I have just installed a used Modine PAH300 gas furnace in a commercial building. This is a three phase furnace, spark flame start, and has a control box that provides sequencing and spark. I am using a digital thermostat and connected the red and white wire to it. I 'think' the red wire gets 24 volt power from somewhere on the transformer. My main question is, where will the white wire get connected to? To be more clear, what device, switch or control does the white wire go to? I have read that in a pilot lit furnace, the white wire goes directly to the gas valve, but where will it go on this furnace? Thanks to all in advance.
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.