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

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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? Why is my gas furnace short cycling so much?

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 explain 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, or clogged 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.

You Can Search Our Site for More Troubleshooting Problems with Answers, Heating, Air Conditioning Parts and Supplies in the Google Search Box Below:

2 thoughts on “Problem: Why does my gas furnace short cycle so much?

  1. Hi Steve, I have a one year old Trane furnace (new furnace installed in a new house last year) and I’m wondering how often is it acceptable for it to retry? I’m inclined to think it should almost never retry but I’d like to get your opinion.

    Last winter we noticed the burners would often fire up and then cut out before the fan started at the beginning of a cycle. This would trigger the fan to kick on for a short time, then turn off and pause briefly before retrying. The furnace would always run through a normal full cycle after retrying like this and it would never retry enough to lock out. Not every cycle was like this, some cycles were normal without any hiccups and there was no obvious pattern.

    However, this malfunction would happen many times throughout the day so we called the local shop that installed the furnace. They didn’t think it should be doing this so they made multiple service calls to try to troubleshoot it. They made adjustments, cleaned the flame sensor, replaced the inducer motor and the controller board and I can’t remember what else…thankfully it was under warranty. But this took a long time and the season changed and we didn’t know if it really got fixed due to warm weather.

    Now that it’s cold again, the furnace is back to retrying frequently, just the same as before, only we’re out of warranty. I’m tempted to just ignore it. Not every cycle retries, it never locks out, it does a good job heating the house. It’s just concerning to hear it go through extra cycles all the time and putting extra wear and tear on the system.

    I’d sure appreciate your opinion. Should we keep chasing after this problem or are we expecting too much?

    1. Hi Steve! Yes, I would not like to hear my furnace short cycle the gas burners as you described. This sounds like a flame sensing problem or a pressure switch problem. I would like to suggest that you test and see if the pressure switch is staying closed the entire time the draft inducer is on. If the pressure switch is opening up then this would cut the gas burners off. You will need to test the pressure switch with a voltmeter. We have a post that discusses and has two YouTube videos about this problem on the following page: If the installers exceeded the vent length requirements set forth by the manufacturer then this could cause the furnace to short cycle. These pressure switch problems and flame sensing problems happen most of the time when the furnace is cold. I would also suggest making sure that all wire connections, especially any ground wire connections are good and tight. Make sure that the flame sensor is engulfed in a good hot blue flame when the burners are burning. Last but not least make sure that your thermostat wire connections are good and tight and if your thermostat uses batteries that the batteries are in good condition. Please let me know if you can find and fix the problem so others can benefit from your findings. Thanks! I sure hope you can find and fix the problem soon. I know this is annoying! God bless you and your family today and always! Steve

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