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?

  • What's the make and model of the heater?
    – Tester101
    Jan 27, 2016 at 20:32
  • Does the heater already have electricity going to it, to run anything on the unit? Or does it run only, exclusively, on gas? Jan 27, 2016 at 21:36
  • It is a Sterling Model #QVF 45. Yes it has 120VAC going to it and it has a transformer in a box that came on the unit going
    – redblur
    Jan 27, 2016 at 22:01
  • The existing thermostat is mounted to the heater itself. Mounted there buy the hvac contractor 20 or more years ago. The heater is LP gas only.
    – redblur
    Jan 27, 2016 at 22:03
  • It looks like the existing transformer is for 24V to the gas valve and the fan time delay switch?
    – redblur
    Jan 27, 2016 at 22:07

1 Answer 1


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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