State University Extension
Home Maintenance And Repair
Maintaining Conventional Residential Gas-Fired Heating Systems
Tuttle, Graduate Student,
Department of Agricultural Engineering
Susan Mireley, Associate
Professor and Extension Housing Specialist
Department of Human Environment and Design
publication covers maintenance of conventional gas-fired heating
systems only. Maintenance and repair of the newer condensing gas
furnaces are not covered. Many of the same procedures and maintenance
activities described in this publication are applicable if propane
(L.P. gas) is used as the heating fuel. The orifices (the mechanism
controlling the amount and direction of the gas flowing to the burners)
and the pressure regulator must be changed to convert a natural gas
unit to propane. As a unit is converted, a label should be added
Periodically, conventional gas-fired heating systems need adjustment to
keep them running in the safest, most efficient and least costly
manner. This publication is designed to help you understand how
gas-fired heating systems work (both forced air and hot water systems),
the periodic preventive maintenance the units should have, and what
special tasks and problems need the attention of a heating contractor.
All gas-fired heating systems are composed of a heat-producing source
(a furnace in the case of forced air systems and a boiler for hot water
systems) that contain both a burner and a heat exchanger; a
distribution system (ducts in the case of forced air systems, pipes in
hot water systems); a burner by-products elimination system (exhaust
stack, flue or chimney); and control mechanisms (such as the
thermostat, gas control valve and safety control devices).
Understanding how these various parts operate and work together is an
important part of any preventive maintenance program.
HOW A GAS-FIRED HEATING SYSTEM WORKS
How the Typical Gas Burner Works
When a room thermostat is turned up or the room temperature drops below
the thermostat setting, the unit signals the furnace or boiler that
additional heat is needed. As a result of the signal, the gas control
valve opens, allowing gas to flow into the mixing tubes (see Fig.
where it mixes with the incoming combustion air. The fuel/air mixture
then flows to the burners, where it is ignited by either a pilot light
or an electronic spark. Ignition occurs only during start-up.
Once started, the flames continue to burn , with additional gas and air
fed to the burners by the gas supply line, the air shutters and
furnace/boiler room air. In turn, the
combustion gases from the flames flow through the flue passages of the
heat exchanger, heating it, and then continue to flow up and out the
exhaust stack (see Fig.
2 for a forced air distribution system and Fig.
3 for a hot water distribution system). In the meantime, heat is
transferred from the flue gases in the heat exchanger to the medium
(either air or water) that flows through the distribution system that
supplies heat to the various parts of the home.
How A Typical Forced Air Distribution
When the air temperature in the heat exchanger reaches a predetermined
temperature, which is controlled by the fan-and-limit control, the
furnace blower begins pulling cool room air through the return air
registers and ducts (see Figs.
4). The air is passed through an
air filter to remove dust. It then passes through the heat exchanger,
as described above, where it is warmed by hot combustion gases passing
through the heat exchanger on their way out of the house. The furnace
blower then forces the warm supply air into a plenum and through the
supply ducts, finally distributing it through supply registers located
in each room in the home. The two air supplies, the combustion air and
the air distributed through the house supply system, should never come
in direct contact with each other.
How A Hot Water Distribution System
When a house thermostat calls for heat from a hot water distribution
system, the circulator pump is activated (see Fig.
3) and supplies hot
water held in reserve in the boiler throughout the distribution system
through the supply main, the supply branches and finally the baseboard
units (i.e., radiators or convectors in some systems) in the various
rooms (see Fig.
5). As cool room air passes over the warmed baseboard
unit surfaces, the air absorbs heat and distributes it throughout the
Individuals, furnishings and objects near the baseboard units are also
warmed by heat given off by the units. The now cool water completes the
cycle, flowing from the baseboard units through the return branches and
the return main and back to the boiler. When the water in the boiler
drops below a predetermined temperature, the aquastat activates the
burner (see Fig.
3). The heat given off by the burner warms the heat
exchanger and rewarms the water in the boiler, which holds it in
reserve until the circulator pump moves it through the distribution
system once again. This two-phase process enables the system to
maintain an on-demand supply of hot water at all times. The homeowner
doesn't have to wait for the water to be reheated and circulated
through the system.
