Fall Furnace Maintenance Guide

Simple maintenance pays big dividends for comfort, efficiency and safety.

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


One day




Under $20

Furnace maintenance basics

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’s dying 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 (Look under “Heating” in your Yellow Pages.)

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).

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.

Forced-air furnace

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.

Video: 6 Tips for Furnace Troubleshooting

Other maintenance chores: belt drives and ducts

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.

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.

Carbon monoxide alarm
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.

Additional Information

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

  • Socket/ratchet set
  • 4-in-1 screwdriver
  • Shop vacuum

Small stiff brush, fine-grit emery cloth.

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

  • Furnace filter
  • 1/2-in. drain line, 2 ft.
  • Drinking straw
  • Lightweight machine oil
  • Metal tape

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