DO-IT-YOURSELF : Repairing Thermostat Is No Sweat, but Be Sure to Shut Off the Power

Thermostats that control home heating and cooking systems seldom break, but when they do, most are easy to diagnose and fix. Even replacing a broken or outmoded thermostat usually is not too difficult.

If a furnace or central air conditioner fails to start, try turning the thermostat’s setting up (on a heating system) or down (on a cooling system) about 10 degrees. Doing so should start the unit. If not, go to the main service panel or fuse box and check for a tripped circuit breaker or blown fuse.

Reset the breaker or replace the fuse if necessary and try again. If the system still does not start, check that the furnace or air conditioner itself has not shut down because of a malfunction. After eliminating these possibilities, suspect a faulty thermostat.

Virtually all room thermostats operate on low voltage that carries little risk of harmful electric shock. However, it is always wise to shut off power to a thermostat at the service panel before examining or working on it.

Don’t tamper with a thermostat that controls an electric baseboard heater; these usually carry full-strength household current and can deliver a fatal shock. Have the unit inspected by a heating technician or an electrician instead.

*

To work on a thermostat, remove the cover and blow away any dust clinging to the inner parts. Look for small metal buttons called contacts that spring together when the thermostat works, completing a circuit that starts the system.

Not all thermostats have contacts. On those that do, adjust the thermostat setting to close them, then slide a coarse paper between the contacts to clean them. Do not use sandpaper or other abrasive material.

Also check for broken parts -- especially the coiled bimetallic spring that is the heart of most thermostats. If you find broken parts, replace the thermostat. Refasten loose wires by wrapping them clockwise around their terminal screws and tightening the screws firmly. Sometimes the screws are on the back of a thermostat plate. Remove the plate to reach the screws.

Burned wires can signal an electrical or fire hazard. Have any such condition inspected by a heating technician or an electrician immediately.

Restore the power and try to start the system again with the thermostat. If nothing happens, bare the ends of a spare piece of any insulated copper electrical wire; then, while holding the wire by the insulated portion, touch the wire ends to the terminals on the thermostat marked “R” and “W.” If the heating or cooking system starts, the thermostat is defective; replace it.

None of the above tests can be made on newer electronic thermostats, which have printed circuit boards. If these fail, consult the owner’s manual and try installing new batteries. Replace the thermostat if the problem remains.

To replace a thermostat, first shut off the power at the service panel. Remove the cover and plate, and then label the wires with pieces of masking tape identifying their terminals.

Unfasten the wires. Unscrew the thermostat from the wall and lift it away, being careful not to let any wires slip into the wall where they will be lost.

Thread the wires through the rear of the new thermostat and attach the thermostat to the wall, using a level to make sure it is horizontal.

Connect the wires by following the labels and the directions supplied with the thermostat. Attach the front plate and any batteries; then attach the cover, restore power and test the thermostat by using it to turn on the system. Finish by programming the thermostat or setting it as required.