When heat starts at furnace the AC compressor outside also starts running

[jaxstraw11]

Just installed the Learning Nest.

Everything seems to work except the thermostat didn't show a COOL mode.

When in HEAT mode though....and furnace turns on.....we can see the AC compressor outside turning on and running also. ( the winter cover is blowing up off unit is how we noticed it was also running).

The old thermostat had a slide switch that we manually switched when we wanted the thermostat to control either heat or cool.

I'll post pictures in the next few replies showing how everything was wired.

My furnace does not have a control panel or a C wire so this is why the Learning Nest was recommended by Google.

PICTURES...

New Nest wiring (blue wire was never attached on original and isn't attached....or even seen....at furnace end.... it's a useless wire)

Old thermostat was wired with only 3 wires.

Old thermostat face showing slide selector for heat and AC

 

 

 

[jaxstraw11]

Furnace wiring at transistor 

 

[zoeuvre]

Hi jaxstraw11, 

Uh-oh! Based on the information and pictures you provided, it appears that your Nest Thermostat is not displaying the COOL mode, and the AC compressor is running even when the thermostat is set to HEAT mode. Could you tell where is the blue wire connected? If not, trace the blue wire on where it is connected to your control board. Also, to ensure the Nest Thermostat's cooling mode is enabled, check its functionality settings through the Nest app or on the thermostat. Ensure the thermostat is receiving sufficient power to operate its functions, as without a C wire, it can draw power from the HVAC system, potentially affecting its performance and cooling control. Consider using a Nest Power Connector for alternative power solutions. By checking compatibility, verifying wiring, adjusting functionality settings, and ensuring a proper power source, you can potentially resolve the COOL mode issue and prevent the AC compressor from running in HEAT mode.

Let us know how it goes.

Best,

Zoe