I'm replacing an old thermostat with a new Honeywell RTH6300B programmable. I have propane heat and central AC. My old thermostat has a black, a red and a yellow leads with no ID on the old thermostat wall plate. How should I wire it up to the new thermostat terminal block? Thanks in advance.

Sad to say but you have bought an non compatible thermostat, what you need is a thermostat that has "Heat Pump" somewhere on the package, Honeywell makes those also in the same models, sometimes they can be tricky if your not looking. It should also say 2heat, 1cool too. Hope this helps and good luck!

treat the blue as if it were yellow

ok to start with if you have a heat pump and not ac over electric heat depending on type but most will be wired as
red to r jumped to rc y to yellow this turns on compressor g to green this turns on fan w to white or brown for electric heat strip or em heat o to orange reversing valve energised in cooling switches from heat to cool or orange to b energised in heating c blue for common
you will have to look at your indoor unit and out door unit to verify what collors the contractor used when they wired it up hope this helps

W- White (heat)
Y- Yellow (condenser)
G- Green (Indoor fan)
C- Blue (Common 24v)
R- Red (24VAC)
O/B (Only needed if system is a heat pump)

you have to configure the thermostat to run the fan when there is a call for heat. look at the installation instructions and they shoud ask you if it is gas or electric heat if its electric then you are gong to have to configure it to makr e the fan come on with the heat. Good luck

Check to see if there is a short between the green wire and the red wire. Green is the fan control wire for AC. Sounds like some wires may be touching. How did the old t-stat get fried?

check to see if the B terminal is on the new t-stat if connect to it , but this terminal refers to heat pumps and is normally use to energize the reversing valve on the outside unit and not used on ac units , but normally the tag O/B is the same terminal on therostats

what kind of system are you hooking it to? heat pump or heat/ac?

the analog unit uses just two wires for heat. red - white. there is a heat anticipator setting that must match the burner current. check your programmable specs and follow instructions.

Is sounds like you wired it correctly, The black is a little unusual perhaps its blue, anyway, try disconnecting the wires from the thermostat and see if it shuts off. If so, then it may need to re calibrated. I will need some more info to help you anymore.