How to Maintain a Furnace


While some aspects of furnace upkeep can be complex and are best left to the professionals, there is a surprising amount of repairs you can do yourself.
While some aspects of furnace upkeep can be complex and are best left to the professionals, there is a surprising amount of repairs you can do yourself.
Comstock/Getty Images

Heating systems are usually trouble-free and easy to maintain. But efficient operation is a function of good regular maintenance. No matter what type of furnace you have, there are several things you can do to keep your heating system in top condition.

While some aspects of furnace upkeep can be complex and are best left to the professionals, there is a surprising amount of repairs you can do yourself. Keeping your furnace running properly can save you a lot of money on heating costs and helps avoid expensive repairs in the future.

In this article, we will tell you how to service and troubleshoot your furnace, including gas, oil and electric furnaces.

Before we get to the troubleshooting, let's start with some steps you can take before your furnace shuts down.

 

Cleaning a Furnace

The three parts of the furnace that should be cleaned: the filter system, the blower and the motor.
The three parts of the furnace that should be cleaned: the filter system, the blower and the motor.
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Dirt is the biggest enemy of your furnace. It can waste fuel and drastically lower efficiency. In some cases, it can even cause the furnace to overheat [source: StartRemodeling.com]. Dirt affects all three basic components of your furnace, so cleaning is the most important part of regular maintenance. The three parts of the furnace that should be cleaned: the filter system, the blower and the motor.

The furnace filter should be replaced or cleaned at the beginning of the heating season and about once a month during periods of continuous use [source: Peterson]. To check the filter, take it out and hold it up to the light. If it looks clogged, replace it with a new filter of the same type and size regardless of the length of time it has been used.

A disposable furnace filter consists of a fiber mesh in a cardboard frame. The size of the filter is printed on the edge of the frame. An arrow on the edge of the frame indicates the correct direction of airflow through the filter. Air flows from the return-air duct toward the blower, so the arrow on the filter should point away from the return-air duct and toward the blower.

A permanent filter is usually sprayed with a special filter-coating chemical, available at hardware stores and home centers. Clean this type of filter according to the manufacturer's instructions, which are usually attached to the furnace housing. Here's how to replace a filter:

Step 1: Look for a metal panel on the front of your furnace below the return-air duct, between duct and blower system. The panel may be marked "filter," or it may form a lid or front of a boxlike projection on your furnace housing.

Step 2: Slip the panel off its holding hooks, or unscrew the panel from the box or furnace housing. On some heating units, filters are exposed; in that case, just slip the filter up and out of the U-shape tracks that hold it in place.

Step 3: Inspect and replace or clean the filter, depending on type.

Step 4: Clean the blower assembly, belts and pulleys to the blower and motor housing. Cleaning the blower is critical if your furnace has a squirrel-cage fan, because openings in this type of blower often become clogged with dirt. To clean your blower, remove the panel that covers the filter to gain access to the blower or panel on the front of your furnace. This panel may be slip-fit on hooks or held in place by a series of retaining screws. Access to the inside of your furnace blower is usually gained by sliding out the fan unit, which is held on a track by screws.

Next, we'll look at troubleshooting a gas furnace.

How to Troubleshoot a Gas Furnace

Remember, there are times when it's wise to call a professional service person.
Remember, there are times when it's wise to call a professional service person.
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Natural gas and propane burn cleaner than fuel oil, and most gas furnaces present fewer operational difficulties than oil burners do. In fact, the problems that affect gas furnaces typically involve the furnace's thermocouple, the pilot light, or some component of the electrical system.

Gas furnaces and heaters have control shutoffs to prevent gas leaks, but they are not fail-safe. If you smell gas in your house, do not turn any lights on or off and do not try to shut off the gas leading to the furnace. Get out of the house, leaving the door open and immediately call the gas company or the fire department to report a leak. Do not re-enter your home.

