I have dual zone heating in my condo and the upstairs is working fine but the downstairs is sweltering hot. (80 degrees) I am not cooking and do not have my clothes dryer running. The thermostat is only set to 68 but the system still keeps kicking on. This just started last night. The thermostsat is a programmable one and was replaced in September. I did change the batteries today thinking that that was a possibility but no change. Please advise what to check next.

Upstairs is 80 but what is the downstairs temp? The damper motor that makes the duel zone may not be actuating. It is usually found on the plunum above the furance where the plunum splits into at least 2 main ducts, there are two arm links that will move to close a door inside the duct, see if yours ever moves

If this is a forced air system. Turn the down stairs thermostat to off and make the upstairs thermostat call for heat. You should not feel any air at the supply registers downstairs if you do you have a stuck damper or a zone control going out.

It is not a forced air system.

What system is it then?