Maintaining an Electric Resistance Furnace

To envision an electric-resistance furnace, think of a giant toaster with a fan blowing through it. As air pushed by the blower moves through the heating elements, it picks up warmth, then continues into the plenum and ducts to registers in each room. Because no combustion occurs in an electric furnace, it doesn’t require the flue or heat exchanger needed for gas and oil furnaces. Therefore maintenance is almost nil. Operating costs, however, usually run substantially higher for electric units.

Check the anatomy drawing at right. An air-circulation switch, often on the house thermostat but sometimes on the furnace, lets you run the blower continuously. Underneath, accessible through a removable cover, there may be fuses or breakers for each of the heating elements. A transformer steps up amperage to the high levels needed for heating. Relays turn the elements on or off according to signals from the thermostat. The chart shows things that can go wrong with electric-resistance furnaces and what you can do about them. Always shut off the furnace’s main circuit breaker before removing the control or access panels. You’ll find the breaker located next to the furnace or in your home’s main service panel. Don’t attempt to work on the heat elements— that’s a job for a professional.

Troubleshooting an Electric Furnace




No heat

Furnace switch or main breaker open; thermostat set too low

Check the switch, the fuse or breakers, and the thermostat. If the blower runs but there’s no heat, check the fuse or breaker block.

Cycles on and off too often

Clogged filter or failing blower, causing unit to overheat

Replace the filter. Oil and adjust the blower.

Not enough heat

Improper thermostat setting; defective heating element; clogged filter or duct

Check the thermostat, then the fuse or breaker; replace a blown fuse or flip on the breaker. If the fuse blows or the breaker trips when you turn on the power, call a professional. Replace the filter.

Heat pump. If your home has a heat pump, it may have resistance-type duct heaters, like the one shown here. These mini-furnaces automatically pitch in when the temperature drops below what a heat pump can handle by itself.

Understanding a Gas Furnace

Burners in a gas furnace are simple, but the controls needed to automatically turn the burners on and off and to provide safety are more complex. A set of tubes called a manifold feeds the burners a mixture of gas and air, which is ignited by the pilot. The blower pushes air through the heat exchanger and up to the plenum. Flue gases exit past the exchanger up the flue and chimney. For safety, a combination valve is connected to the pilot. If the pilot goes out, the valve shuts off the gas. A second safety—the limit switch in the plenum—turns off the gas if the plenum gets too hot; it also stops the blower when the temperature in the plenum drops to a certain level after the burners have shut down.

Checking the Flue and Chimney

Flues can develop leaks, releasing highly lethal carbon monoxide into your home’s air. The same can happen if the chimney becomes partially clogged with debris or animal nests. These dangers warrant an annual checkup. Inspect all the points illustrated at far right, paying special attention to the pipe between the furnace flue and the chimney. For more about chimneys and flues.

For no charge, the gas company will probably inspect your flue and test for carbon monoxide. If they find a problem, they may shut off your furnace. Ask for an exact description of the problem, as well as the steps you should take to make your flue safe. If the problem cannot be solved by a simple step like sealing joints in the flue, call in a professional.

Test the draft. Hold a candle to the opening in the top access panel with the furnace running. A flue leak will blow out the flame.

Possible sources of leaks. Any of these problems may cause a carbon monoxide leak. If you suspect a damaged flue pipe, replace the pipe. An improperly installed flue pipe can also be the culprit. The flue must be the correct size; the slope and number of bends of the pipe are also factors.

Lighting a Pilot, Replacing a Thermocouple

Some gas furnaces, boilers, and water heaters ignite with an electrical spark system. Most, however, depend on a gas pilot light for firing. When the unit fails to operate, it’s usually because the pilot has gone out; you may need to replace the thermocouple or make adjustments. To relight a pilot, follow the steps on the instruction plate attached to the furnace. With most, you’ll find a gas cock with three settings—off, pilot, and on. Turn the cock to “off,” wait a few minutes for residual gas to clear, then switch to the “pilot” setting. Hold a long lighted match or stick to the pilot, depress a reset button, and hold it down for a minute.

If the pilot stays lit, turn the cock to “on” and the burners will fire. If the pilot goes out again after you release the reset, repeat the entire procedure, holding the reset down a little longer. If you can’t get the pilot lighted after two or three tries, most likely you’ll need to replace the thermocouple, a copper tube with a bulbous end that gets heated by the pilot flame.

To replace a thermocouple, shut off the gas inlet valve and remove the access cover so you can see the burners. Disconnect the thermocouple from the control by unscrewing a hold-down nut. At the other end, it may be held in place with a nut or it may simply pull out. Purchase two replacements of the same length (so you can have one on hand for the next replacement). When you install the replacement, take care not to kink the tube and make sure the bulbous end is touched by the pilot flame. A weak or wavering pilot flame also may shut everything down. The flame should be blue, with the tip barely flecked by yellow. If the pilot flame doesn’t fit this description, make sure that it isn’t being buffeted by a draft. Then try cleaning out the opening of the pilot tube with a needle or nail.

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