Keep your furnace running efficiently and safely and prevent the hassle of breakdowns with a few simple maintenance procedures. We'll walk you through a series of simple steps that will keep it in tiptop shape. The entire maintenance operation takes less than three hours and costs only a few dollars
By the DIY experts of The Family Handyman Magazine
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Furnace maintenance basics: Natural gas heating system service
Photo 1: Remove the combustion chamber door
Flip the electrical power switch to Off. Remove the combustion chamber door by lifting up and pulling it out, and remove the burner cover (if you have one). It’s usually held in place by two screws.
Photo 2: Inspect the burner flames
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.
Photo 3: Vacuum the burners
Turn off the power switch again and shut off the gas by giving the valve one-quarter turn (see Fig. A. for approximate gas shutoff valve location). 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 (see Symptom 5 in “Symptoms that call for a heating pro” below). Lift off the lower door (blower door) and vacuum the blower compartment.
Photo 4: Remove the blower
Remove the blower (also called a squirrel cage) in order to clean it. If you have a control panel in front of the blower, two screws will loosen it and you can let it hang. Next, using a 7/16-in. socket and ratchet, remove the two bolts that hold the blower in place, then gently lift it out.
Photo 5: Clean the blower
Clean the blower blades thoroughly with a vacuum and small brush. Take care not to stress the wiring or disturb the counterweights that will be on the fan blades. If you can’t clean the blower thoroughly, don’t clean it at all; you could throw it off balance.
Photo 6: Change the furnace filter
Change the furnace filter every one to three months. A cheap 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.
Photo 7: Clean the pilot
Blow dust off the pilot. Direct air to the exact spot by blowing through a drinking straw. A dirty pilot can cause the flame sensor (or thermocouple) to get a false reading that the pilot isn’t lit. Some newer furnaces have hot surface igniters instead of pilots and electronic igniters (Photo 9). (Note: One burner was removed for clarity.)
Photo 8: Clean the flame sensor
The flame sensor occasionally becomes coated with residue and will prevent your furnace from lighting. Remove it by pulling it down out of its bracket. Lightly clean the surface with fine emery cloth and slip the sensor back into its bracket.
Photo 9: Clean hot surface igniters
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.
When it comes to furnaces, an ounce of prevention truly is worth a pound of cure. To help you avert the hassle of your furnace failing or simply not putting out enough heat—just when you need it most—we’ll walk you through a series of simple steps that will keep it in tiptop shape. The entire maintenance operation takes less than three hours and costs only a few dollars—pretty cheap insurance.
In this article, we’ll focus on natural gas and propane-fueled furnaces. The maintenance tasks involving the blower chamber also apply to oil furnaces; however, oil furnace combustion chambers are very different and should only be worked on by professionals. Heat pumps, on the other hand, work more like a central air conditioner than like a furnace, so we won’t deal with them here.
Routine furnace maintenance and cleaning don’t require special skills. If you’re handy with a few basic hand tools, you can do it. We won’t be doing tricky or potentially dangerous stuff like adjusting the gas burners. Leave that for a pro. See “Symptoms That Call for a Heating Professional,” for more details.
We should warn you that your furnace may look somewhat different than the one we show here. If you don’t feel confident about taking some of the steps we show, skip them. And pay close attention to the safety precautions in this article, in your furnace service manual (if you can find it!) and posted on your furnace. Even if you follow our maintenance steps, call in a heating professional for a thorough furnace checkup at least every three years.
Follow Photos 1-9 for a complete guide to furnace maintenance basics.
Tip: If you’re faithful about changing your filter, you won’t have to clean the blower (Photo 5).
While working on your furnace, do not remove burners, stick anything into the pilot orifice or make adjustments. Misaligned burners can pose a serious hazard by allowing gas to build up before the burner ignites, causing a flash fire. Poking a sharp object into a pilot can widen the orifice, turning the pilot into a flamethrower.
