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I have a boiler system that uses 2 wires to the thermostat. I am going to buy a new thermostat that needs a C wire. There is a spare wire not being used and it is not hooked up to anything.

How can I use this spare wire and hook it up to a transformer I will install near the boiler, where the wire is?

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    possible duplicate of How can I add a "C" wire to my thermostat? – mjohns Jul 19 '15 at 18:58
  • There should already be a transformer in the boiler control circuitry. One leg of this transformer supplies the R terminal of the thermostat, C is the other leg. – Tester101 Jul 20 '15 at 2:15
  • Please include the make and model of the boiler. – Tester101 Jul 20 '15 at 11:50
  • Post the model of you boiler, you may have a C (common) terminal on it already. – Joe Fala Apr 7 at 20:33
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Installation of a WiFi thermostat to control a forced hot air furnace, and you have a spare wire available to use for (C). Rather than get into the wiring of the furnace, I install a Taco SR501 single zone relay outside the furnace, near the service switch. The 120 volt AC for the SR501 can be connected to the furnace (usually at the service switch). The SR501 then can be wired using the T T terminals (red and white from the T-stat) and the green (or spare wire) connect to the C terminal in the SR501. Connect two wires from the burner head (T T) to terminals 5 and 6 on the SR501 (normally open). This works well, and the SR501 relay is about 50 bucks...

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