Thermostat - R, RC, RH confusion

[geoffv@magma.ca]

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 *

Thanks.

Geoff

[Freon]

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.

[geoffv@magma.ca]

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

[Freon]

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 * 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 *

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.

[geoffv@magma.ca]

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