I have an older Rheem HVAC system and when in the AC mode, my system works just fine, however when in the Heat mode, the a/c compressor comes on outside when the furnace comes on. I tried replacing the thermostat and the problem persisted.

I'm not sure when this problem started as we have only been living in the house for a little over a year and we just recently discovered the problem. As a temporary fix, I disconnected the 24v line to the compressor.

I guess my question is: Do I need to replace the control center in the unit, is this a wiring problem or is something else going on?

Is it a heat pump? Reversing valve is bad or low voltage to it is not being supplied, maybe thermostat misswired?