furnace short cycling, how to find cause?


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Old 01-12-20, 10:47 AM
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furnace short cycling, how to find cause?

I have a 28 year old Carrier furnace, which has worked pretty well trouble free save for a motherboard replacement around 20 years ago.

I noticed this morning it is short cycling. It's extremely cold out here at -30C and it was short cycling about every 5-7 minutes. All my air intakes are clear, and filter is new. On occasion the large hose will fall off the humidifier and blow warm air in the limit switch area, I think that causes the limit switch to overheat, and I noticed it was down again when I checked it this morning.

I put it back on and it was still short cycling, so I took the cover off the add-on humidifier unit to make sure it's getting lots of air on the intake side. Last winter I swapped the pricey filter I was using out for a less restrictive one, and that seemed to help. Today I took the filter right out, and so far it has not short cycled since I cranked the thermostat up to test it.

My furnace never used to short cycle, even with finer filters and the humidifier enclosed, and I am wondering why it is happening now. Could it be the limit switch needs replacing? Or some other components not working at 100%?

I am not sure which switch I should look at testing and/or replacing, here is a photo of two items that look like they could be limit switch.

https://www.dropbox.com/s/7p5p3men6w...pture.JPG?dl=0

 
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Old 01-12-20, 11:50 AM
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The humidifier running will cool the air causing the high limit switch to not trip as frequently.

It's very rare for a heat switch to go bad. They don't open early. You can use a meter to find out which one is opening. Set the meter to the scale that can read 24vAC. A normal switch will be a short or 0 volts. A tripped or open switch will show 24vAC.

Typically the A/C condenser coils gets dirty causing poor air flow thru it.
That will cause the furnace to short cycle.
 
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Old 01-12-20, 01:01 PM
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Thanks for the reply. I have no AC so there is no condenser coil in this one. The humidifier is one of those add on units that is mounted on the air intake side and has a large hose connecting to the side of it from the heating side. It seemed to have zero effect on the house humidity so I just stopped using it.
 
 

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