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HELP! MY GAS FURNACE WON’T STAY LIT!

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Your furnace pilot light or burner keeps going out and it’s so cold!

If you want to troubleshoot the problem, first you need to understand the workings of a furnace ignition system. Then, when you watch your furnace you can find where the problem is happening.

Furnace ignition systems

There are many kinds of furnaces that have slightly different ignition systems.  However, this is how the average furnace ignites:

  1. The thermostat calls for heat.
  2. This turns the draft inducer fan on which brings fresh air to your furnace.
  3. The gas valve then opens and gas flows to the pilot burner.
  4. An electronic spark ignites the pilot light.
  5. The flame sensor tells the furnace that the pilot is lit.
  6. The gas valve opens so gas can flow to the main burner(s) and be lit by the pilot light.

Direct ignition furnaces do not have a pilot light. They skip steps 3, 4 and 5 and light the burner directly. A flame sensor is still present but it is over the burner’s flame.

Furnaces with a standing pilot light do not perform step 4. You need to manually light the pilot light before any of the other steps. These kind of furnaces are very old and inefficient.  This would be a good time to start shopping for a new furnace.

  • If you are having trouble trying to light a standing pilot light on an old furnace make sure you are holding the pilot button long enough.
  • Once the pilot is lit keep the button depressed for at least 30 seconds (sometimes up to 1 minute). You need to give the flame enough time to heat the flame sensor that tells your furnace it’s OK to light the burner.
  • If it’s still not working, it may be one of the problems below.

Now that you understand the basics, watch your ignition system and take note of when the flame goes out.

The pilot light ignites and then goes out.  The burner does not ignite.

You can see the furnace light the pilot and then shut off without lighting the burner. This can happen several times before the furnace stops trying.

Possible causes:`

  • Incorrect flame sensor alignment. The flame sensor could be working but isn’t positioned correctly to sense the pilot’s flame. The flame sensor lets the furnace know when the gas is lit. Without it your furnace would continue to allow gas to flow into your home, even when the pilot light was out.
  • Bad flame sensor. Flame sensors occasionally go bad and need to be replaced.
  • Not enough gas pressure. If there isn’t the right amount of gas for your pilot light, it won’t be able to get the flame sensor hot enough. You may need to adjust or replace the gas valve.
  • Bad ignition board. The small computer control board that controls the ignition sequence may need to be replaced.

The burner ignites but goes out before your home is heated

When the thermostat is turned up, the furnace ignites and the burner comes on. The furnace stays on for a short time (1 to 10 minutes) and then goes out. The flames and pilot light go out but the fan continues to run.

Possible causes:

  • Overheating. The limit switch keeps the furnace from running if the heat exchanger is too hot. Airflow blockages like closed or blocked vents or dirty filters are common causes of an overheated furnace.
  • A bad limit switch. The limit switch could 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 flame sensor over the burner that shut off the gas if it doesn’t sense a flame.

Get a San Antonio furnace professional’s help

The Cool Guys serves San Antonio and the surrounding area. Contact us today to have a professional furnace technician make sure your furnace will keep you safe and warm.

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