How may I replace a old thermostat if it does not have wire label, it has four wires ( red, green, orange and white. The new Thermostat model RTH221B Honeywell,

Thanks

All thermostats will have a letter designation beside the wire terminal, connect the new thermostat the same as the old one, normally the colors of the wire will match the letters, but this is not always the case, in your case the orange wire might be a substitute for the yellow "Y". When you see "RH" and "RC" connect the red wire to both of these with a jumper wire. Good luck, Mike

What type of system is it (heat pump,gas heat, oil heat etc) SINCE YOU SAID IT ONLY HAS 4 wires I take it for granted its not a heat pump which would need a minimum of 5 wires and 6 if the t stat does not have battries which could be used for common. Never the less Green wire is normally the fan which goes to G TERMINAL , WHITE is normally heat which goes to w-1 terminal,Yellow is normally a/c which would go to y terminal but someone could have substituted orange for yellow. And there should be ajumper going between rc and rh