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

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.

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.