Gas furnace planned maintenance
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Gas Furnace Planned Maintenance PowerPoint PPT Presentation

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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.

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Gas Furnace Planned Maintenance

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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 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


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