Honeywell makes a wide variety of thermostats for controlling the heating and cooling of buildings. Most modern thermostats are designed for modern heating and cooling systems and therefore require three, four, five and sometimes more wires. Still, many furnaces and air conditioners can be controlled with a basic two-wire thermostat and because hundreds of thousands of homes were originally wired for their use, Honeywell continues to sell these units. The Honeywell CT30 and CT33 low-voltage thermostats will replace a wide range of two wire thermostats.
Turn off the power to the heating and cooling system by removing the fuse or switching off the circuit breaker in the service panel.
Remove the cover plate of the old thermostat.
Remove the mounting screws that hold the thermostat base to the wall.
Remove the thermostat base from the wall while reaching behind it and gently pulling the wires in the same direction.
Stop if any wires are connected to terminals labeled Y, Y1, M, G, F, Rc. This thermostat will not work for systems using these wire designations.
Disconnect and remove the wires from the old thermostat. Wrap a piece of tape around each wire and label them with the letter and/or number that corresponds to the terminals they were removed from.
Connect the wire labeled R, RH, 4, or V to the "R" terminal on the back of the new thermostat base.
Connect the wire labeled W, W1, or H to the "W" terminal on the back of the new thermostat base.
Attach the new thermostat base to the wall while feeding the wires back into the hole.
Snap on the thermostat cover.
Restore the power.