Gas Furnace
Planned Maintenance
Annual Fuel Utilization
Efficiency (AFUE)
• Percentage of the annual average efficiency
of a furnace
• All furnaces manufactured after 1992 must
have a AFUE of at least 78%
Types of Gas-Fired Furnaces
Natural Draft
Natural Draft Furnaces
• Rely on the buoyancy of the hot combustion
products to create the draft needed to draw
combustion products through the heat
exchanger and out the vent
• Difficult to obtain a 78% AFUE rating
• Not being produced by most manufacturers
Induced-Draft Furnaces
• Have an AFUE of at least 78%
• Use an inducer fan to draw the products of
combustion through the heat exchanger
• Inducer fan restricts the flow of warm air
out the vent during the off cycle
Condensing Furnaces
• Have an AFUE rating of at least 90%
• Have an additional heat exchanger which
removes the latent heat from the flue gas by
condensing the water vapor
• Side wall venting
• Draws in 100% combustion air from outside
Furnace Configurations
Gas Furnace Components
Gas Valve
Gas Manifold and Orifices
Gas Burners
Heat Exchangers
Burner Ignition Devices
Gas Valves
• Most furnaces will use a combination gas
• The basic function of a combination gas
valve is:
– Automatic and manual shut-off of gas flow to
the main burners & pilot
– Pressure adjustment and regulation of the gas
supplied to the gas manifold & pilot
Gas Valves continued….
• Natural gas furnaces use a combination gas
valve--it should be adjusted to regulate the
outlet gas pressure to 3.2” w.c. to 3.8” w.c.
• LP furnaces may not have a pressure
regulator adjustment; if it does, it should be
adjusted to regulate the outlet pressure to
10.5” w.c. to 11” w.c.
Gas Manifold & Orifices
• Spuds can be changed in the field to obtain
the correct burner input rate
• The spud controls the flow of gas to each of
the burners
• The exact orifice size of the spud is
determined by the manufacturer
Gas Burners
• Properly mix the gas with the combustion
air to the combustion chamber
• Two types:
– Multi-port
– Mono-port
• The amount of primary air supplied to some
multi-port burners can be changed by
adjusting the primary air shutter
Burner Ignition Devices
Three basic types
Standing Pilot
Pilot Re-ignition
Direct Ignition
Standing Pilot
• Two types of safety devices:
– Thermocouple
– Thermally-actuated switch
• Clean orifice carefully
• A properly adjusted pilot flame will have a
soft blue color with some yellow at the tip
• Should have a height of 3/8” to 1/2” to
impinge on the thermocouple
Standing Pilots continued...
• Normal output voltage of thermocouple is
26 to 32 millivolts DC
• The minimum acceptable output voltage is
12 millivolts DC
• If pilot flame goes out the gas flow to the
pilot assembly should drop out within 2-1/2
Spark Re-ignition
• Used on mid-efficiency & high-efficiency
• Output spark typically 15,000 volts
• Pilot flame is proved by either a set of
thermally-actuated switches or a flame
rectification circuit (more common)
• Never attempt to manually light
Spark Re-ignition continued...
• Clean the components of any dirt, scale,
soot or carbon using a soft-bristle brush
• Check high voltage cable for any cracks or
poor connections
• Check safety lockout operation
– With gas valve off set thermostat to call for
heat, see if gas valve shuts down if no flame is
Direct Ignition
(Hot Surface Ignitor)
• Used on high efficiency furnaces
• Hot Surface Ignitor directly lights burners
• Uses flame rectification circuit to prove
• Made of ceramic - fragile
• Visually inspect for cracks or breaks
• At room temperature the resistance should
equal 45 to 90 ohms (if over 110 ohms
Vent Checks
• Visually check for any obstructions
• Visually check for any worn or damaged
• If any problems observed, immediately
notice the proper personnel
• Inspect vent pipe to other appliances that
may be tied into the furnace(s)
Combustion Testing
Category I Appliance
Category I - Appliance Test Point
Category I Appliances
Fan Assisted Furnaces
Category IV Furnaces
Temperature Rise Checks
• Determine temperature rise for data plate
• Generally low-efficiency furnaces will have
a temperature rise of 75 to 100 degrees
• Generally high-efficiency furnaces will
have a temperature rise of 40 to 70 degrees
• Always check supply temperature out of the
line of sight of the heat exchanger
Temperature Rise Checks continued...
• Before checking the temperature rise of a
furnace make sure the filter is clean and the
airflow is not being blocked
• Use the same type of thermometers for
measuring the return and supply
• Before checking the temperature rise,
compare the accuracy of each thermometer
against each other
Safety Controls Checks
• Verify the operation of the high temperature
• Block return airflow completely and
observe the gas valve shutting down and at
which temperature
• On induced draft furnaces remove one end
of the rubber hose from the air pressure
switch and observe the gas valve shutting

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