I am installing a new thermostat (Honeywell model no. RTH230B) and need help understanding how to wire it. Upon removing my old thermostat (Honeywell model no. CT87A), I have four wires coming out of the wall: Red, black, green, and yellow. I don't remember how they were connected to the old thermostat, but there are 3 spots labeled as R, W, and Y. The new thermostat has the following connectors: Rh, Rc, W, Y, and G. There is also a jumper between Rh and Rc that can be removed. My heating system is a gas-fed hot water baseboard Laars Endurance. I only use the thermostat to regulate heat. Help is greatly appreciated!! Thanks!

Old thermostat

http://customer.honeywell.com/techlit/pdf/69-0000s/69-0274.pdf

New thermostat

http://customer.honeywell.com/techlit/pdf/PackedLit/69-1800.pdf

For heat only you use RH for red wire and W for White wire. You can leave the RC jumped since it makes NO difference.

Now what your other two wires are for I have no idea unless there was a sub-base under the old thermostat. You said you have no cooling so the Y=yellow wire would not be used and the G-green wire is for the blower in a forced air system so you do not have that also.

Now this is taking into consideration no sub-base existed and your wiring is common standards.

Make sure you do all the setup's as described in the manual you have or is posted here.