Dan has been in the HVAC industry for 23 years, with experience ranging from installation and service to sales and distribution.
Is Your Furnace Not Lighting?
Do you feel it getting a bit chilly? Do you hear the furnace running, but you're getting only cold air and it keeps turning on and off? And then . . .
Exactly. You hear nothing.
Well, depending on what time it is, you may want to get some extra blankets, but the good news is that you might be able to figure this one out for yourself and even fix the problem pretty easily.
Diagnosing a problem with your hot surface ignitor (silicon nitride or carbide ignitor) is rather simple and replacing it is just as easy. So strap on your work boots, get out your toolbox, and let's get this thing running. (Just kidding. You won't need work boots at all and probably only one tool.)
What Is a Hot Surface Ignitor?
The hot surface ignitor is a device used in most modern furnaces to ignite the gas you use for heat. It's a relatively simple component that works quite like a toaster in that its heat is generated by forcing 120 volts through an exposed, resistant material. In this case, that material is usually silicon carbide or silicon nitride.
Now, I used the word resistant lightly. Make no mistake, this component is very fragile. It is electrically resistant, not impact resistant at all. When handling this component, be very careful not to whack it against anything, and don't touch it with your bare hands. Even the slightest crack in the ignitor can cause it not to work, and the oils from your skin can cause failure too.
Diagnosing the Problem
Diagnosing a faulty hot surface ignitor is really quite simple. There are very few times where your eyes and ears will fool you, so odds are, if you watch and listen, you'll know if your ignitor is bad.
Simply watch the furnace the next time it tries to start up. There is a chance that if your furnace was trying to start but didn't, it may be in a lockout phase. This is a safety default on your furnace and can be overridden by shutting off the main power to the unit, waiting approximately 60 seconds, and then turning it back on again. Now your furnace should try to restart, so watch and listen.
As the furnace begins to do its thing, you'll hear the fan come on. Then, perhaps—if you have one—you'll hear the smaller inducer fan turn on. Shortly thereafter, you should see your hot surface ignitor begin to glow followed by a "tick" sound. This "tick" is the gas valve opening. Finally, you should see the flames ignite.
No glowing? No fire? Your hot surface ignitor is bad. At least that will be the case most of the time.
Replacing a Furnace Ignitor
Replacing the hot surface ignitor is simple. The key, again, is to be careful because of how fragile these can be. I don't mean to dumb it down, but you just take out a screw, remove the part, and undo the clip. That's it. The new part goes back in the same way the old part came out, only more carefully.
I do recommend buying two; I did this when I worked on furnaces full time. This saved me a trip more than once, and when repairing my own, I ended up with a spare that I know I'll use someday. These tend to go out about every five or six years, and you never know if it might be sooner, so it's good to have one on hand.
Another important point is being sure you get the proper replacement. You won't find these at your big-box home improvement stores, so call a local wholesaler for a replacement. Be sure to call one that deals in your unit's brand or sister brand, and have the model number ready. Take the old part with you along with its mounting bracket and electrical connector. Sometimes, the only difference in the ignitors will be these two items, but they are an important difference, so you want them to be correct, too. You can expect this part to cost somewhere between $25 and $50—still much cheaper than a service call.
Furnace Repair Tips
If you see the ignitor is working like it should, then you likely have a different problem. Try checking a couple of these items for other easy fixes.
- Fuse: There is a 3-amp fuse on your circuit board. Of course, if it blows, then you likely don't have anything going on at all in the furnace.
- Flame Sensor: This is a very common problem with furnaces and, again, another easy fix. With this, your furnace will likely fire and then shut down almost as fast.
- Thermostat: Perhaps it's time to replace your old thermostat with a newer, more accurate one. Not that your furnace shutting down has to do with accuracy, but an aged thermostat can cause a furnace to do funny things, too.
I hope that something here has the home-fires burning again.
As always, my aim is to help you, and my hope is that you'll help others.
Need More Furnace Help?
- How to Figure Out What is Wrong With Your Furnace
Do you know how your furnace is supposed to work? If not, how do you intend to fix it? This hub will help you understand the proper working order of a furnace so that you can pinpoint problems and repair them or know what to tell the people who will.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Question: I bought a new furnace igniter but it is longer than the old one; is it safe to use?
