[warrenpeas]

My AC has always worked fine. A couple months ago I replaced a battery powered thermostat with a Honeywell Wifi smart thermostat. The new thermostat needed a "C" wire for power and there was an unused dark blue wire behind the old thermostat so i connected it to the "C" port at the furnace along with the white wire that was already connected to that "C" port. The new thermostat would not power up until i used a jumper wire to connect the R and RC ports of the thermostat then it worked fine. i do not have a heat pump.
The thermostat has been working great on Heat for a couple months but now when i switch to Cool it will turn on the fan but the AC compressor never turns on. I think it is a wiring issue any advice is really appreciated.

original furnace setup with only white wire connected to C

Blue wire attached to C with white wire (is this the issue?)

Old battery powered thermostat setup with RH-RC jumper wire

New Thermostat setup with blue C wire and R-RC jumper wire

 

[57TinkerMan]

Looks like it's wired ok, I would shorten the exposed copper on your blue wire and reconnect it to the thermostat C terminal. Some thermostats have a time delay on a call for cooling, how long have you waited for the unit to come on? Check your circuit breaker and/or fuses to the outdoor unit. If you have a volt meter you can check for 24VAC on the 18/2 wire from the furnace at the outdoor unit when the thermostat is set to cooling. Good luck...

 

[warrenpeas]

yes it is wired per the instructions so i hooked the old thermostat back up the same way it was before and its doing the same thing. the fan runs for 5 to 10 minutes then shuts off and the AC compressor never comes on. i tried all the breakers and they are fine. i think its an outside connection issue

 

[surferdude2]

yes it is wired per the instructions so i hooked the old thermostat back up the same way it was before and its doing the same thing. the fan runs for 5 to 10 minutes then shuts off and the AC compressor never comes on. i tried all the breakers and they are fine. i think its an outside connection issue

There would have to be a control board problem for the fan to shut down while the stat was still calling for cool. That same problem may be affecting the compressor control although I have a hard time figuring how that could be.

Measure the 24 volts at the transformer or on the board between the C and R terminal. A low voltage there could cause an odd problem that would affect both of those functions.

 

[surferdude2]

Thinking more (dangerous pursuit that) I think the compressor contactor coil or associated wiring (including the wire from the furnace to the condenser unit) could be partially shorted and pulling the control voltage down to the point where it is causing the problem with the fan as well. If you find that the 24 vac is sagging down under 20 volts when the stat calls for cool, then disconnect the 2-cond. wire (red-white with green wrapped around it) running to the condenser unit and see if that restores the 24 volts and allows the fan to stay on as it should.

Don't run in cool mode any longer than necessary to do the test since you may be frying the transformer.

 

[warrenpeas]

There would have to be a control board problem for the fan to shut down while the stat was still calling for cool. That same problem may be affecting the compressor control although I have a hard time figuring how that could be.

Measure the 24 volts at the transformer or on the board between the C and R terminal. A low voltage there could cause an odd problem that would affect both of those functions.

So you are saying when I set it to cool the fan should come on and stay on and not turn its self off after 10 minutes when set to Auto?

 

[BayouRunner]

Mistake

 

[surferdude2]

That's what I'm saying... and also don't run it any longer in that mode except for testing as outlined previously, in case it may be overloading the control transformer and may burn it out.

Removing the control wire running out to the condenser will eliminate one possible source of the problem and help narrow it down.

 

[warrenpeas]

I couldn't figure out the issue so i called out a tech. They replaced a capacitor in the AC unit and the contactor. There was no wiring issue.

 

[surferdude2]

Thank you for the feedback. We were on the right trail... sounds like excess compressor contactor coil current was the root problem since the capacitor wouldn't cause the furnace blower to cycle off.