Is there a way to test for the existence of a common wire at the thermostat using a multimeter, or does it have be tested at the furnace itself?
The system is pretty old (heat only); the current thermostat uses only 2 wires. I found a third wire in the wall that wasn’t connected to anything and was wondering if I am lucky and this is a common. Is there a way to tell just by using a multimeter, without going down to the furnace and mucking around in there? (it’s a cramped space, not fun to work in)