and is very noisy at the air intake. Is the installation ok or does it need to be corrected?

    Bob

  • Hi, Joe. The furnace doesn’t measure temperature at the furnace location, the thermostat measures it at the thermostat location. So no, a warm attic would not be a reason for the system to not heat. I’d check the other suggestions in the post first.

  • Debra Yeshua’s Daughter Collin

    Hi was reading about the ignitor. However the furnace is burning. Its actually lit up quite well through the little window. It is blowing like crazy but NO heat. PLEASE help!

  • Greg

    Thanks for your blog. It was the first one I read this morning at 4am when I woke up to a cold house. I fired up the furnace and nothing but cold air coming out. I had to go in to the spider-infested crawl space to check on the furnace. Did not see the igniter glowing and noticed a small crack in it (looked just like your picture). Seeing that it was Sunday morning, I figured there was no way I was going to find an igniter today. After about an hour of googling and searching, I found a place open on Sunday mornings! I called them when they opened at 7am. They said they didn’t have the exact replacement, but told me to bring the old one with the bracket. I drove across town and we were able to match up a similar one. The one they had didn’t have the quick-connect wire harness so I had to hard wire it with wire nuts – no big deal. They charged me $55. I came home and installed it and had heat by 9am! Wife is happy. Thanks for your help and guidance in helping me save some money. Service call would have been a lot on a Sunday.

    • Hi, Greg! YES! I love a good “husband saves the day” story. Congratulations on a $55 fix! You’re right — WAY cheaper than a service call. Now you see why I keep a spare ignitor sitting on top of my furnace! 🙂

  • anthony antz

    I have a trane xe 80 my furnace won’t kick on at all it flashes one red light when I press the pressure button so I replaced the control board the flame sensor and the hot surface ignighter as soon as I changed the hot surface igniter I fired up but when I put he door back on it did nothing did I fry the hot surface ignighter did I order the wrong one maybe my furnace is trane xe80 model number is Tud060c924h4 any help will be great thanks

  • Great advice. Thanks, John! 🙂

  • Cease

    I just want to thank you for this article and the detailed information. I am a new homeowner and my furnace went out leaving my family cold for 2 nights. Fortunately, my furnace is the same as what you posted with the photos and details. I checked inside the vent cover of my furnace and discovered the surface ignitor was the issue! I was able to connect with an HVAC contractor who was able to give me a new ignitor for $20. My family was cold for 2 nights but I was able to fix it myself for $20 instead of having a contractor come in for $100+. This information really helped and I just want to thank you!!! God bless.

    • That’s fantastic news, Cease. So glad your family is warm again. 🙂