Modine gas furnace thermostat wiring
I figured out how this works, and thought I would add the info here for someone else. You don't control the heater directly with the thermostat, you have to add a 3 pole contactor to it, that has a 24 VAC coil, and switch the contactor coil with the thermostat. Basically you are cutting the mains power on and off to control it. I bought a 16 amp 4 pole Allen Bradley electro mechanical contactor with a 24 VAC coil. Possibly this is how all 3 phase powered furnaces run, I don't know.