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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.

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    Is there not a W terminal on the control box? Can you post the furnace's wiring diagram for that matter?
    – ThreePhaseEel
    Dec 1, 2016 at 4:53
  • The control box has all it's terminals populated now, and I did not see a W terminal. I have an identical furnace at another location and looked at it today and it did not have traditional thermostat wiring, red and white wires, so i could not tell, but none of those wires went down to the control box. I can't seem to find a wiring diagram for it, nothing on the furnace itself, or anything online.
    – DGA
    Dec 1, 2016 at 4:56
  • Can you post photos of the connections on the control box?
    – ThreePhaseEel
    Dec 1, 2016 at 12:41
  • 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.
    – DGA
    Dec 1, 2016 at 20:09
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    post that as an answer and I'll upvote it
    – ThreePhaseEel
    Dec 1, 2016 at 23:04

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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.

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