AC condenser fan ran when thermostat was in heat mode on "fan" setting

In my most recent inspection I turned the thermostat on its “fan” setting in heat mode with the temperature setting well below the air temperature to take a control reading for CO, and noticed the AC condenser fan was running.

The thermostat did have “heat”, “off”, and “cool” settings. The AC condenser was relatively old, manufactured in 1992.

There was cool air coming out of the condenser and the coolant line was not noticeably cold, but I found it alarming, given that even the unseasonably warm air temperature was still in the mid 50sF.

The heat mode operated normally when the thermostat fan setting was on “auto” and the mercury tipped over almost right on the air temperature.

I’ve never come across anything like this. Anybody know what was going on? Thermostat malfunction or improper wiring, or AC condenser issue?

Sounds as if ya had a heat pump with a gas secondary furnace. Hard to tell from here what ya had. If no heat pump ya got thermostat wiring problems.

Bad thermostat sub-base.

Do you have photos of the HVAC?

Appeared to be a typical 80% efficiency gas, forced air furnace of a relatively modern design for a 1993 manufacture date (Bryant). Probably thermostat issues.

I have a 80% gas furnace on my own home with a heat pump outside don’t assume anything