Thermostat - R, RC, RH confusion

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Thermostat - R, RC, RH confusion

Postby geoffv@magma.ca » Mon Oct 31, 2011 8:49 pm

G'Day All. I've been looking at the thermostat posts and haven't found and answer -- my apologies if I missed it.

I'm replacing a Honeywell T87F with a Honeywell VisionPro 8000. Our system is 1994 vintage Rheem furnace and A/C. Here's the - the old thermostat has RH with 2 wires under it (a red and a white) and R with one wire. The new thermostat has RC and R with a jumper between them. Where should the 3 “R” wires go, and should the jumper be removed?

Thanks.

Geoff
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- Thermostat - R, RC, RH confusion

Postby Freon » Tue Nov 01, 2011 3:25 pm

To be safe, you need to go to the furnace control board and see where the thermostat wires connect. Tell us at the furnace control board the terminal letter for each of the 3 wires (red, white and the other unspecified colored wire at the R terminal). No sense guessing and ruining your new thermostat.
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- Thermostat - R, RC, RH confusion

Postby geoffv@magma.ca » Wed Nov 02, 2011 7:49 pm

Thanks for the quick reply Freon. At the furnace control board, the terminals are wired as -

C - white
W - white
R - red
G - black
Y - no wire

Thanks again.

Geoff
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- Thermostat - R, RC, RH confusion

Postby Freon » Thu Nov 03, 2011 2:02 pm

Okay, first task is to differentiate between the 2 white wires. See if there is any subtle way to know which wire goes where. We need to get these identified. The one white wire to the C terminal is your common line and connects to the C at the thermostat. The other to the W is the call for heat and connects to the W terminal at the thermostat. The R is the +24 volts and connects to the R at the thermostat .

Is your system a simple heat and AC system or do you have a heat pump? I ask because usually there will be a wire from the Y terminal of the thermostat to the Y terminal of the control board and another wire from the Y terminal of the control board to the outdoor AC condenser unit. The Y terminal at the control board is a simple junction and is not connected to the board electronics. But if you have AC, how do you activate it?

Usually the jumper between Rc (AC power) and Rh (heating power) is left in place UNLESS you have 2 separate transformers, which I doubt. But until we learn what white wire is what and why there is no Y connection at the control board, we can't connect things. It is possible there is a Y (usually yellow but not always) wire from the thermostat that's connects to the AC condenser using a wire nut. Look around the control board area for that type of connection.
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- Thermostat - R, RC, RH confusion

Postby geoffv@magma.ca » Thu Nov 03, 2011 8:33 pm

Thanks for the response.

The system is a single stage Rheem Criterion furnace with regular AC. I'll get the multimeter out and trace the wires, but I'm away for a few days, so it will be next week. Thanks again.

Geoff
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