We purchased a older home with boiler heat. There old thermostat was not connected. There are 3 wires: white, red and black. I purchased a Honeywell RTH2300 model and connected the wires to Black - B, white - W, Red - R. The Booiler stays on and the room is now 81 despite have the thermostat set at 65 HOLD. The oly way to control the heat is to switch off the heat switch on the thermostat. My guess is the back wire must go into one of the other colored Jumpers or does the black wire not get used?

Go to the other end, at the boiler, what is the black wire connected to?

I will check tonight when I'm home. I really appreciate the help.

I will check tonight when I'm home. I really appreciate the help.

I disconnected the black wire and all seems to work! I could not see the black wire connecting at the boiler.

Good deal, I was wondering what the black wire was used for, its maybe grounded somewhere,, its good it didn`t fry your tstat

Well I spoke too soon. I just found out the temp is down to 51 degrees (outside temp is 28). Theromstat is set to hold at 65. Thermostat says heat is on but nothing is happening. The black wire must go to a valve or something. Any suggestions?