Hi All,

I'm trying to replace my old mercury thermostat (from 1979 it seems) with a new 4+1+1 thermostat. I took off the old 'stat and I had 3 wires connected to it brown, green(maybe grey), and black. The brown and green wires were connected to the same terminal while the black one was connected to the other terminal on the old thermostat. My furnace is an old gas furnace with fan I guess. The instructions for the new thermostat says to connect power to R/RC, heater relay to W, and fan relay to G. I assume my green wire is fan relay, but how do I tell which of the black/brown wires is power or heater relay?

TIA.