I installed a programmable thermostat this morning. Simple enough, even for me: pull out the wires from their connections, and place into the same respective connections on the new thermostat.
Except the furnace hasn't turned on all day. When we pull the batteries, the thermostat display doesn't come up, indicating it's not getting power from the wiring. So we tried hot wiring and bypassing the thermostat by just direct connecting the red power wire to the white furnace wire. Nothing.
The fuse box has been checked repeatedly. Nothing.
A voltmeter shows power at the furnace switch in the basement... but mysteriously, only when the switch is in the off position. That one's a real head scratcher. Especially since the switch had been in the on position with the furnace running just fine, prior to swapping the thermostat.
So, we have power to the furnace when the switch is in the off position, but no power to the thermostat upstairs under any condition. I give up. Thank god for gas fireplaces.
I just realized something. When I went to turn off power to the thermostat at the fuse box, I turned off power to the hallway the thermostat is in... which would have done nothing. I didn't kill power to the furnace itself, from which I assume the thermostat derives its power. Which means I was playing with live wires. Sweet. Is it possible I blew out a fuse in the furnace?