Answer: So long as it is mounted securely and directly in the path of the gas as it was then you should be fine. Is it the proper voltage? That is important.
Question: After refueling my furnace it won't kick back on. I have turned it off and on a few times trying to bleed the air out. What else can I do?
Answer: I don't believe kicking the furnace off and on to bleed is the proper way to do so. I think it's more like bleeding a broken line if you ran it empty. I would have a professional take care of this because I do know if done wrong you can cause damage to the unit and potentially a hazardous situation.
Question: What is the correct orientation of the ignitor shield? Does it go between the ignitor gas source, or does the ignitor go between the shield and gas source?
Answer: There is no igniter shield that I am aware of outside of a grill to protect it from grease. I'm not sure what you have or are working on, but nothing should be between the igniter and gas supply. That could slow ignition, thus creating a build-up of gas before ignition and that's bad.
© 2012 Dan Reed
Emily on December 08, 2019:
So is my ignitor bad if I have to fan it on its second light? I already cleaned the sensor off and it seemed to work for a while but then it stops. I am not sure if how I am wording this makes sense but I don't want to buy the wrong part. Odd thing is it seems to work sometimes after turning the switch on and off a couple of times.
Michael Langan on November 12, 2019:
with an igniter with a heat sensor, does the heat sensor serve the same purpose as a flame sensor? I can't find a flame sensor on my unit, yet it is behaving like a flame sensor issue exists.
Brian on October 12, 2019:
So my ignitor is turning red but the gas won't open and catch fire. The LED diagnostic says "improper main ground or line voltage below 75 volts. Or broken ignitor, or open ignitor circuit." Should I replace the ignitor?
Dan Reed (author) on February 26, 2018:
Be sure the igniter you're using is the proper voltage...typically 120V or 80V and also that the power being supplied by the board is proper. If not, it could be causing the problem.
Richard M Cortellini on February 25, 2018:
since replacing the last ignitor 2.5 years ago, I've replace the ignitor 2x in the last 2 weeks.
Dan Reed (author) on December 30, 2017:
kalifornia - I'm leaning towards the high limit/fan switch or one of the roll out switches. It's sure acting like a flame sensor but with that ruled out these would be my suspects. They are rather inexpensive parts. I hope it is as those are easy fixes but still hard for me to be sure from here. The video was very helpful though there is no real way to be sure without running electrical tests which it sounds like you're capable of doing so try testing those components. Best of luck and Happy New Year!
kailfornia on December 26, 2017:
furnace (rheem criterionII ~ 1999) shuts off after 7 seconds.
1. replaced control board
2. replaced flame sensor and checked with microamps / its working.
3. checked connections, all seem good.
stumped on where to go for troubleshooting from this point.
video of issue here:
any help very appreciated. thanks!
Scott on February 10, 2017:
My igniter went out.
Checked it and had a small break in It.
Took a very thin piece of copper sheathing I had left over from trimming wood joist outside.
Gently wedged it in crack on ignitor.
Turned on furnace and the copper actually vaporized into a carbon like sunstance.
Ignitor lit and has worked for days.
Long enough for me to get a replacement.
Dan Reed (author) on December 21, 2016:
It sounds as if it's maybe a pressure switch which would be the cheapest easiest place to start.
George Wetherby on December 18, 2016:
I have a Tran XE78 gas hot air furnace. The unit runs almost a full cycle but the flame turns off the inducer slows down , then it speeds up, the flame turns on again and the cycle continues for approx. 3-5 minutes then turns off? Any ideas?
Dan Reed (author) on July 30, 2012:
Thank you kindly vespawoolf. I realize how exciting HVAC is to read about so I do try to sprinkle it with a bit of humor to help ease the pain. LOL! I do hope that should you ever need it, this will come in handy.
Vespa Woolf from Peru, South America on July 30, 2012:
Normally I wouldn't read a hub about repairing a furnace, but your writing made it enjoyable! I'm sure this is very helpful information for a DIYer with a broken furnace. Voted up!
Marcy Goodfleisch from Planet Earth on July 28, 2012:
I'm afraid I had to vote 'no' on this one - I think my safety and the safety of maybe my entire neighborhood might hinge on that choice. I didn't know this part of the furnace existed! As always, I learn so much from your hubs! Voted up and up!