Figure A: Gas Furnace Details
A forced-air furnace has four main sections: (1) the blower chamber; (2) the combustion chamber; (3) the return duct; and (4) the supply duct. When your thermostat calls for heat, the burners will kick on and begin to heat up the heat exchanger. The heat exchanger contains all the dangerous gases produced by combustion and vents them through the exhaust stack. When the heat exchanger gets hot enough, the blower starts. The blower pulls cooled air through the return duct, passes it over the warm heat exchanger and returns the warmed air to the rooms. Furnaces vary quite a bit in design, so yours may be somewhat different from this illustration. If confused, consult your service manual or a heating professional.
Note: You can download and print Figure A from the Additional Information section below.
Video: 6 Tips for Furnace Troubleshooting
Natural gas heating system service other maintenance chores: belt drives and ducts
Photo 10: Test fan belts
The belts on belt-driven blowers need occasional adjustment or replacement. Inspect the drive belt for cracks or frayed areas. A new belt is inexpensive. When you install the new belt, tension it so it deflects 1/2 to 3/4 in.
Photo 11: Oil bearings
Some older furnaces have two motor bearings and two blower shaft bearings that require annual oiling. Clean the area around the oil caps, then remove the caps. Apply two to three drops of lightweight machine oil (like 3-In-One oil) and replace the caps. Don’t over lubricate!
Photo 12: Reset dampers
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.
Photo 13: Seal leaky ducts
Seal leaky ducts, especially return air ducts, with special metal tape (available at home centers) or high-temperature silicone. Then conduct the following 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 (see Fig. A, above). 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. Gently squeeze the exhaust stack with your hand. It should be firm but slightly flexible. Call a heating professional or plumber to fix all these types of problems.
Photo 14: Test your water heater for backdrafting
Test your gas water heater for backdrafting while your furnace is Off. Turn up the water heater thermostat until the water heater burner comes on. After a minute or more, hold a smoking stick of incense or match up to the exhaust stack. The smoke should be pulled into the stack. Conduct the test with all exterior doors and windows closed and bath and kitchen fans running. If the vent doesn’t draw, call in a heating specialist or plumber to find the problem. Turn the thermostat back down.
Some homes have older furnaces with belt-driven fans, which may need maintenance (Photos 10 and 11). Sometimes dampers in ducts need seasonal adjustment (Photo 12) or air sealing (Photo 13). And make sure your furnace doesn’t cause your gas water heater to backdraft (Photo 14).
Tip: If your furnace has a standing pilot (a pilot that burns all the time), turning off the gas to the furnace when the heating season is over will save you as much as 5 percent per year on your gas bill. To relight the pilot, consult the instructions on your furnace’s gas valve.
Symptoms That Call for a Heating Professional
Symptom 1: Short cycling
When your furnace runs for only short periods (less than three minutes) before shutting off, the problem is called short cycling. This happens when the thermostat is out of adjustment or when the heat exchanger overheats and the burner automatically shuts off to prevent damage.
Symptom 2: Irregular flame
Properly functioning burners have fairly even rows of flames. If the flames are uneven or lean toward the back of the furnace, call in a pro. It could be a sign of dirty burners or a cracked heat exchanger.
Symptom 3: Odd noises or rumbling
While rumbling and popping aren’t cause for concern in a hot water or steam heating system, they shouldn’t be present if you have forced-air heat.
Symptom 4: Chronic illness
Frequent headaches or flu-like symptoms can be a sign of combustion gases leaking from a cracked heat exchanger or carbon monoxide leaking from an exhaust stack. With these symptoms, have your heating system checked out even if your carbon monoxide alarm remains silent.
Symptom 5: Soot deposits
Soot is a fine black powder that collects when combustion is incomplete. Its presence may indicate that your burners need adjusting or that you have a cracked heat exchanger that needs replacing.
Carbon Monoxide Alarm
Install a carbon monoxide alarm on each floor. If you already have these alarms, test them. Carbon monoxide is an odorless, colorless gas sometimes produced by oil-, gas- and wood-burning appliances (furnaces, stoves, fireplaces, etc.). If this gas spills into your home in high enough concentrations, it can be fatal. Plug carbon monoxide alarms into electrical outlets or directly wire them to the electrical system. Do not install them in utility rooms, garages, kitchens or bathrooms.