On some gas furnaces and heaters, a plug-type door covers the pilot light assembly. To gain access to the pilot burner, pull the door out of the furnace housing. On other units, remove the panel that covers the pilot and gas burners.

The pilot light controls, reset buttons, gas valves and thermocouple are usually contained in an assembly at the front of the furnace. The furnace limit switch is located on the plenum (main chamber) or main duct junction on the upper housing of the furnace.

Next, let's take a look at the pilot light.

Gas Furnaces: The Pilot Light

To relight the pilot, follow the manufacturer's instructions exactly.
To relight the pilot, follow the manufacturer's instructions exactly.
Emanuele Taroni/Getty Images

The pilot light on a gas furnace can go out because of drafts. To relight the pilot, follow the manufacturer's instructions exactly; they're usually fastened to the furnace. If instructions for relighting the pilot are not provided, follow this general procedure:

Step 1: Find pilot light assembly. It typically has a gas valve with ON, OFF and PILOT settings.

Step 2: Turn the valve to the OFF position and wait three minutes.

Step 3: Switch valve to PILOT setting. Hold a lighted match to the pilot opening while you push the reset button on the pilot control panel. Keep this button depressed until the pilot flame burns brightly, then set the valve to the ON position.

Step 4: If the pilot flame won't stay lit, the opening may be clogged. Turn the gas valve OFF and clean the opening with piece of fine wire. If it won't stay lit after several attempts, you may have a faulty thermocouple. If the pilot flame still won't stay lit, call a professional service person.

Some furnaces have an electrical system to ignite the gas; in these systems there is no pilot light. Instead, an electric element heats up and ignites the burners. If this electric ignition system malfunctions, call a professional service person.

Now, let's check the thermocouple and limit switch.

Gas Furnaces: Thermocouple and Limit Switch

If the blower runs continuously, either the blower control on the thermostat has been set to the ON position or the limit control switch needs adjustment.
If the blower runs continuously, either the blower control on the thermostat has been set to the ON position or the limit control switch needs adjustment.
Tom Grill/Getty Images

The thermocouple is a gas furnace component located near the pilot light burner. It is a safety device that shuts off the gas if the pilot light goes out or the electric igniter fails.

If the pilot light won't stay lit, the thermocouple may be faulty and should be adjusted or replaced. To adjust the thermocouple, you must tighten the thermocouple nut with a wrench. Take care not to apply too much pressure to the nut -- just tighten it slightly. Then try lighting the pilot. If the pilot won't stay lit, replace the thermocouple with a new one of the same type. Here's how to replace a faulty thermocouple:

Step 1: Unscrew the copper lead and connection nut inside the threaded connection to the gas line. Under the mounting bracket at the thermocouple tube, unscrew the bracket nut that holds the tube in place.

Step 2: Insert a new thermocouple into the hole in the bracket. Be sure the steel tube is up and the copper lead is down. Under the bracket, screw the bracket nut over the tube. Push the connection nut to the threaded connection where the copper lead connects to the gas line. Make sure the connection is clean and dry.

Step 3: Tightly screw the nut into place, but do not over-tighten. Both the bracket nut and connection nut should be only a little tighter than if hand-tightened.

The limit switch is a safety control switch located on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It also shuts off the blower when the temperature drops to a certain level after the burner has shut off. If the blower runs continuously, either the blower control on the thermostat has been set to the ON position or the limit control switch needs adjustment. Check the thermostat first. If the blower control has been set to ON, change it to AUTO; if the blower control is already on AUTO, the limit switch needs to be adjusted.

To adjust the switch, remove the control's cover. Under it is a toothed dial with one side marked LIMIT; don't touch this side. The other side of the control is marked FAN. There are two pointers on the fan side; the blower goes on at the upper pointer setting and turns off at the lower pointer setting. The pointers should be set about 25 degrees apart. Set the upper pointer at about 115 degrees Fahrenheit (46.1 degrees Celsius) and the lower one at about 90 degrees Fahrenheit (32.2 degrees Celsius).

