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

Induced-Draft

Condensing

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

Upflow

Downflow

Horizontal

Multipoise

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 valve

• 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 minutes.

Spark Re-ignition

• Used on mid-efficiency & high-efficiency furnaces

• 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 proved

Direct Ignition(Hot Surface Ignitor)

• Used on high efficiency furnaces

• Hot Surface Ignitor directly lights burners

• Uses flame rectification circuit to prove flame

• 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 replace)

Vent Checks

• Visually check for any obstructions

• Visually check for any worn or damaged pipes

• 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 temperatures

• Before checking the temperature rise, compare the accuracy of each thermometer against each other

Safety Controls Checks

• Verify the operation of the high temperature switch.

• 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 down

THE

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