I am trying to install a new Honeywell rth221 thermostat. I have three wires that are white, red and green. I must be installing them incorrectly because the heat does not seem to come on no matter how I change them. Please help.

White to W
Red to Rh
Green to G

You may have popped the fuse in the furnace replacing the thermostat witout turning off the power first. KISS is correct in his recommendation.

OOPs, forgot about that one (popping the fuse).