Hi, I bought a Honeywell programmable thermostat to replace my old thermostat that controls both hotwater furnace and central AC. There are 5 wires now connected to the old thermostat, red, black, yellow , green and white. In my new Honeywell thermostat, there are also five slots as follow:
Rh: heating power supply
Rc: colling power supply
W: heating signal
Y: cooling signal
G: Fan

I assume I can connect the W, Y, G to the wires with the corresponding color, but what about the Rh and Rc, which is for red and which for black? Thanks for your help.

The red wire should connect to RC and be jumpered to RH. I am not sure about the black. Some times there is a blue common wire connected to a B or C terminal. You might check at the furnace and see what the black does there. I don't quite understand how the programmable thermostats work without a common, but many do.

Dear Labman

Thank you for your reply. I was a bit absent-minded (and the light was dim) when I cheked the color of the wires. Now I realized that the "black" wire that I saw on my old thermostat is actually blue. Also, the intallation manual of my new thermostat mentioned that "the jumper wire between the Rc and Rh terminal must be removed in 2- or 5-wire installation", which seems different from what you described. Could you let me know why? Thanks again.

I don't quite understand why the thermostats have both an RC and an RH terminals and usually you jump them. In some installations, there is more than one transformer, and it may be to power the thermostat with one and switch the other for heat. Wire yours up according to its directions. If it doesn't work that way, post back with the details.

I have just installed the same thermostat. I have Rh/RC jumped and then W, Y, G. There are a Blue and an Orange remaining. Whenever I set the machine to autocool OR autoheat it blows hot air. But if I set the a/c to off anf the fan to on it blows cool, or at least not hot...

I'm at a loss, what if I un jumoper Rh/Rc and use the blue for Rc?

With Heat/Auto on and the temp set to 85, the heater turns on.
With Heat/Auto on and the temp set to 60, the cooler turns on.

With Cool/Auto on and the temp set to 60, only the fan turns on...

Does that help narrowing down at all?

Aruuuuuuuuugh. You have an outside compressor unit? It should have 2 wires connected to it running from the Y and B terminals at the furnace. On decent A/C systems, they are blue and yellow, but any color, or colors will work. If the thermostat is working right, the cooling should never come on when the thermostat is set to heat.

The old mercury thermostats had the red wire connected to the common of a double throw switch. With the heat/cool switch in the heat position, below the set temperature the contact connected to the W terminal was connected to the red terminal. In cool, above the set temperature, the red was connected to the yellow. One contact of the mercury switch was always connected to either the white or yellow. However the heat/cool switch broke one circuit, only allowing heating or cooling. The new thermostats do the same thing, only electronically, not a tube where mercury runs from one end to another depending on the temperature. Your thermostat could be bad, but seldom are out of the box. Keep poking around and let me know what you find. Easy problems I can fix in one post. Others, the best I can do is explain how things work and help people figure out the problem. Nobody else seems to have many answers either. Since you can get heat, I guess we can keep trying some more before calling in a professional.

I loved the one question. I did little more than suggest the furnace had a bad circuit board and how to take it out. The guy took it to work, found a little capacitor that was bad, replaced it, and his furnace worked fine. Great victory for DIY.