Please help me wire my thermostat!!

[m1chael57]

I have two thermostats - one for heat and one for A/C and would like to eliminate one. I'd like to keep the Honeywell T8112 which runs my A/C but is designed to run A/C and heat. There currently are four wires connected to it: red, white, yellow and green. On the other wall, I have an old dial-type thermostat running my baseboard hot water heat. It has only two wires: red and a white. My question is...can the T8112 run both? I'm guessing it can. If so, where do I connect the red and white for the heat? The T8112 has two screws marked "R" and obviously only one is taken. I guess the red from the heat goes there. But where the heck does that white wire for the heat go?

[Phelps]

Out of curiosity: If you have hot water baseboard heat, what kind of a/c system do you have?

[m1chael57]

It's a Trane XL 1200 installed back in 1999.

[Phelps]

I'm a general practioner who knows theory of operation, and models don't really help me. What I really wanted to know is how such a unit runs without a forced air furnace. How does the a/c blow inside the house, for example? Was there ducting run just for the a/c to blow out of?

[m1chael57]

Hey Phelps. Thanks for the inquiries. No one else seems interested in helping. They are two seperate units. The hot water, baseboard heat came with the house. Several years ago, I had the A/C installed. The unit is in the attic. 2nd floor ductwork was easily installed...obviously. Additional ducting runs through the 2nd floor closets to the first floor.

I don't know why they didn't combine systems on one thermostat - probably didn't want to be liable for any potential problems with the heat down the road. I'm refinishing the hallway where both thermostats coexist on adjacent walls and want to combibe them.

[suprheat]

Where does the white wire in 4 wire bundle hook to? Usually, only 3 of the 4 wires are used in situations like yours. In your type of installation the normal hook up is: 4-wire bundle - Red to Rc, Yellow to Y, Green to G, and White is a spare. And for the 2-wire bundle - Red to Rh and White to W. Make sure there isn't a jumper wire between Rh and Rc. Power to both systems should be shut off while you do this. But I'm curious about that White wire.