[quote="blaine"]RH is power for heat and RC is power for cool; some systems have seperate heat and cool 24VAC power. Yours has a single power lead so must connect to both.[/quote]
Yes, but I'm not sure what you mean by "power for heat". My system has a separate furnace and A/C handler, with two separate power supplies/transformers.
I have two wires coming from the furnace to the thermostat: a white one (which was connected to my Aprilaire's W terminal) and a red one (which was connected to my Aprilaire's RH terminal).
I have four wires coming from the A/C handler, red (which was connected to R and jumpered to RC), green (was G), yellow (was Y) and blue (was C).
So the only way I can get 24VAC is across C-RC. There's (obviously) nothing across C-RH, since they're coming from two different transformers.
My contention is that while the Aprilaire was more than happy to be powered from the A/C handler, the 3m-50 is not, and since my C-wire is from the A/C handler and not the furnace, the "5 wire heat/cool" setup does not apply. I would have to run another C-wire from my furnace in order for that to work.
I wouldn't be at all worried about jumpering RC-RH in the absence of one or the other wire. But I'm jumpering RC-RH even though I have both.