MAINTENANCE AND INSPECTION YOU CAN DO
During the heating season, furnaces and boilers can accumulate a
buildup of dirt In addition, the various moving parts wear. Dirt and
water can lead to a loss of system efficiency, system failures, and
health and safety problems. Annual inspections and maintenance are
required to keep a gas-fired heating system operating efficiently and
You can do a number of maintenance tasks yourself. You may want to ask
your heating contractor to show you some of the procedures during his
or her next visit, or consult your heating system owner's manual. It
should provide an excellent guide to the activities do-it-yourselfers
can do. This publication describes some of those activities in a
general way, but the owner's manual will give you specifics for your
If the owner's manual is not available, write to the manufacturer and
request that one be sent to you. You will find the manufacturer's name
and probably the address on the nameplate affixed to your unit
somewhere..If the manufacturer's address is not available, consult your
heating contractor or search the yellow pages to find the name of a
heating contractor that sells the brand you own. In your letter, give
the model and serial number of your unit so the correct owner's manual
can be sent to you. Both numbers can be found on the unit's nameplate.
If you are a novice do-ityourselfer, follow the manufacturer's
recommendations carefully and do only those tasks the manual explains
clearly. All others should be done by a heating contractor. Gas is a
clean, efficient and safe fuel if the equipment burning it is well
maintained by knowledgeable people. Novices and inexperienced tinkerers
should be aware of the potential for creating problems.
1. Inspect the pilot flame height and color (see Figs.
1 and 6). Check
your unit's owner's manual for the procedure and the correct height.
The flame should be blue and fairly sharp. If it isn't, or if it seems
too high or too low, have a heating contractor adjust it. Some gas
burners may have a spark ignition unit in place of a standing pilot
light. This is an energy-saving feature that helps cut gas consumption
and so saves on heating bills.
2. Inspect the flame color of the burners and the pilot light (see Fig.
1). A natural gas flame should be primarily bluish and it should be
fairly sharply defined. (It you are burning propane, the tips of the
flame should be yellow.) If theflame is lazy or yellow, the burners
should be cleaned or the gas-to air ratio in the mixing tubes needs to
be adjusted. Contact a heating contractor to clean the unit and adjust
the amount of air entering the system through the air shutters.
3. Gas is a clean-burning fuel. If during your inspections you notice
greasy dirt or soot built up on the burners, a problem is present. Call
your heating contractor to locate and repair it.
4. Clean the draft hood (see Fig.
1), the mechanism that controls the
rate at which combustion gases are pulled up and out of the flue or
chimney. Dust buildup can interfere with the hood's efficient
operation. Vacuum any loose dust. Note: on some gas burners, the draft
hood is incorporated into the body of the unit and may not be directly
visible. Your owner's manual will explain the location and any care it
5. Inspect the exhaust stack for bad connections and damaged or
corroded pipes (see Figs.
Have a heating contractor
replace any damaged parts.
6. Inspect the furnace's/boiler's electrical system (master switch and
electrical cable, for example). Electrical problems affect the
performance of a unit and also present a safety hazard. Bad
connections, bare wires, blown fuses or tripped circuit breakers
indicate that an electrical problem is present. Contact your
heating and cooling contractor to correct it. (Warning: electricity is
potentially dangerous for people not familiar with its operation. Use
care in trying to correct any problems yourself.) If you ever notice
damaged wires in front of the burners, contact your heating contractor.
7. Clean the furnace room periodically. Dirt and lint from areas
surrounding the unit can be pulled into the burners along with
combustion air. Eventually they will slow the movement of combustion
air to the unit, causing it to burn inefficiently and give off soot.
8. Clean the room thermostat(s). Dust buildup interferes with efficient
thermostat operation. Remove the cover and wipe away dust with a soft
brush, such as a watercolor paint brush. Work carefully-thermostats are
9. Inspect any visible sections of the chimney and the chimney top. If
they are worn or damaged, consult your heating contractor.
10. Safety note: gas-fired furnaces and boilers require an adequate
supply of air to ensure proper and safe burning of the fuel. In
addition, adequate clearances are necessary between combustibles such
as walls, doors and framing members and the furnace/boiler, the vent
stack and the chimney. Never enclose a unit unless you check with a
heating contractor about the unit's combustion air needs. Never store
combustible materials near the unit.
Forced Air Distribution System
1. Clean the blower fan and the blower housing and clean and lubricate
the blower fan shaft and the blower motor (see Fig.
