Turn the power switch on and activate the burners by turning up your thermostat. Inspect the burner flames. The flames should be fairly even and blue. Yellow flames indicate dirty burners. (Don’t breathe on the flames because the extra oxygen will also make them turn yellow.) Don’t adjust the burners yourself. Call in a pro.
Turn off the power switch again and shut off the gas by giving the valve one-quarter turn. Vacuum the burners and the furnace base. To get at the back of the burners, tape a 20-in. length of 1/2-in. drain line to your vacuum hose. Vacuum everywhere you see dust. While everything is open, use a flashlight to look for signs of soot (fine black powder), which often indicates poor combustion. Lift off the lower door (blower door) and vacuum the blower compartment.
Change the furnace filter every one to three months. A $1 fiberglass filter will adequately protect the blower and blower motor. If you want to install a more expensive, high-efficiency filter, check the owner’s manual for the manufacturer recommendations. High-efficiency filters can restrict the airflow, strain the blower motor and make your furnace less efficient. If you want cleaner air, the best option is a separate air-cleaning system.
Hot surface igniters are the most common ignition system on furnaces being manufactured today. They take the place of standing pilot lights and electronic igniters. Clean the dust off the hot surface igniter by leaving the igniter in place and blowing air through a straw. This part breaks very easily; don’t even touch it. In fact, when you replace the furnace doors, do so gently to avoid breaking the igniter.
If your furnace heating ducts also serve as air conditioning ducts, they may have dampers that require adjusting for seasonal changes. The seasonal settings should be marked. Two-story homes often have separate supply trunks to serve the upstairs and downstairs. To send more warm air downstairs (winter setting) or more cold air upstairs (summer setting), adjust the damper handle on each supply trunk.
Seal leaky ducts, especially return air ducts, with special metal tape or high temperature silicone. Then conduct a backdrafting test to make sure the combustion gases go up the flue: Adjust the thermostat so the burners come on. Hold a smoking stick of incense beside the draft hood. The smoke should be drawn into the hood. Also inspect the exhaust vent pipes on your furnace and water heater (while they’re cool). White powdery residue can indicate corrosion. Call in a pro to fix these problems.