Right now I have two separate thermostats, one obviously controlling the A/C (heat pump), and the other controlling a gas furnace. To save money, I've upgraded the heat thermostat to a programmable thermostat (using two wires, red and green).

I would love to have one thermostat controlling the whole house. I would love some idea of what is involved in combining two separate thermostats?

FYI - The thermostats are not close to each other, so pulling wires would be quite a bit of work.

I guess my big question is whether it's just a matter of combining all the wires in one thermostat? If it's much more involved, I can just add a second programmable thermostat.

Thanks in advance for your help!

Yes, just connect all the wires to one. But it must be one with the correct terminals. Perhaps you should have looked at wireless, programmable thermostats. To make the one you have work, It must be capable of the extra functions of a heat pump. Read its directions. If it doesn't explain how to hook up a heat pump and at least 2 stages of heat, it just won't work.

You should be able to instal a heat pump capable, programmable thermostat in place of the one controlling the heat pump now. It would be a matter of figuring out which of its wires to connect to the W terminal that activates the gas furnace. All the furnace controls would also need to be connected to the common from the transformer feeding the heat pump controls. If the heat pump isn't using the furnace blower as its air handler, you will need to run wire between the 2.

Study your system, and the directions for the thermostat. Post back for more details. Expect more help on hooking up the heat pump from the directions than from me.

If you like the furnace thermostat's location, pulling wires into interior walls isn't that much of a job if you have an unfinished area above or below.

labman, thanks for the help!

The new thermostat is one of the newer Honeywell models, and I'm almost certain it handles a heat pump.

The wireless idea sounds promising. I'm sure I could return the current themostat and go with a wireless model. I'll do some research, but I assume with a wireless model I would need two thermostats with one receiver? Or would it be the other way around?

Wireless would be easier than pulling wires.

You would have one thermostat, and would only need one receiver if you could locate it where you could easily run wires to the furnace, heat pump valves and outside unit. There should already be wires running between the heat pump valves and the outside unit.

Perhaps I was a little cryptic. A heat pump thermostat will have options for heat using at least 2 of W, W1, W2, and AUX. If it has AUX, that is the one to run to the W on the furnace.

Put RTH7400 + "heat pump" into the search box at the top of the page. It may find some threads with the answers you need. It could be as simple as moving the RTH7400 to the heat pump thermostat location.
Connecting an unused wire from the AUX terminal to the w terminal on the furnace.
Plus the common of the heat pump transformer to the common of the furnace.