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 DraftInduced-DraftCondensing

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

UpflowDownflowHorizontalMultipoise

Gas Furnace Components

Gas ValveGas Manifold and Orifices

Gas BurnersHeat 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 PilotPilot Re-ignitionDirect 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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