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I'm trying to install a wireless thermostat for my church building's HVAC system. Currently there is no C wire, so I tried to see where the circuit board was so I could install one. I couldn't find anything that looked like a C wire though

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If this isn't possible, I'm guessing a second solution would be to power it with an external transformer, but I'm trying to avoid that route if I can.

  • What type of system is this (hot water, forced air, etc.)? What is the make and model of the unit? Can you add some context to your photo? That's definitely a transformer, but there's no way for us to know where any of the wires go. – Tester101 Feb 21 '15 at 15:32
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    Note that if it's an older system (and church systems are always ancient and held together by prayer ), there may not be a single "circuit board", just relays and thermostats wired together. – keshlam Feb 21 '15 at 15:35
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You may not have a C wire. The C is a common used to provide a constant power circuit (C is the Negative or Ground) for the thermostat to use.

In this furnace there are only 2 wires that a manual thermostat or battery operated on will connect which turn the furnace on.

Your only option is to run another wire (Or use another if there are extras) and connect it to the negative pole on the transformer (LOW VOLTAGE SIDE).

Its often the blue wire but I would need a better angle on the transformer to tell for sure on that particular one.

You should also check that the output is no more than 24v. If its anything else it could damage the thermostat.

CAUTION: There are live 110 lines into that transformer. Make sure your power is shut off at the breaker before touching anything in there. Make sure its covered when your done.

  • *24v AC - not DC. – TFK Feb 1 '16 at 14:42
  • You are correct, I removed the Current type all together. I do know there are some appliences like pellet stoves that use a DC control voltage. It does not matter to the thermostat in most cases. As long as it under 24V – JpaytonWPD Feb 1 '16 at 14:50

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