I'm trying to replace my parents mechanical thermostat with a White Rodgers digital one. The house used to have a heat pump with backup electric furnace. The heat pump was removed, so all that is left is the electric furnace.

The mechanical thermostat had wires labeled W,B,G,R,O,Y. I hooked up the digital one using the same wires for the same terminals as the mechanical one, but it won't kick on the blower unit. I hear the relay switch, but nothing from the furnace.

My question is, which of the wires are needed and which are not, considering the heat pump is not there anymore. Also, the digital thermostat has another terminal labeled C, which is supposed to get continuous 24V AC. I used the wire from R on the mechanical thermostat for C on the digital since it was the only one I could find with 24V. Hope someone can point out the error of my ways, because this is driving me crazy.

 

R = Red (24vac control power supply to the thermostat)
G = Green (energizing this commands the indoor air blower to run).
W = White (energizing this commands the electric heat to come on).
Y = Yellow (energizing this brings on the compressor)
O = Orange (energizing this has the reversing valve of a heat-pump activate allowing heating or cooling pending the system design).


Notes ~

Your thermostat may have . . .

- Rh & Rc. These are separate power wires to separate heating and cooling machines. More commonly one jumpers these two together. Your thermostat may not have them as such, just the basic R terminal.

- C This amounts to the common side of the 24vac control power (Red 24vac). It is used with the red 24vac wire to provide power for the digital thermostat and not have to have a battery (baring a battery back-up). The thermostat wire commonly connected to it should be Blue or Black.
There's a very good chance the original installer did not connect it as in the original install with the mechanical thermostat didn't need it.

Oops.


If you're lucky, the original installer ran a thermostat cable that has a spare conductor. If one is there, connect it to the C terminal of the stat and at the machine, connect it to the C terminal (if there is one, there may be nothing more than a blue or black wire among all the others). You'll do well just looking at the schematic diagram that's hopefully still stuck inside one of the machine panels.

Many of the newer digital thermostats are built so as to not need the common return wire, so be sure to RTFM.


Other than that, you'll need to call a pro to do it for you.

 

You need the R...you don't need the C...

Hook the wire back to R, and leave the C alone.

At best it will work fine, or you may have blown a fuse if the unit it's controlling is equipped with one...

At worst, you blew a transformer...

 

Hey, great info guys. Thanks! Wish the manual would just state these things as simply as you BC847.

One last question:

My digital has the RH, RC but no R. Currently I have nothing connected to these terminals. Is that correct? I Hooked the red wire from the mechanical thermostat to C, as it was the only one showing 24v.

Still, the blower will not kick on when I select blower. What is B? I have B, R hooked to C, G and W hooked up.

 

If you are only running electric heat, connect the Red thermostat wire to the Rh terminal.

To help simplify things, think of the Red wire as the power "+" or positive (in this case though we're using Alternating Current, AC).

If you don't have a black or blue wire to connect to the C terminal, as megacabdad suggests, don't connect anything there for now.
As I posted earlier, the new digital stats are able to operate without a dedicated "Common" wire to power the stats workings. One could say they leach the power they need from the heat/cool/fan circuits. Hopefully, yours will be of that type . . . . maybe.