I just had a 75000 btu Reznor UDAP garage heater installed. Can I run a "C" wire for a smart thermostat if I don't have a C connection on the terminal strip? There is a C connection on the circuit board and an unused W2 connection on the terminal strip.

There are 4 connections on the terminal strip R, G, W1, W2. The current thermostat is connected to the R and W1 terminals. The G is connected to the circuit board from the terminal strip, but looks to be unused tight now. The W2 has nothing hooked to it, i.e bare terminals.

My question is, Can I use the G terminal (since it's connected to the circuit board already) to connect my C wire up (to provide 24V power to my smart thermostat), or should I connect a wire from the unused W2 terminal and connect it to the C terminal on the circuit board, and us that to run 24V power to the smart thermostat. Or is the heater just incompatible and I'll need to buy a battery operated wifi thermostat?


2 Answers 2


After more carefully reading your question:

Neither G nor W1 are suitable to use as a common (C). The C connection is there in the unit as you said, but not present on the rear terminal strip.

Based on the manufacturer schematic, it does appear that "W2" on the rear terminal strip of the single stage unit is truly not connected to anything, so you should be able to extend your "C" terminal from the interior circuit board to W2 on the rear terminal strip without any adverse consequences as you mention. It would probably be a good idea to relabel "W2" on the rear to "C" as well.


After reviewing the wiring diagram, it looks like there's a couple places you can connect a C wire.

Reznor UDAP wiring diagram
Click for larger view

The secondary side of the transformer connects to the circuit board via the SEC and COM terminals. Without seeing a schematic for the board, I'd have to assume that SEC feeds to R, while COM feeds to C. This could easily be tested, by measuring the voltage between the R terminal and the COM terminal. If you measure ~24 volts, then my assumption is correct, and you could connect a C wire to the COM terminal.

A safer bet would be to connect the C wire, to the C terminal on the board. Again, you could measure the voltage between the C terminal on the circuit board, and the R terminal on the terminal strip to verify.

If you want to expose a C terminal at the terminal strip, and you're not using the W2 terminal on the strip. You could connect the C terminal on the circuit board, to the W2 terminal on the terminal strip (just make sure you remove anything else that's connected to the W2 terminal). Then relabel the W2 terminal to C, and connect the C wire from the thermostat to that terminal. WARNING: Make sure the W2 terminal is not connected to anything else, and that it's just an empty, unused terminal. Otherwise you could damage the system.

Reznor UDAP circuit board C terminals

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