Hi, We're replacing our 1970's mercury thermostat with a modern programmable. The existing wire is 2 conductor, and has 24vAC (I confirmed with voltmeter ). We replaced the wire with a 7conductor-18AWG "thermostat" wire, and wil l properly dispose of the mercury t-stat.
I know we need to connect to the two original wire terminals on the furnace (with 24vAC difference), but I understand these thermostats need _3_wires to work. This is for heat only, no AC (but maybe in future). I know one i s designated the "common wire" but it's confusing because if you short the
24vAC, the furnace comes on (of course, like with the old mercury tstat). So not sure where the 3rd wire is going to go, and instruction booklet is a bit confusing. Has anyone been through this process and can offer guidanc e? ThanksTheodore.