Gas Furnaces: Burner Adjustment and Gas Leaks

The flames on the gas burner should be full and steady, with no sputtering and no trace of yellow.
The flames on the gas burner should be full and steady, with no sputtering and no trace of yellow.
Peter Dazeley/Getty Images

The flames on the gas burner should be full and steady, with no sputtering and no trace of yellow. To adjust the flame height on the main burners, call a professional service person. To adjust the height of the pilot flame, turn the flame adjustment screw until the flame is from 1 1/2 to 2 inches (3.8 to 5.1 centimeters) high. The adjustment screw is located near the gas valve on the pilot assembly, if the control has this adjustment feature.

If you suspect leaks around the furnace unit, stir up a mixture of liquid detergent and water. Paint this mixture on the gas supply line along its connections and valves; the soapy water will bubble at any point where there's a leak. If you find a leak, try tightening the leaking connection with a pipe wrench, but be careful not to over tighten the connection. If the pipe connections or valves still leak, call a professional service person.

An oil furnace has an entire new set of the problems from the ones we've just conquered in gas furnaces. Move on to the next section to learn how to clear a blocked fuel line or change an oil filter.

Now, let's examine how to troubleshoot and repair an oil furnace.

How to Troubleshoot an Oil Furnace

Check the smoke rising from the chimney. If the smoke is black, the furnace is not burning the oil completely and fuel is being wasted.
Check the smoke rising from the chimney. If the smoke is black, the furnace is not burning the oil completely and fuel is being wasted.
Lucas Allen/Getty Images

Oil-fired burners are used in many parts of the country as the basic heat source for warm air and hot water heating systems. Most of the home oil systems in use today are called pressure burners. In this type of system, oil is sprayed into a combustion chamber at high pressure, propelled by a blower and ignited by an electric spark.

The oil continues to burn as the mist is sprayed. While there aren't many quick fixes you can undertake yourself on these types of furnaces, good regular maintenance can help eliminate many problems. Here are a few oil furnace maintenance tips:

  • During the heating season, check the smoke from the chimney. If the smoke is black, the furnace is not burning the oil completely and fuel is being wasted. Call a professional service person for adjustments.
  • Clean the blower at the beginning of the heating season and again about midway through the season.
  • Clean soot from the stack control about midway through the heating season. If the blower motor has grease or oil fittings, lubricate the fittings midway through the heating season with cup grease or 10-weight non-detergent motor oil (not all-purpose oil), available at hardware stores.
  • Clean the thermostat before each heating season.

An oil furnace is a complex assembly. The maintenance and repair work for this type of furnace is limited to simple parts: the filters, the blower, the motor belts, the switches and the thermostat. Electrodes, an oil nozzle, air tubes, a transformer, a pump, and other components require special tools and testing equipment and are best left to a professional for service [source: WarmAir.com].

To become familiar with your oil furnace, remove the access panel covering the burner blower by removing the retaining screws around the rim of the housing. You can access the air blower and filter through a metal panel on one side of the furnace. The panel is held by either hooks or retaining bolts; slip the panel up and off the hooks or remove the bolts and lift the panel off. Most furnaces have switches and reset buttons located on the motor or in a switch box outside the furnace housing. These are usually identified with stampings or labels, such as DISCONNECT SWITCH, RESET, and so on. The stack control sensor, a safety device that monitors burner operation, is positioned in the stack and held with a series of retaining bolts.

Next, learn how to check the oil filters.

Oil Furnaces: Oil Filters

Remember to close the oil shutoff valve between the fuel tank and the filter.
Remember to close the oil shutoff valve between the fuel tank and the filter.
Dorling Kindersley/Getty Images

The oil filter should be changed or cleaned at the start of the heating season and about midway through the season -- three or four times a year [source: NationalFurnace.com]. Here's how to clean or replace the filter:

Step 1: Close the oil shutoff valve between the fuel tank and the filter.

Step 2: Unscrew the bottom or cup of the filter housing and remove the filter.

