It’s a nuisance; your furnace burner or pilot light keeps going out and you’re freezing cold!
So why isn’t your furnace staying lit?
To troubleshoot the problem, you first need to understand how your furnace ignition system works. Then you can watch your furnace and pinpoint where the problem is happening.
How furnace ignition systems work
While there are different kinds of furnaces with slightly different ignition systems, here’s how an average furnace ignites:
- Your thermostat calls for heat.
- The draft inducer fan kicks on. This fan gets fresh air into your furnace to create a safe combustion chamber.
- The gas valve opens and sends gas to the pilot burner.
- The pilot light is lit by an electronic spark.
- A flame sensor lets your furnace know that the pilot was successfully lit.
- The gas valve sends gas to the main burner(s), which is lit by the pilot light.
If you have a direct ignition furnace, you won’t have a pilot light. These furnaces skip steps 3-5 and instead light the burner directly. Your furnace will still have a flame sensor, but it will be over the burner’s flame.
If you have a furnace with a standing pilot light, step 4 won’t happen. Your pilot light will be lit (by you) before any of the other steps happen. Side note: you may want to consider getting a new furnace since yours is very old and inefficient.
Now that you know the correct sequence of your ignition system, watch it. When does the flame go out?
The pilot flame lights but goes out before lighting burner
Situation: You see your furnace light the pilot when your thermostat calls for heat, but then it quickly shuts off without lighting the burner. It may try a few times before giving up altogether.
- Bad flame sensor. The flame sensor tells your furnace that the gas is lit. Without this device, your furnace would simply continue to allow gas to flow into your home, even when the pilot light was out. Flame sensors can go bad and occasionally need to be replaced.
- Poor flame sensor positioning. The flame sensor might be fine, but isn’t positioned right and so cannot sense the pilot’s flame.
- Defective ignition board. The small computer control board that controls the gas flow, spark and the rest of the ignition sequence may be bad.
- Insufficient gas pressure. If there isn’t the right amount of gas for your pilot light, it won’t be able to keep the thermocouple happy. Your gas valve may need to be adjusted or replaced.
A note about furnaces with standing pilot lights:
If you are trying to light a standing pilot light on an old furnace but it keeps going out, make sure you are holding the pilot button long enough.
Once you light the pilot, you need to keep the button depressed for at least 30 seconds (sometimes up to 1 minute). This gives the flame enough time to heat the flame sensor (thermocouple) that tells your furnace it’s OK to light the burner.
If it’s still not working, it may be one of the problems above.
The burner ignites but then goes out before heating up your home
Situation: You turn the thermostat up, the furnace ignites and the burner comes on. The furnace stays on for a while (maybe 1 minute, maybe 10 minutes) and then goes out. The fan continues to run even though the flames and pilot light go out.
- Overheating. Your furnace has a limit switch that keeps it from running if the heat exchanger is too hot. Dirty filters, closed or blocked vents and other blockages in airflow are common causes of an overheated furnace.
- A bad limit switch. The limit switch may be bad and need to be replaced.
- Bad thermostat. Your thermostat may be incorrectly telling your furnace that your home is warm enough, which shuts off the burners.
- Bad flame sensor. A direct ignition furnace has a flame sensor over the burner that shuts off the gas if it doesn’t sense a flame.
Get a Phoenix furnace professional’s help!
George Brazil Air Conditioning & Heating serves all of the Valley of the Sun. Call us or schedule online to have a professional furnace technician make sure your furnace stays lit.