Furnace Not Igniting? 4 Common Causes and How to Fix It

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 IgniteOne 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!

4. Thermostat

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:

  1. 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.
  2. 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.
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10 thoughts on “Furnace Not Igniting? 4 Common Causes and How to Fix It”

  1. Ducane Ng furnace, sometimes fails to light, but turn circuit off and back on always restores operation. When not lighting the gas valve clicks are much softer sounding. When working the clicks are well defined.

    • I have a 2001 Haier gas heater gas ignition want come on I replaced thermostat green light is on having problems with the gas not coming though ignition

  2. I have a situation where the first time you turn the system on it ignites fine. It uses a thermocoupler system. The second time you turn it on the pilot lights the sensor gets hot, the gas valve then opens but there is no ignition at that time if you wait too long there is a great ignition, that scares us. It just started after cleaning the sensor. If you leave it off for an hour or so then it starts normal. I am puzzled. This is an old electric start westinghouse heat and air unit. Thans if you can shed some light on this problem.


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