Step 3: If the filter is disposable, insert a new one of the same size and type. If your furnace has a permanent filter, clean the filter according to the furnace manufacturer's recommendations.

Step 4: Replace all old filter gaskets with new ones.

Step 5: Screw in the bottom of the housing and open the oil shutoff valve.

Some oil furnaces have a pump strainer, which is located on the pump attached to the burner/ blower unit. Clean this strainer when you clean the oil filter. Here's how:

Step 1: Unbolt the cover of the pump housing (where the oil line enters the burner) and lift off the cover.

Step 2: Remove the thin gasket around the rim. Find and remove the strainer, which is a cylindrical or cup-shape wire mesh screen.

Step 3: Soak the strainer in kerosene for several minutes to loosen any built-up sludge. Carefully clean the strainer with an old, soft toothbrush.

Step 4: Inspect the strainer. If it's torn or badly bent, replace it with a new pump strainer of the same type.

Step 5: Set the strainer into place on the pump, place a new gasket on the rim and then bolt the cover of the pump housing back on.

Next, we'll take a look at the switches and stack control on the furnace.

Oil Furnaces: Switches and Stack Control

A worker fixes a filler at a tank to refuel a residence heating system with heating oil in Ruesselsheim near Frankfurt, Germany.
A worker fixes a filler at a tank to refuel a residence heating system with heating oil in Ruesselsheim near Frankfurt, Germany.
Ralph Orlowski/Getty Images

Some oil furnaces have two master switches. One is located near the burner unit, and the other is near the furnace housing or even at a distance from the furnace. Make sure these master switches are both turned to the ON position.

The stack control of the oil furnace, located in the stack, is a safety device that monitors the operation of the oil burner. If the burner fails to ignite, the stack control shuts off the motor. Frequently, however, a furnace shutdown is caused by a malfunctioning stack control rather than by the burner.

If the burner fails to ignite, first check the fuel tank and refill it if necessary. If the tank doesn't need to be refilled, press the reset button on the stack control. If the burner doesn't ignite after you've pressed the button once, clean the control, as detailed below. Then press the reset button again. If the burner still doesn't operate, call a professional service person.

The stack control gradually becomes coated with soot during the heating season. To keep it working properly, clean the control every month or as soon as it becomes soot-covered. Here's how to clean the stack control:

Step 1: Remove the bolts that hold the control in the stack. Pull out the sensor and its housing.

Step 2: With a brush dipped in soapy water, remove all of the soot from the control. Wipe the control dry with a soft cloth.

Step 3: Before replacing the control, clean the stack. Spread newspaper to protect your floor, and then disassemble the stack. As you work, remove the soot and debris from each section by tapping them firmly on the newspaper-covered floor.

Step 4: After cleaning each section, reassemble them in reverse order. Make sure the stack sections are properly aligned and firmly connected.

Step 5: Finally, reposition the stack control in the stack, and reseal the connection to the chimney with refractory cement.

Some oil furnaces have an electric-eye safety switch instead of a stack control. This switch serves the same function as the stack control. If the burner has an electric-eye safety, remove the access cover over the photocell; it is held by hooks or retaining screws. Wipe the cover clean to remove accumulated soot. Reassemble the switch, replace the cover and turn the power back on. If the burner still doesn't ignite, or if it is especially dirty, call a professional service person.

Oil Furnaces: Draft Regulator, Limit Switch and Burner Adjustment

If the blower control has been set to ON, change it to AUTO; if the blower control is already on AUTO, the limit switch needs to be adjusted.
If the blower control has been set to ON, change it to AUTO; if the blower control is already on AUTO, the limit switch needs to be adjusted.
Don Klumpp/Getty Images

The draft regulator, located on the stack, is closed when the burner is off but opens automatically to let air into the chimney when the burner is turned on. Accumulated soot and rattling are signs that the draft regulator needs to be adjusted. Too much air in the chimney wastes heat; too little air wastes fuel by failing to burn it completely. To increase the airflow, screw the counterweight inward. To decrease airflow, turn the counterweight outward. The draft regulator should be adjusted by a professional service person as part of regular annual maintenance.

