Not enough information really.

Red is the +24
White is typically Heat (connected to Red to get heat)
Green is typically Fan (connected to Red to blow fan)

Blue is not really a standard color.

We need to know the brand/model furnace.

You'd probably do better to ask this in the Heating & Air Conditioning forum.

This post was edited by ronnatalie on Thu, Jan 29, 15 at 18:53

Black is usually what will power the digital thermostat, otherwise the thermostat would run on a battery.

At least that is how mine operates.

Black is the return and frequently isn't present. If it is there, yes you can power the thermostat from the red/black lines. The thermostat actually doesn't need black to work. Your traditional mechanical thermostat will just close red and white together to call for heat and close red and green to get the fan.

For this reason many of the digital ones as you point out have a battery.

1. Red wire is ~24VAC
2. When testing voltage set your multi-tester to AC, if you select DC you won't get a reading.
3. Go to your thermostat website and download a manual so you can wire it correctly.

http://www.electrical-online.com/thermostat-wiring-explained/

This post was edited by moviegeek on Mon, Feb 2, 15 at 21:29