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Problem: Why does my gas furnace short cycle so much?

Troubleshooting Gas & Electric Furnaces

Problem: Why does my gas furnace short cycle so much? The furnace gas lights, but before the thermostat is satisfied the gas goes off. I do not understand what could be causing the gas to go off in our furnace before our thermostat has been satisfied. I noticed this happening for the past week. What could be the problem?

Answer: We have this question asked quite often. We have an excellent Youtube Video made by AC Service Tech LLC that explains why gas furnaces short cycle below. Craig Migliacco does an excellent and thorough job explaining four reasons why gas furnaces short cycle! Thanks to AC Service Tech LLC and Craig Migliacco for making this excellent video!! Please click here if you would like to see the AC Service Tech LLC written post with pictures that explains The four top reasons why a gas furnace short cycles. In short detail the 4 reasons are:

  • The thermal limit switch is tripping and opening up the electrical circuit. The thermal limit switch tripping could be caused by a dirty filter, dirty blower wheel, dirty or stopped up evaporator coil, a slow blower motor (might need a new capacitor or the gas input might be set too high (might need to adjust gas pressure).
  • The pressure switch is tripping and opening up the electrical circuit. This could be caused by a stopped up vent or a slow, stopped up draft inducer. You might need to clean the furnace’s condensate drain line. Please click here if you would like to see our furnace pressure switch troubleshooting page.
  • The control board could have a relay with pitted contacts that are failing.  You might need a new control board.
  • The flame proving process might be off because of a dirty flame sensor, gas burners or heat exchanger. Please click here if you would like to see our flame sensor troubleshooting page.

This excellent video below explains these 4 problems in detail.

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