I have a forced air propane furnace and central air. A few days ago it cooled down here to a comfortable temperature so we turned the A/C off, by flipping the heat/off/cool to the off position. While turned off the ac has been cycling. It will kick on, run a short amount of time like maybe 30 seconds, kick back off, repeat. I haven't timed it but the cycle time seems random. So I first replaced the battery in the thermostat, then replaced the whole thermostat with a new digital non-programmable unit. I got home from work yesterday and my wife said it was doing it again. She said that after the phantom cycles she turned the switch to cool but turned the temp way up so it wouldn't command on and that seemed to help. Then it got hot out, we actually ran the ac and it seems to be functioning correctly when in use. Any thoughts as to what would be the cause considering I already replaced the tstat?
Thanks
wae
PowerDork
7/12/22 9:56 a.m.
I wonder if you could have a fault in the wiring to the thermostat? Is it the just fan that comes on or is it the compressor fan outside as well? Maybe try disconnecting the green and yellow wires from the furnace side and see if that fixes the phantom cycle?
When the A/C runs, is it just the outside unit or is it the whole furnace with the blower and whatnot.
If its the outside unit, the contactor (relay) is likely faulty. Its inside the compressor under a cover.
If the whole thing is ghost-running, then look at the thermostat wiring to see if there is something amiss there, or it may be the main board in the furnace that sends signals out to the compressor to turn on.
I caught mine cycling on/off. It was due to the fan start capacitor having taken a crap, but the compressor was still kicking on when told to and then shortly overheating due to no fan.
It is both the condenser unit and the furnace fan. My wife said that it's only doing it with the thermostat in the off position. If it's set to cool but with the temp set high enough to not need to run it doesn't do it.
TJL (Forum Supporter) said:
Janky thermostat?
When this started happening I replaced the battery in the thermostat, then when that didnt fix it I replaced the whole thermostat. I know its possible for both thermostats to have the same issue, but not likely. I think I will try wae's suggestion fo unhooking the green and yellow at the furnace and see if that fixes it to eliminate the wiring and the tstat.
carguy123 said:
capacitor
Which capacitor are we talking about?
I don't know why he said that. A capacitor isn't going to cause random starting when the system isn't even calling for cool.
Since you have what is known as a straight cool AC, you will have two low voltage wires running from the air handler out to the condensing unit. They are pretty simple. When the board calls for cool through those wires it closes the contactor and starts the compressor and condensor fan. When the system has satisfied, the call for cool is stopped and the contactor springs open and shuts off the outside unit.
