Central air - odd problem I think

MaxMercy

This morning when I woke up, the AC was off and the thermostat was not calling (fine so far), but outside, I could hear the compressor running. I went back to my thermostat and although it wasn't calling (normally indicated by the temperature flashing), I moved the Heat - Off - AC switch to the middle off position and the compressor continued to run. I moved the fan switch from auto to on and the air handler fan in the attic started up. I put the switch back to auto and then flicked the H-O-C switch a few times and the compressor turned off.

I opened the back door and looked down at the now stopped (HEIL) compressor and didn't see any icing anywhere, so I went back upstairs and turned the thermostat on and set it to call for AC, and the compressor started along with the air handler and the AC was working. Right now, everything seems fine.

This is a basic digital Hunter Energy Monitor II four wire thermostat from the mid 90s, so my question is whether the thermostat could have caused the compressor outside to run without running the air handler fan at the same time, or is it more likely it was a stuck contactor outside at the compressor?

I also can't be sure if it was the compressor or just the compressor fan that was running outside because I (stupidly) didn't check to see what was happening before the problem resolved itself.

If the problem returns, I'll go outside and check the compressor directly.

JUGHNE

4 wire stat…..does it have batteries?

MaxMercy

https://forum.heatinghelp.com/discussion/comment/1806373#Comment_1806373

Yes. Recently changed. It uses two AA cells in parallel.

JUGHNE

If the comp ran for any length of time after the last cycle without the inside fan then frost/ice up would be obvious.

If the outside fan was the only thing running them it could have a shorted wire giving the fan partial voltage.

Possible if the contactor is a 1 1/2 pole type….switching only one side of the 240 volt supply……the other supply line would be "hot" thru out the AC wiring.

Ironman

If the compressor and condenser fan were both running, then you probably have a sticking contactor.

mattmia2

Cold be a control wire nicked and touching somewhere too. Or the relay in the t-stat sticking.

Compressor may have been cycling on overload or if the issue is with the control wiring or t-stat or possibly control board in the air handler/furnace depending on what system it is, the pressure switches on the condenser could have been causing it to cycle so it wasn't icing. If it wasn't icing I would be inclined to think the pressure switches were deenergizing the contactor and cycling the compressor.