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?
Wiring – Want to Control 2 thermostats separately from one gas fired heating only furnace
wiring
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You'll need a transformer to step down mains voltage, to 24 volts that the thermostat can use. And you'll need a relay that can work with 24 volts.
You'll wire it up something like this.
When the thermostat calls for heat, the relay contact will be pulled closed. This will signal the fireplace to turn on.
NOTE: the three contacts used on the thermostat are R, W, and C. In the drawing the C may or may not look like a C, so I just wanted to make that clear.
You can only share the C wire, if you're also sharing the R wire. If the R wires come from different equipment, the C wires must also come from the different equipment.
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Best Answer
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
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.