The limit switch is a safety control switch and is located on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It also shuts off the blower when the temperature drops to a certain level after the burner has shut off. If the blower runs continuously, either the blower control on the thermostat has been set to the ON position, or the limit control switch needs adjustment.

To determine the problem, check the thermostat. If the blower control has been set to ON, change it to AUTO; if the blower control is already on AUTO, the limit switch needs to be adjusted. To do this, remove the control's cover and find the toothed dial underneath. One side is marked LIMIT; don't touch this side. The other side is marked FAN. There are two pointers on the fan side; the blower turns on at the upper pointer setting and shuts off at the lower pointer setting. Pointers should be set about 25 degrees apart. Set the upper pointer at about 115 degrees Fahrenheit (46.1 degrees Celsius) and the lower one at about 90 degrees Fahrenheit (32.2 degrees Celsius).

Do not try to adjust the burner of an oil furnace; call a professional service person.

Finally, we have electric furnaces and heaters. Electric heat is in some ways much simpler than either gas or oil, but it can still have its fair share of problems. Keep reading to the next section to learn how to maximize the efficiency of your electric furnace.

How to Troubleshoot an Electric Furnace

Electric furnaces are fused at a building's main electrical service entrance.
Electric furnaces are fused at a building's main electrical service entrance.
Comstock/Getty Images

Although an electric heating system does have advantages, its operating cost generally makes it less desirable than any of the other furnace systems available today. The high cost means that minimizing heat loss caused by improperly installed ducts or inadequate insulation is even more important than with other types of systems. However, their maintenance costs tend to be lower over the lifespan of the furnace than other types of heaters [source: Trained Eye Home Inspection].

For maximum energy efficiency, have a professional service person clean and adjust your electric furnace every year before the beginning of the heating season. Do not attempt any repairs to the heating elements, electrical connections, relays, transformers, or similar components of an electric furnace; repairs to these components must be made by a professional service person.

The controls of an electric furnace may be mounted on the surface of the housing or installed behind an access panel on the front of the furnace. The access panel may be slip-fit on hooks fastened to the furnace housing with a series of sheet-metal screws. To remove the access panel to the blower, filter and blower motor, slip the panel up off hooks or remove a series of sheet-metal screws.

Electric furnaces are fused at a building's main electrical service entrance. Many electric furnaces are on separate circuits, sometimes located in a separate fuse box away from the main panel. The heating elements of the furnace are also fused, and these fuses are located on a panel that is on or inside the furnace housing.

If changing the fuses or resetting the breakers does not restore power to the furnace, call a professional service person. Do not attempt to repair heating elements, the transformer, heating relays, or power relays. Repairs to these components must be made by a professional service person.

Now that we've run the entire furnace gamut, your home should be nice and toasty all winter long. Though major furnace repairs to a furnace are best left to a professional, you should be able keep those house calls to a minimum with our maintenance tips.

And if you're looking for more information about home repairs, furnaces or other related topics, follow the links on the next page.

Related Articles

More Great Links

Sources

  • NationalFurnace.com. "Safety and Maintenance." (May 6, 2011) http://www.nationalfurnace.com/advice.htm
  • Peterson, Josh. "How to Clean a Furnace Filter." PlanetGreen.com. (May 6, 2011) http://planetgreen.discovery.com/home-garden/clean-furnace-filter.html
  • StartRemodeling.com. "Fall Furnace Cleaning." (May 5, 2011) http://www.startremodeling.com/clean_furnace.htm
  • Trained Eye Home Inspection. "Electric vs. Gas Furnace." (May 6, 2011) http://www.trainedeye.ca/articles/heating/electricVsGasFurnace.html
  • WarmAir.com. "Oil Furnaces." (May 6, 2011) http://www.warmair.com/html/oil_furnace.htm