I have a gas-fired forced air furnace which I need to control two separate heating zones from. One zone is to heat a crawl space only to 45 degrees and the other is the space temp of the cabin. I want to wire a switch between the (2) separate thermostats in order to switch which zone I want to heat. The system is 24 Volts and has three wires from the thermostat, W, R, and G.

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    What kind of logic are you looking for? Furnace comes on when either call? When both call? Or you decide which one is in control at any given time? Who will be there to do that when you are not around? This seems like it may be an XY problem, and I fear you'll get bad advice. – Harper - Reinstate Monica Jan 29 '19 at 21:12
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    Usually when you talk about zones with a forced hot air heating system, that implies the ductwork is divided so it can heat each zone independently, and usually there is one thermostat per zone. Is this the case - when one zone is active, the crawl space receives heat, and when the other zone is active, the main living space in the cabin? – batsplatsterson Jan 29 '19 at 22:30
  • Why can you simply not have a small supplemental heater for the crawl space? – ThreePhaseEel Jan 30 '19 at 0:39

I think you need a (dpdt) double pole double throw switch g is the fan, and w the heat both are connected to R 24v using the common of the switch back to the furnace on one side connect thermostat w on the other g. Do the same with the second thermostat and when you flip the switch it will change the input from one thermostat to the other. Trying to make it clearer Many switches will have the marking NC and NO and common. Stat 1 to NC contacts and stat 2 to NO contacts and the common goes back to the furnace.

  • So from the switch I would add an additional R wire from each common terminal of the switch back to the furnace and the W, G, & R wires from each thermostat just going to each side of the terminals on the switch? In other words the 2 thermostat G and W wires just go to the switch and not back to the furnace? – Matthew Benoit Jan 30 '19 at 15:53

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