How to Use Two Wires for Low Voltage on a Honeywell RTH221B Thermostat

Loosen the locking screw at the bottom of the new thermostat. Take it apart by lifting the face from the bottom, removing it from the wall plate or base. Familiarize yourself with the new thermostat's five terminals, which are labeled Y, G, Rh, Rc and W. A jumper ties Rh and Rc together; keep the jumper connected.

Turn off electric power to the furnace or air conditioner by switching off the associated circuit breaker. Check whether or not the electricity is off by activating the home's current thermostat. If the thermostat works, then the correct circuit breaker wasn't shut off.

Remove the home's current, or old, thermostat without disconnecting the wires. Check whether or not the wires are clean. If they are corroded, pull out fresh wire and cut the corroded portion with wire strippers. Strip 1/4 inch of insulation off each wire.

Find the label of one wire of the old thermostat, and write down its letter designation and how the wire is hooked up. Tape the label to the wire about 1 inch from its tip. Remove the wire, and wrap it around a pencil so you will not lose it in the wall. Use the same procedure for the old thermostat's second wire, including writing its letter designation and how it is connected.

Mount the new Honeywell RTH221B thermostat to the wall with the screws that were provided with the device. The new thermostat has five terminals, but only two will be used. Match the letter of the wire labeled Y or W to the corresponding slot on the wall plate, and attach the wire to that slot by loosening the screw and sliding 1/4 inch of the wire's tip into the slot. Tighten the screw, and the wire is clamped. Do the same for the second wire, choosing either Rh or Rc.

Determine the unit that the thermostat will control. Match Y for air conditioning or W for heating, and attach it to the appropriate slot of the terminal of the new thermostat. This step is applicable if the letter in the old thermostat does not match or the label is unreadable. Attach the second wire coming from the wall to the new thermostat choosing either Rh or Rc.

Insert two AAA batteries in the thermostat, and mount the thermostat's face to the wall plate. Tighten the screw at the bottom of the thermostat, and turn on the electric power to the furnace or air conditioning system.