2). Dust, dirt and debris buildup affects the efficient operation
and useful life of these components.Remove it with a vacuum or soft
cloth and lubricate according to your owner's manual. (Note: some newer units may not
require lubrication because the bearings are sealed.) Warning: shut off electricity to the
unit before starting these procedures.
2. Inspect the blower fan belt for wear and correct tension (see Fig.
2). It is fairly common for the belt to be too tight, which can
lead to failure of the motor or fan bearings. On the other hand, a
loose belt can slip, causing faster belt wear and premature belt
failure. Replace the belt when slippage can no longer be corrected or
belt failure seems likely. Adjust the belt tension according to your
furnace's owner's manual. Warning:
shut off electricity to the unit before starting belt adjustment. (Note: many newer gas furnaces do not
have a fan belt-a motor drives the fan directly.)
3. Replace the air filter periodically to ensure that air is
circulating freely and cleanly (see Fig.
2). Dirty air filters slow air movement, make other furnace
components work harder (which shortens their useful life) and waste
energy. Inspect them monthly to determine how often they should be
changed. Follow directions provided in the furnace owner's manual.
4. Inspect duct work for air leaks that result in heat loss and wasted
money (see Fig.
4). Locate them by running your hand over areas where you suspect
leaks. Seal leaks with duct tape.
5. Clean and clear room supply and return air registers of any dust or
obstructions (see Fig.
4). They reduce air circulation and waste energy dollars. A vacuum
will do a good job.
6. Any warning signs noted during the heating season such as unusual
odors from the registers, discoloration over registers, excessive dirt
in the house air supply, or unusual cycling of the burner or fan or
both-should be reported to your heating contractor.
7. Periodically throughout the heating season, make a conscious effort
to listen to your unit as it goes through a heating cycle. The normal
procedure is: the room thermostat calls for heat, the burner goes on,
the fan then starts. Both should remain on until the thermostat
temperature is satisfied. Once it is satisfied, the burner stops first,
followed by the fan. If the burner or the fan or both cycle on and off
frequently before the thermostat is satisfied, a problem exists.
Consult your heating contractor. Likewise, if the burner ever goes on
but the fan does not follow, call a heating contractor.
Hot Water Distribution System
1. Clean the circulator motor and lubricate the motor and circulator
pump (see Fig.
3). Dust and dirt buildup inhibits efficient operation. Consult
your owner's manual for instructions and lubrication needs.
2. Clean and bleed baseboard units (convectors or radiators in some
systems; see Figs.
7). Both dust and dirt buildup on the baseboard unit's fins and air
in the pipes decrease the heat transfer efficiency of the baseboard
unit. A vacuum and a softbristle brush work well together for cleaning
the fins. If air is present in the pipes, you'll hear a sound similar
to water trickling. Bleed the unit by opening the air valve until water
runs freely from the unit. Then close the valve. (Note: many modern hot water systems
contain automatic bleeders and do not require this step. Consult and
follow your owner's manual for the correct procedures.)
3. Inspect baseboard units to ensure that adequate clearance exists
between them and the floor, particularly carpeted floors. Clearance is
necessary to allow air to flow freely around the unit. If your hand
cannot slip easily into this area, consult a heating contractor about
raising the unit.
INSPECTION A SERVICE PERSON SHOULD DO
Though you can do the maintenance and repair tasks described in your
owner's manual, other jobs require the knowledge of a heating
contractor. Inexperienced homeowners attempting these tasks may make
mistakes that expose them and their families to unnecessary hazards and
possibly damage the equipment. We mention these tasks because a heating
contractor should do them during a regular maintenance visit. When
selecting a contractor, ask about the standard service procedures
followed and see if they are similar to those described here. We
suggest that a heating contractor service your system every two to
three years and perhaps annually if the system is old. The cost will be
approximately $50 per visit.
1. Inspect and clean the burners (see Fig.
1). They can easily become plugged by dust or soot. Cleaning
requires special equipment.
2. Adjust the air shutters (see Fig.
1 ). The air shutters control and regulate the air/fuel mixture
released for ignition and combustion. Their correct adjustment improves
overall furnace efficiency and thus helps reduce energy costs.
3. Adjust the gas pressure to the burner. The pressure affects the
volume of gas delivered to the burner and, thus, the air/fuel mixture.
4. Inspect the gas control valve (see Fig.
1). This valve controls the amount of gas flowing into the system.
