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I have a 24V gas-fired furnace heating only, 3 wire thermostat and I want to use 2 separate thermostats to control the furnace. 1 thermostat will be used in warmer heating months to control the space temperature of a cabin and the other thermostat will be used in below zero weather to heat a crawl space containing pipes to prevent freezing. Only one thermostat will be able to control the furnace. Someone suggested a DPDT switch but how do I wire that switch?

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You could just wire them in parallel and use the main one at your leisure. Just set the crawlspace thermostat as a low limit stat. Many thermostats will only go down to 15°C (60°F) so if you desire a set point just above freezing for the crawlspace, you will need a non residential thermostat like a A419

Honeywell A419

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www.grainger.com

The A419 is probably a little expensive and a little too feature packed for your needs but it's a great, reliable and reasonablely priced(for what you get) unit. You can try a mechanical refrigeration thermostat but they often close on temperature rise. Some have a normally open and a normally closed terminal.

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You should be able to wire the thermostats in parallel. When either one is closed, the furnace runs.

Run wires from both thermostats to the furnace. (You likely have one set there already.) From thermostat A the two wires (usually one black, one white, sometimes other colours) are connected to two screws on the furnace. With thermostat B's wires connect one each to the same two screws. So both white ones are on one screw and both others are on the other screw.

Most thermostats don't care about current direction, so one wire to each screw.

Therm A -----------------------0-------------------------------- Therm B
        -----------------------0--------------------------------
                               ^
                       Screws on Furnace
  • How do I wire the thermostats in parallel? – mattylou Apr 4 at 14:46
  • With the thermostats wired in parallel then won't both thermostats be able to control the furnace? I really want just one thermostat to control the furnace. Someone else suggested a DPDT switch which I could use to change control from one thermostat to the other. Would this work? If so how would the switch be wired for the furnace and two, three wire, 24V thermostats? – mattylou Apr 5 at 15:13

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