1. Gas Furnace Planned Maintenance

2. 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%

3. Types of Gas-Fired Furnaces
Natural Draft
Induced-Draft
Condensing

4. 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

5. 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

6. 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

7. Furnace Configurations
Upflow
Downflow
Horizontal
Multipoise

8. Gas Furnace Components
Gas Valve
Gas Manifold and Orifices
Gas Burners
Heat Exchangers
Burner Ignition Devices

9. 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

10. 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.

11. 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

12. 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

13. Burner Ignition Devices
Three basic types
Standing Pilot
Pilot Re-ignition
Direct Ignition

14. Standing Pilot Two types of safety devices: Clean orifice carefully
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

15. 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.

16. 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

17. 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

18. 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)

19. 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)

20. Combustion Testing

22. Category I Appliance

23. Category I - Appliance Test Point

24. Category I Appliances

25. Fan Assisted Furnaces

26. Category IV Furnaces

27. 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

28. 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

29. 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

30. THE END