Seems like a simple question, but I've had a very hard time finding a definitive answer to this.
I went to the hardware store to buy a replacement for a faulty thermostat and when I arrived and looked at what was available there were specific TYPES of heating & cooling system (single stage, multi stage, heat pump, electric baseboard heat etc) that each thermostat supported or didn't support. I ended up not purchasing anything because I didn't want to buy the wrong one - and to be honest didn't trust the recommendation of the employee.
In my specific case, I have a Rheem Criterion ii heater, but can't find any description online of which of those types of heating system types mine is. For my benefit and others, it would be great if someone could detail how to determine this?