The heating contractor will determine if it is operating correctly and
inspect for leaks in the valve.
5. Inspect the thermocouple, a safety device designed to sense and shut
down the gas supply to the pilot flame if the pilot light goes out (see
1 and 6). If it is worn, the heating contractor will replace it.
6. Inspect and adjust safety devices incorporated into the unit, such
as the fan-and-limit switch or aquastat, burner safety devices and any
power flue devices, if the unit contains a power draft.
7. Clean the heat exchanger surfaces (see Figs.
3). Cleaning improves heat transfer between the heat exchanger and
the household air or water supply.
8. Check for gas leaks, which are extremely dangerous. Serious health
hazards are possible if sufficient quantities of gas are released into
the home. Gas has a peculiar odor. If you ever suspect a leak,
immediately leave the home and contact your gas company or service
person from a nearby phone.
9. Check for combustion leaks. Combustion leaks allow dangerous gases
to escape into the furnace room and duct work.
Forced Air Distribution System
1. Align the blower pulley and the blower motor pulley (see Fig.
2). Improper alignment can cause abnormal belt wear and slippage.
(Note: most newer gas furnaces do not have pulleys-a motor drives the
2. Adjust the blower fan speed (see Fig.
2). Overly high fan speed will cause duct noise and waste
electricity. If the fan is set unnecessarily low, an excessive amount
of heat will be lost through the supply ducts before it reaches the
registers to provide room heat.
3. Adjust and test the fan-and limit control, which measures the
temperature of the air surrounding the heat exchanger. It automatically
turns the furnace blower on and off during each burning cycle and shuts
the burner down if the heat exchanger becomes overly hot. The control
monitors three temperatures: a fan-on temperature, a fan-off
temperature and a temperature limit, which is the safety device
designed to shut the burners off if the heat exchanger becomes too hot.
It is never adjusted or changed-the other two can be, however.
For example, the fan-on temperature is usually about 135 degrees F.
When the air around the heat exchanger reaches this temperature, the
blower fan comes on and moves the heated air throughout the home. The
fan-off temperature is usually set at approximately 100 degrees F. When
the air surrounding the heat exchanger reaches this temperature, the
fan blower stops so it doesn't circulate cooler air through the home
and cause uncomfortable drafts.
To save energy, you may want to ask your heating contractor to lower
these two temperature settings to a level closer to the house
thermostat setting. As a result, the furnace will supply more heat to
the house but the air will feel cooler. You may experience some
draftiness and discomfort after the changes until you become accustomed
to the new settings.
Hot Water Distribution System
1. Check the circulator pump coupler for wear and broken coupler
springs (see Fig.
3). Normally, if the coupler springs are broken, a loud racket
occurs; they have been known to be broken, however, with little noise.
2. Check the operation of all safety controls (see Fig.
3), such as the safety relief valve, which would relieve the system
if overly high pressure should occur because of overheating, and the
limit thermostat, which would turn the unit off if the boiler should
3. Check and adjust air and water levels in the expansion tank, which
provides a reservoir for the safe collection of water as it expands
during the heating cycle (see Fig.
3). Note: many new expansion tanks contain bladder or diaphragm
devices that control water and air levels automatically.
Problems and their Causes part 1
Problems and their Causes part 2
Problems and their Causes part 3
Problems and their Causes part 4
Conventional Residential Gas-Fired Heating Systems, Michigan
Brotherson, Donald. ''Heating the Home,'' G3.1. Small Homes
Council-Building Research Council, University of Illinois, 1976.
Energy, Mines and Resources Canada. "The Billpayer's Guide to Heating
Systems.'' Minister of Supply and Services, Canada, 1983.
Knight, Paul A. Mechanical Systems Retrofit Manual.- A Guide for
Residential Design. New York: VanNostrand Reinhold Company, 1987.
Mireley, Susan and Don D. Jones. "The Indiana Do It Yourself Home
Maintenance Checklists: Space Conditioning Systems," HE-
63. Indiana Cooperative Extension Service, Purdue University, July 1986.
Other references part 2
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June 30, 1914, in cooperation with the U.S. Department of Agriculture.
Thomas G. Coon, Director, MSU Extension, East Lansing,MI 48824. This
information is for educational purposes only. Reference to commercial
products or trade names does not imply endorsement by MSU Extension or
bias against those not mentioned.
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