I am trying to add a C-wire for a smart thermostat on an existing gas furnace heater (no cooling). The control board is a HK42FZ065 and the condensate pump a cb151ul.

I tried measuring the voltage between both the R terminal and COM on the control board directly, then again between W and COM ... no voltage.

I anyway connected the spare wire on the thermostate side to COM on the control board (along with all the other wires (R,W,G,C) and no surprise the thermostat did not power on.

I traced the wires a bit more closely and notice that the "Rc" wire from the thermostat goes to "W" on the control board. "W" on the thermostat goes to the condensate pump and then to the "R" terminal on the control board. Is this normal? I don't understand the wiring logic behind this at all. The green wire appears to not be connected to anything? Again is this common?

In any event, I can't seem to pull ~28v from anywhere. I even tried "R" and "W" to the screw connecting the control board to the case.

I double checked that the case door safety switch is completely depressed. I even put the case back on and checked the thermostat... no luck. The old thermostat is has AA batteries and works fine.

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1 Answer 1



The original installers weren't counting on a C wire so the R/Rh and W are basically interchangeable. If I swap R and W on my thermostat, I will have constant power over R and C and W is triggered when heating is called for.

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