[munny9068]

Can't determine if there is a 24vac common on my old Bryant. How do you add one ?

[hvac1000]

How many wires on the old thermostat and what colors?

[munny9068]

Yellow to Y, green to G, white toW and red to 4/R. there is a blue wire in the thermostat sub-base that is trimmed off and not used and no blue wire is used or visible on the wire going to the air handler terminal board.

[acetc]

You don't have a common connected but you could use the blue wire for a common by connecting it at the air handler to the "C" on the terminal board or to the transformer, Mike

[munny9068]

Thanks. Sorry to be so stupid, but I don not see a 'C' terminal on the connection board at the handler.The wire from the thermostat comes through the closet wall and attaches to a terminal board on the side of the air handler. There are 5 screws there labeled, Y,G,W,4/R and X/F. Only 4 wires of the house wire are visible with the yellow connected to the Y,green to the G, white to the W and red to the 4/R. I do not see any blue wire on this end of the thermostat wire. It appears that the 1976 installers just cut off wires they did not need or use. There is also a short segment of thermostat wire running from this same terminal board to the bottom of the compressor unit. Two wires here; a red wire connected to the X/F terminal on the air handler and a white wire connected to the Y terminal (along with the yellow wire from the thermostat). I have found internet pictures of this same old thermostat but the terminals in mine are labeled differented. For instance, mine is G-F instead of G, Y-C instead of RC, 4-M instead of Rh, RV instead of Y, and W/H instead of W. Is one of these terminals the common or is it located inside the unit? Any further help in determing what terminal could/is used as the 24V common will be appreciated. I certainly don't want to have to run this new TH8320 solely on battery power.

I found the schematic for the wiring in the air handler and it most definitely shows a 24vac transformer along with all the heat relays etc. I'm just not good anymore at reading schematics especially old ones that don't matcg newer labelling.