Thermostat Replacement Help

[redpoint5]

PGE offered free/cheap Nest thermostats for customers, so I got one for my grandmother's house.

The thermostat is an old mercury Honeywell q674f that has a bunch of wires I'm not familiar with.

The Google installation guide doesn't list some of the wires (X, O, L) and says the Nest might not be compatible, but I don't know why it wouldn't be.

It's a single fuel (electric) heat pump, so it should be pretty simple. Signal for fan, signal for heat, signal for cool.

You think I can replace this mechanical with the Nest? Which wires go where, and which don't I need anymore?

[Piotrsko]

Match the colors or the letters. You have two whites and greens however. Should be hookup data for the old system somewhere on gaggle unless someone saved the install package.

Can't be that hard, all the HVAC people I know understand zilch about electronics

Btw: wasn't difficult to build a timer thermostat for my pellet stove even though the pellet people said it couldn't be done.