Another C-Wire Question

[wbradney]

OK, it turns out I had a power problem and didn't realize it.

My old Aprilaire unit didn't need batteries and had an existing c-wire connection, so I though the standard "5-wire Heat/Cool" setup would be all I needed.
After I wired the 3m-50 everything seemed to be working except communicating with the radio from my network.
I could take out the batteries and appear to still have power, but it turns out I wasn't taking out the batteries for long enough - it took a full 30 secs or so for the 3M-50 to fully discharge and power off.

So I actually did NOT have power to the thermostat.

With a meter I could see that I had a solid 24VAC across C-RC, but only ~1V across C-RH, so the old Aprilaire seemed to be drawing it's power from the A/C air handler in the attic and not from the furnace in the basement.
The 3M-50 seems to want power only from C-RH - is that correct?

To solve my problem I jumpered RC and RH. Now heat and cool still work and after reprovisioning the radio the Thermostat for Windows program can see the thermostat.

I'm not sure what are the implications of jumpering RH-RC, but I was getting no power from the furnace, only from the A/C air handler.

Can anyone give any guidance?

[wfisher]

You were correct in jumpering RH and RC. Did your 3M-50 come with a jumper already installed on RH and RC?

[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.

[wbradney]

[quote="wfisher"]You were correct in jumpering RH and RC. Did your 3M-50 come with a jumper already installed on RH and RC?[/quote]

Yes, there was a jumper already in place, but the "5 Wire Heat,Cool" instructions said to remove it if you have both RH and RC wires, which I do. I just don't actually have any power coming from RH.

[wbradney]

[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.

[blaine]

The furnace transformer has the RH wire and its C wire. The A/C has the RC and its C wire. If both systems C wires are connected, and then you connect RH to RC, the transformers would be in parallel. If they were out of phase, you could lose both transformers; if they are in phase and not identical transformers one of them could overheat and eventually quit.

If the two systems C wires are not connected, then connecting RC to RH and using the A/C's C wire would work just fine. The problem is some systems "ground" the C wire to the HVAC metal and if that is in your case, the two C wires could be connected hence very probable troubles.

You could measure the continuity between the two C wires and if they are not connected together (open circuit) go for it.

If possible, plug in a 12V, AC or DC power adapter at the hvac and connect to the furnace R wire at the HVAC. Run the other adapter wire (no polarity) to the thermostat C terminal via any spare wire in either heat or A/C cable.

[wbradney]

OK, I think I'm starting to get it.
I don't believe it possible that the C terminals of the furnace and the A/C could be connected - if they were, wouldn't I already be getting 24V across C-RH?

[blaine]

Yes, good point.