Over the years gas furnaces have proved to be some of the most reliable means of home heating. That, however, does not mean they are bulletproof and will always come on and work when you need them. Like any type of mechanical appliance, they are prone to occasional breakdowns. One of the most common problems on forced air gas furnaces is their failure to ignite and come on. It can be a very frustrating experience that literally leaves you out in the cold.
Of course, when your gas furnace fails to ignite you can pick up the phone and call your HVAC repair person. That service call is not going to be free for sure; this is especially true if it’s after normal business hours. The alternative is to save yourself some money by trying to repair your furnace ignition problem yourself.
This guide is mainly talking about furnace not igniting problems. Read our How to Troubleshoot Top 12 Furnace Problems for more info.
Furnace Ignition Failure Easy Troubleshooting
There are some very simple troubleshooting steps, that anyone with even a light mechanical background can do themselves. If you are successful with one of these and get your furnace to come on, you may just save yourself quite a bit of money on an otherwise costly HVAC service call.
1. First Things First – Check Your Gas Supply
Troubleshooting a Gas Furnace When It Fails to Ignite One of the first steps in determining why your furnace is not igniting is to make sure it’s getting gas to it.
- Natural Gas Users
If you are on a city gas line you will not be able to do this step because trying to tell if you are getting natural gas to your heater is dangerous and should only be done by a professional HVAC technician. If you smell gas near your furnace when you turn it on there is a good chance that your gas supply is not the problem
- Propane or LP Gas Users
It’s much easier to determine if your furnace is getting gas if you are a liquid propane user. First of all, you need to go out into your yard and locate where your gas storage tank is. In the top center of it, there will be a domed area where its fill valve is. There will also be a gas pressure gauge.
If this gas pressure gauge reads 25 or above then you are ok; if not call your gas company for a fill-up and you have probably solved the reason why your furnace did not start up. This is an important step for propane gas users because if you call an HVAC technician before doing this they will charge you for a service call. This is true even if being out of LP gas was the only problem with your furnace.
2. Check Your Air Filters
You would be surprised how many gas furnace users do not do this step when their heater does not start up and end up getting charged for an expensive HVAC service call. A gas furnace burns a mixture of air and natural or LP gas. If the air in your system is not flowing it cannot produce the required gas/air mixture and your home heating system will not ignite because of it.
Often times the reason air does not get to your furnace’s igniter is because it is restricted. The main reason for this air restriction is usually traced back to dirty air filters. So check them if your gas furnace fails to ignite when you turn the system on. It’s also a good idea to replace your furnace’s air filters at least once a year.
3. Ignition Source
There are two ways in which a furnace ignites the gas air mixture that comes into it. The oldest way is by a pilot light and over the last 30 years or so these have been replaced by electronic ignition systems. If these fail to do their job your furnace will not come on.
- Pilot Light
This is a small flame which is constantly burning inside your furnace. It’s so small it does not consume much gas. If you look at the bottom of your furnace you will see a tray where the burners are. Your pilot light is located to one side of this tray just slightly above it usually.
Look under your furnace to see if it’s lit. If not, you can use a long match to try and carefully light it again. If that does not work then you have a bigger problem.
- Electronic Furnace Ignition
With an electronic furnace igniter switch, your furnace is safer and does not use the small amount of gas that a furnace with a pilot light does. These igniters are located in about the same position as where we described the pilot light as being. They are made of a very lightweight metal alloy that gets super-hot when an electric current goes through it and this ignites the gas/air mixture coming into your furnace.
Look under your furnace and see if your igniter glows when your furnace tries to start up. You will also hear a clicking noise that is a sign your heater system is trying to come on. Repeated clicking in small intervals usually means your igniter is not coming on and working.
The only simply remedy for this that anyone can do is to very gently reach in and clean the igniter and the area around it. We can’t emphasize ‘gently’ enough because the igniter tip is very fragile. This will sometimes get rid of carbon that has built up around some sensitive ignition points and is keeping the system from coming on.
Turn off the furnace before trying to do this step!
If you turn your heater on and then move the thermostat to a higher setting several things should happen that will make the furnace ignite. The first sounds you will hear are that of the system’s fan which brings air into it to start the burning process. After that, you will hear the swoosh sound of the gas igniting. After a few seconds, hot air will then flow out of the vents in the rooms of your home.
None of this will happen of course if your thermostat is not working properly. What’s the easiest way to tell you to have a defective thermostat? None of the sounds we just described will take place because your broken thermostat did not communicate with your furnace that it’s time to come on. All will be silent near your gas furnace unit.
Fortunately, the thermostat on your gas furnace controls is pretty easy to replace yourself. Just open your control box and locate it. Take it out and then go to your local HVAC supplier and have them match it up with a new thermostat. To install it just reverse the process you used to take it out.
When it’s Time to Call an HVAC Technician?
