Wiring – How to power Honeywell wifi thermostat with external transformer

hvacthermostatwiring

I want to install A Honeywell RTH 6580 WF thermostat in my garage for heat only . My heater is a simple LP gas, hanging unit heater; heat only and my thermostat has two wires labeled W and R . I have no C wire which I understand is necessary to power the Wi-Fi thermostat . I have also read that instead of a C wire that an external transformer can be installed and used to power the thermostat . A transformer that I looked at at my local Menards is actually a doorbell transformer and can be wired as 24 V AC output and is rated at 20 VA.

Question 1: Is this an acceptable transformer for my application?

Question 2 & 3: There will be two wires (I believe) that will go from the load side of the transformer to the thermostat. Which terminals do these wires connect to on the thermostat? Then do my existing "W" and "R" wires connect to the corresponding "W" and "R" on the new thermostat?

Best Answer

There should already be a transformer on the unit. I believe it's on the back of the unit, mounted to a junction box. There should be two exposed terminals on the transformer. One is R, and the other is C. To determine which is which, use a volt/multimeter to measure AC volts between the R wire going to the thermostat and the transformer terminals. One terminal will measure 24 volts, while the other will measure 0 volts. The terminal that gave you 24 VAC, is the C terminal. Run a wire from that terminal, to the thermostat C terminal.

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