Gas Furnace Burner on & off

[morwoll123]

Does anyone know why my gas furnace will turn on and work normal for a few minutes, and then the burner will turn off before it reaches the temp on the thermostat and then before the fan turns off the burner will come back on??? This will happen at least 5 times of on & off just by turning the thermostat up 3-4 degrees. One other note, If I pull the air filter out about 2-3 inches, it will then continue to run correctly. Any suggestions would be much appreciated.

[LazyPup]

Sliding the air filter out was the key to your answer. When the air filter is in place the return air is restricted which lowers the velocity of air into the plenum. With the velocity reduced, the air in the plenum is getting too hot and tripping out the "High Limit" switch. When you slid the air filter out a bit it allowed air to bypass the filter thus increasing the velocity to its normal level and all is well.

Solution: First replace the air filter. The normal airborn dirt and particulates trapped by air filters are microfine, therefore even a visible layer of the dirt on the filter is indicating the filter is saturated and should be changed.

Next, check all return air duct openings to make sure none are restricted by furnishings or personal items sitting in front of the grilles.

If changing the filter does not resolve the problem you may need to consider having a larger filter installed to allow more cross sectional area of airflow.

[Hube]

Welcome to the forum.
This is more than likely caused by a DIRTY or restricted FILTER, or some other restriction to the air flow such as closed dampers, closed registers, etc.
also, this problem can be caused by an improper setting of the "High limit" control. If its faulty or set too low it will shut off the burner and not allow it to re-fire again until the temperature in the furnace has dropped to a proper safer level.
Normally, a 'high' limit will be set to kick off at approx 180, but it depends on your furnace and ducting specs.

[morwoll123]

Thanks for the quick responses!! I did just replace the air filter with a cheap filter to try to get more airflow and that didn't do it. I also checked all the cold air returns and they are not blocked. There are also no heat registers closed, everything is opened all the way. What is a good way to test the high limit switch?? or is there a way to test it?? One other thing, the furnace is about 11 years old and has worked great up to 3 months ago when this started. Thanks again!!!

[imeduc]

If you have an AC coil on this furnace can be it might need cleaning also to let air through there. New filter every 30 to 90 days is what we tell people all the time .

ED

My mistakes dont define me they inform me.

[LazyPup]

The problem is definitely resulting from a restricted air flow. If you have insured that all duct openings are open, and you have replaced the filter, it could, as Imudec pointed out, also be arising from a dirty AC coil. Aside from all of that the problem is associated with the high limit switch.

Most high limit switches use a bi-metal sensor that will weaken in time which would throw it out of calibration, or it could be dust and dirt on the sensor which would trap the heat against the sensing element.

Because the high limit switch is a primary safety control to prevent the furnace from overheating and becoming a potential fire hazard it would be best to have a trained HVAC tech check it.

[morwoll123]

Well, just an update at where I'm at in my process of elimination. I replaced my high limit switch and that did not make any difference. I also removed my grates over my cold air return to see what the flow was like. I could definatly feel it sucking in air, but not sure what it was like a few months ago when everything was working correctly. My next step will probably be to check the AC coils. However, I'm not sure of the best way to get into that area. It looks like the front comes off of the duct work, but that's where the drain and freon pipe enter. Wish me luck Any suggestions about my issues would be appreciated again.