Once you have done all of the above steps and your furnace still is not igniting you don’t want to try any other furnace troubleshooting unless you really know what you are doing with furnace heating systems. At this point, it’s best to call a trained HVAC professional. They have the experience and equipment to pinpoint most heater problems in just a matter of minutes.
Here is a word of caution though that you really need to pay attention too. If your HVAC technician does indeed find a problem don’t give them the go-ahead to fix it until they show you a written estimate of what that furnace repair will cost. You also need to tell them if they find anything additional wrong while working they need to talk to you about it before proceeding to fix it.
This will keep you from getting any big surprises when they give you the furnace repair bill.
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Tips On How Not to Be Left Out in the Cold with Your Furnace
The most common time to have your furnace fail to ignite is the first time you go to use it after it has not been used in a while. If you are turning it on it probably means it got pretty cold outside and now you have no heat. There are some things you can do to avoid this scenario and to keep your furnaces ignition in tip-top working order.
Here are some suggestions for doing it:
- Don’t wait until it gets cold to turn on your furnace for the first time. Most people know when the cold season in their area is. It’s recommended that about 1 month before this cold season starts turn on your furnace to check and make sure its running right. If it isn’t you have plenty of time to get it fixed before you need to use it.
- Schedule a routine maintenance furnace check once a year. Again the best time to do this is a month before your areas cold season hits. During this routine maintenance session, your HVAC service person will check over the entire system to make sure it's running OK and clean critical furnace ignition parts. Many times they will catch small problems too and repair them before they become much bigger and more expensive furnace repairs.
These are the common questions we receive regarding a furnace that won’t ignite.
Can you be more specific about how to clean a furnace igniter? Also, is that the same thing as a hot surface igniter?
First, yes, the igniter is commonly called a hot surface igniter or even a glow plug. When they’re getting power, they begin to glow, eventually becoming red-hot, hot enough to ignite gas.
Cleaning your hot surface igniter requires a small screwdriver and fine-grit sandpaper made for metal.
- Turn off power to the furnace.
- Take off the furnace door, and locate the igniter.
- Remove the two screws holding it in place, and gently pull it out with or without detaching the wires.
- Be careful NOT to touch the “stick” part of the igniter. Oil from your hands will burn extra hot and might damage the igniter when it lights.
- Gently sand all sides until the soot has been removed. Re-install and secure it.
Pro Tip: It’s not a bad idea to pre-emptively replace the igniter. If you can clean it, you can replace it. The part is cheap - $8 to $30 for most models – and it can save you from being without heat at some point in the middle of winter due to a failed igniter.
I have a multimeter. Can I test the igniter?
Yes, you can. Great idea, in fact. This furnace troubleshooting step will help you rule out whether the igniter is the cause of the gas furnace not igniting or starting.
Our guide “How to Tell if Your Furnace Ignitor Is Bad & Where to Buy One” has step-by-step instructions that are easy to follow, if you need them. Perhaps if you already own a multimeter, you know how to test an igniter.
The essential steps are to:
- Check if the igniter is getting power. While the igniter is connected, turn the furnace to On.
- When the inducer motor comes on, use the leads on the meter, one on each igniter connection. If it shows about 120 volts, there is power to the igniter. If it is getting power but won’t light, it is bad.
- Check the resistance of the igniter. Switch the meter to Ohms. Good igniters typically read in the 40 to 150 ohms range, or perhaps up to 200. If it reads higher, consider it bad and replace it.
Here’s the full guide. It tells you where to find a new igniter and how to replace it.
I’ve heard about furnace flash codes. What are they, and could they help me solve an ignition problem?
Flash codes are a series of light “blips” using one or two lights. Each furnace’s flash codes are different. They’re also called fault codes.
Some codes definitely relate to furnace ignition problems. These error codes include a bad flame sensor or an electric ignition module.
Take the cover off your furnace and look for one or two lights. They might be steady, which usually isn’t an error code. Or it/they could be flashing in a series such one light flashing twice and the second light flashing once.
Near the lights or on the inside of the cover, there should be a key showing you what the flash codes mean. If there’s no key there, like the one below, then perhaps the manual has it.
Pro Tip: Try to get a new furnace filter in the next 24-48 hours to prevent a lot of dust from coating the blower and other internal parts. If those items get really dusty, use a shop vacuum with a brush head to gently remove the dust.
If you still can’t find a chart showing what the flash codes mean, search online for your furnace’s manual using the model number.
Is there a way to check if a dirty filter is the problem when I don’t have a new one to replace it with? I don’t want to pay for a service call only to find the filter is the problem.
Yes. It’s OK to run a gas furnace for a little while without a filter in place. Remove the filter or filters, turn off power to the furnace, and then turn it back on. This resets the furnace.
Then, turn up your thermostat past the room temperature, and see if the furnace fires up.