Reverse Cycle HVAC Issue

[Lee Hadjiosif]

My cooling system has been working fine all summer, but when I was in need of some heat the other night, the heating was not staying on. The compressor would come on for 30 seconds producing good heat and then go off (The thermostat was set to 4 degrees higher). After about 3-5 minutes, it would come on again and off in 30 seconds, and it would repeat this until I shut it off. The condenser coil was recently cleaned. I have another similar unit on the front that shares the pump, and it worked fine. When put back on cool both systems worked fine. Any ideas on what may keep the compressor from staying on on heat only? Maybe a malfunctioning heat sensor? Thanks
Lee

 

[finding41]

Sounds like it is in "furnace limp mode". My house furnace did the same thing. I called the repair guy and he opened the furnace and watched a red flashing light and said it was my flame sensor.
I asked him how he knew that as I stood there and watched the 30 second diagnoses. He said he just counted the flashes on the circuit board and looked on the list beside it.
He then undid one screw and pulled out the sensor. ( It looked like a pencil led.) He took a piece of emery cloth and gently sanded it and put it back in.
He told me he didn't have one in the truck or he would have replaced it! I've fixed it once last year myself. (10 years after the original fix!)
Maybe open it up and have a look.

 

[SEMPERAVANTI41]

First, I would want to eliminate the thermostat as the cause of the problem. You could hook up a test light to see if the thermostat is calling for the compressor when the compressor goes off, or better you could swap thermostats from your forward unit to see if the problem persists ( I assume that the thermostats are the same).

Once you eliminate the thermostat as the cause of the problem, my guess is the reversing valve. Perhaps the electrical connection to the reversing valve has corroded, causing high resistance, or maybe the reversing valve is tired. It appears from looking at Grainger.com (part # 3A439) that some reversing valves have an electrical coil that can be changed without opening up the sealed part of the refrigeration system. Again you might be able to swap the coil from the forward unit to the rear unit to test. I would also want to eliminate the wiring from the thermostat to the reversing coil. Perhaps the wiring has a loose connection or corrosion.

Hope this helps!

 

[Chuckbear]

Lee, We had the same issue. For us it was a loose connection on the reversing valve. Yours may be the same or the valve is going bad. If the AC works in cooling mode that that is a good indicator. Chuck

 

[Lee Hadjiosif]

Chuckbear, thanks. Can you tell me what the reversing valve looks like and how to locate it?
S/V Adagio