I took the old thermostat off, and had G,Y,W,Rc and '4' terminals connected to the Green, Yellow, White, Red and Black wires respectively.

Voltage from Rc to G|Y|W was 24VAC as well '4' to G|Y|W was 24VAC

all other combos read less than one volt, so I'll assume that's just noise.

The new Thermostat requires GYW, Rc and Rh (or a wire to Rc and a jumper to Rh)

So, what's the Black wire for? Do I just tape it off and ignore it?

2 Answers 2


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RC and RH terminals on the thermostat are for old fashioned equipment that used 2 transformers 1 for Cool and 1 for Heat, RC was Red Cool, RH was Red heat and no jumper between RC and RH, thermostats come with the RC and RH terminals jumpered so modern single transformer systems can run their Red wire to either RC or RH as they are jumpered.


I found the manual for the old thermostat on the web.

Turns out '4' is an alternative label for Rh on some old models.

I guess I needed that.

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