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Solution 1: Check the thermostat to make sure it's on

A furnace can be intimidating—especially when it's not working. However, there is good news from furnace repair pros. Roughly a quarter of all service calls could be avoided with easy fixes that cost little or nothing. In this article, we'll focus on the common culprits and show you what to do about them.

Before you assume you have a furnace problem, check the thermostat to make sure it’s actually telling the furnace to come on. Thermostats, especially programmable ones, cann be complicated, and the more options a thermostat has, the more that can go wrong.

  • Make sure the switch is on “Heat” rather than on “Cool.”
  • Check the temperature setting.
  • Compare the temperature setting to the room temperature. Set the temperature five degrees higher than the room temperature and see if the furnace kicks on.
  • Make sure the program is displaying the right day and time, as well as a.m. and p.m. settings.
  • Trace the thermostat wires back to the furnace to check for breaks, especially if you've done any remodeling recently. If you find a break in one of the thin wires, splice the line back together and wrap it with electrical tape.
  • Replace the battery. If you have a power outage with a dead battery, you'll lose your settings and the thermostat will revert to the default program.
  • Open the thermostat and gently blow out any dust or debris. Make sure it's level and firmly attached on the wall, and that none of the wires coming into it are loose.
  • If you can't make the program settings work, you can bypass them altogether. Simply punch in the temperature you want with the up/down control and then press the hold button. That will switch on the furnace if the thermostat programming is the problem.
Tip: Lost your owner's manual? Most major-brand manuals are on the Web—just go to the manufacturer's Web site.
Furnace trouble spots

Furnace trouble spots

High efficiency furnace trouble spots

High efficiency furnace trouble spots

Figure A: 8 Things to Check Before You Call a Repair Service

You can check and correct all eight items in just a few minutes. We show a gas-fired, forced-air furnace here, but most of the same checks apply to electric systems and hot water boilers.

Note: You can download Figure A and enlarge it in Additional Information below.

Solution 2: Check shutoff switches and breakers

It sounds unbelievable, but furnace technicians often find that the only “repair” a furnace needs is to be turned on. Look for a standard wall switch on or near the furnace—all furnaces, no matter what age or type, have one somewhere. Check the circuit breaker or fuse for the furnace as well. Make sure the front panel covering the blower motor is securely fastened—there's a push-in switch under it that must be fully depressed for the furnace to operate.

Solution 3: Change filters

Dirty filters are the most common cause of furnace problems. Dust and dirt restrict airflow—and if the filter gets too clogged, the heat exchanger will overheat and shut off too quickly, and your house won't warm up. If the blower is running but no heat is coming out, replace the filter. A dirty filter also causes soot buildup on the heat exchanger, reducing the efficiency of the furnace and shortening its life.

The owner's manual shows where the filter is and how to remove it. Change inexpensive flat filters at least once a month. Make sure that the arrow points toward the furnace. Inspect pleated filters once a month. Hold them up to the light and if you can't see the light clearly through them, replace them. Manufacturers say pleated filters are good for three months, but change them more frequently if you have pets, kids or generate lots of dust.

Caution!

Always turn off the shutoff switch (see Solution 2) and turn the thermostat off or all the way down before changing the filter or working on the thermostat or furnace.

Solution 4: Make sure the gas is on

Just as with switches, someone may have turned off a gas valve and then forgotten to turn it back on. Trace the gas line back from the furnace to the meter, and if you see a handle that's perpendicular to the gas pipe, turn it so it's parallel. If you have an old furnace or boiler, you may have a pilot light. Remove the front panel and the burner cover and check to make sure it's lit.

Solution 5: Make sure the chimney exhaust flue is clear

Drawn by the warmth, birds sometimes fall into the chimney exhaust flue. Turn the furnace off and the thermostat all the way down, then dismantle the duct where it exits the furnace and check for debris. Be sure to reassemble the sections in the same order and direction that you took them out.

Solution 6: Flush out drain lines

High-efficiency furnaces can drain off several gallons of water a day in heating season. If the drain lines become restricted by sediment or mold growth, the furnace will shut down. If the drain hose looks dirty, remove the hose, fill it with a mixture of bleach and water (25 percent bleach), then flush it after several minutes.

Solution 7: Look for blocked or leaky ducts that can restrict airflow

If your furnace comes on but one or two rooms are cold, first make sure all the room registers are open. Then examine any ductwork you can get access to and look for gaps between sections or branching points. Seal any gaps between sections of duct with special metal duct tape. Don't use standard cloth duct tape—it quickly deteriorates, and it may also cause ducts to leak if it was used to seal sections in the past.

Also check for handles protruding from the ductwork. These are dampers or air conditioner bypasses—make sure they're open.

Solution 8: Clean away leaves and debris from heat pumps or intake and exhaust vents.

If you have a furnace that vents out the side of the house, make sure nothing is blocking the intake or exhaust. If either of the pipes is covered with screen mesh (like window screen), replace it with 1/2-in.-mesh hardware cloth. If ice is clogging one of the pipes, you have a bigger problem somewhere in the system. Clear it off and call a technician to find out why it's happening.

If you have a heat pump, clear away grass and leaves from the fins of the outdoor compressor unit. Before heating season starts, hose it down gently from the top to rinse dirt and debris out of the housing.

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