Just moved into a new house and would like to incorporate my central air controls and heat control together. I got a thermostat that will handle both. Now the ac part went fine with just 3 wires. Taking the heat only lux thermostat seemed promising. On the panel it says .15 to 1.2 amps max 30volts ac. It is an old mercury type. OK so low voltage right? Hook it up to the new t-stat and turn the breaker on. Bam smoke. turn it off and scratched my head. Do I have a 24volt or do I need a line voltage thermostat? Thanks!

Jay...

Need to tell us exactly what kind of heat you are trying to control,, brand and model numbers, am I understanding you smoked the tstat? If you have a forced air furnace you must have wired something wrong,, if you are talking about baseboard electric heat, yep you need line voltage tstat,, tell us more

Since you said the systems are separate, they likely each have their own 24 VAC transformer.

You either need to know what you are doing or purchase a stat with separate Rc and Rh terminals. Rc is R(cooling) and Rh is R(heating).

The tstat controls the circulator ron the burner. Regular hot water baseboards.

Does the tsat that runs the curculator only have two small wires?