Gas and Oil Home Heating Furnaces - Thermostat Question

I just recently bought a Honeywell T8624D thermostat and cannot figure out the exact wiring. Most of the wiring is straightforward and clearly marked. The problem I'm having is that the thermostat calls for a wire designated as "C" and I do not have that wire available at the furnace control module (White-Rogers 50A51, Trane XL80). The Honeywell wiring diagram is showing this as one leg of the 24vac transformer. I just have no idea where to pick this up off of the furnace control module.

Any help would be greatly appreciated

Thanks

Tom
:confused:

common at the tstat. you will need another conductor if you are using all you have presently. i would get it from the circuit board low volt tie in, of the outdoor unit tstat wires, one is y to the tstat and the other is common.

You are correct I do need to pull another wire and you are also correct there seems to be no "C" at the furnace control module. What I have on the control module is as follows.

W1 - 1st stage
W2 - 2nd stage
Y - AC compressor contactor
R - 24vac
G - Fan
B - ????

"W1, W2, Y, R and G" go to my tstat
"Y and B" head out to the AC

Is the B terminal my common at the module??

Thanks

Yes the B should be your common return wire. To prove it..put one lead on the R terminal and the other on B, if it reads 24V your OK, also check B to ground , this should have continuity if it is a grounded transformer, as most are... When you run your new T'stat wire, get wire with two extra wires in the event something changes in the future or a wire breaks...Look at the schematic on the furnace, it should show it.

however checking from r to b is not fail safe. if you were to check r to w1 with no call for heat you would also read 24 volts, finding common back through the heat relay. you appear to have an electric furnace? i assume this is this a 2 stage heat tstat?

actually taking the lead to the transformer directly will eliminate altogether the question of b being the return, if it has continuity with the C side of the trasnsformer, it is common.

I'll check this tonight. I have the schematic to the Funace but it shows the furnace control module with the terminals as depicted above and no idication of what is internal to the module. I'm assuming it is generating the 24vac. I'll poke around with my meter to verify and look for the transformer. I have to go to my daughters vball match so will not get to check till tonight. Thanks to all for the help and I'll post what I find.

All Good, "B" was the terminal I needed. Confirmed this with a merter measuring back to transformer. New wire pulled and tstat all wired, works great. I now have a 2 stage tstat to make better use of my 2 stage furnace.

thanks again