1. AIR CONDITIONING (HEATING AND HUMIDIFICATION)
SECTION 6
AIR CONDITIONING (HEATING AND HUMIDIFICATION)
UNIT 31: GAS HEAT

2. UNIT OBJECTIVES
After studying this unit, the reader should be able to
describe each of the major components of a gas furnace.
list two fuels burned in gas furnaces and describe characteristics of each.
discuss a multipoise furnace and its safety devices.
discuss flame rollout switches, auxiliary limit switches, and draft safeguard switches.
discuss gas pressure measurement in inches of water column and describe how a manometer is used to make this measurement.

3. UNIT OBJECTIVES discuss gas combustion.
After studying this unit, the reader should be able to
discuss gas combustion.
describe a solenoid, diaphragm, and heat motor gas valve.
list the functions of an automatic combination gas valve.
describe the function of a servo-operated gas pressure regulator.
discuss the meaning of a redundant gas valve.
discuss different gas burners and heat exchangers.

4. UNIT OBJECTIVES
After studying this unit, the reader should be able to
describe the difference between induced-draft and forced-draft systems.
describe and discuss different ways of controlling the warm air fan.
state the function of an off-delay timing device for a warm air fan control.
describe the standing pilot, intermittent pilot, directspark, and hot surface ignition systems.
list three flame-proving devices and describe the operation of each.

5. UNIT OBJECTIVES (cont’d)
After studying this unit, the reader should be able to
discuss reasons and the systems used for the delay in starting and stopping the furnace fan.
state the purpose of a limit switch.
describe flue-gas venting systems.
discuss flame rectification and how it pertains to local and remote flame sensing.
apply flame rectification troubleshooting and maintenance procedures.
discuss the components of a high-efficiency gas furnace.

6. UNIT OBJECTIVES (cont’d)
After studying this unit, the reader should be able to
describe direct-vented, non-direct-vented, and positive pressure systems.
explain dew point temperature as it applies to a high efficiency condensing furnace.
discuss excess air, dilution air, combustion air, primary air, and secondary air.
describe the condensate disposal system of a highefficiency condensing gas furnace.
identify furnace efficiency ratings.

7. UNIT OBJECTIVES (cont’d)
After studying this unit, the reader should be able to
discuss electronic ignition modules and integrated furnace controllers.
describe a two-stage furnace, a modulating gas furnace, and a variable output thermostat.
explain gas piping as it applies to gas furnaces.
interpret gas furnace wiring diagrams and troubleshoot flowcharts or guides.
compare the designs of a high-efficiency gas furnace and a conventional furnace.

8. UNIT OBJECTIVES (cont’d)
After studying this unit, the reader should be able to
describe procedures for taking flue-gas carbon dioxide and temperature readings.
describe typical preventive maintenance procedures.

9. INTRODUCTION TO GAS-FIRED FORCED HOT AIR FURNACES
Heat producing system
Manifold, burners, ignition, controls, heat exchanger and venting system
Venting system removes flue gases
Heated air distribution system
Blower and controls
Ductwork and air distribution system

10. TYPES OF FURNACES Upflow – Stands vertically, top air discharge
Low Boy – Used where there is little headroom
Both supply and return are at the top
Blower located behind the furnace
Downflow – Stands vertically, bottom air discharge
Horizontal – Left or right discharge
Multipoise or Multipositional – Multi-positional

11. Return Air
Supply Air
Floor
THE UPLOW FURNACE
Heat Producing System
Blower

12. Return Air
Supply Air
Floor
Blower
THE LOWBOY FURNACE
Heat Producing System

14. THE HORIZONTAL FURNACE INSTALLED IN A BASEMENT
Return Air
Supply Air
Floor
THE HORIZONTAL FURNACE INSTALLED IN A BASEMENT

15. TYPES OF FUEL Natural gas Manufactured gas Liquefied petroleum (LP)
Propane
Butane

16. Natural Gas About 95% methane and other hydrocarbons
Lighter than air (specific gravity = 0.60)
Colorless and odorless
Odorants are added for detection purposes
Displaces oxygen and can lead to suffocation
Produces about 1,000 btu per cubic foot

17. Manufactured Gas Liquefied Petroleum
Lighter than air (specific gravity = 0.60)
Produces about 540 btu per cubic foot
Liquefied Petroleum
Liquefied propane, butane or a combination of both
Kept in the liquid state until used
Utilizes a tank regulator

18. Propane Butane Produces about 2,500 btu per cubic foot
Heavier than air (specific gravity = 1.52)
Displaces oxygen
Can explode when allowed to accumulate
Butane
Produces about 3,200 btu per cubic foot
Not very popular, difficult to work with

19. MANIFOLD PRESSURES Expressed in inches of water column (in. W.C.)
27.7 in W.C. = 1 psig
Natural gas and Propane/air mixture: 3.5 in. W.C.
Manufactured gas: 2.5 in. W.C.
Liquefied petroleum (LP): 11 in. W.C.
Gas pressures are measured with manometers
Water manometers or digital manometers

20. Connected to outlet of the gas valve
Open to atmosphere
Connected to outlet of the gas valve
Outlet
Inlet
1.75
3.5 in. W.C.
1.75
Gas Valve
WATER MANOMETER

21. GAS COMBUSTION Combustion requires fuel, oxygen and heat
Ignition temperature for natural gas is about 1,100°F
Perfect combustion produces carbon dioxide, water vapor and heat
Imperfect combustion produces carbon monoxide, soot and other products
Flame should be blue with orange tips
Yellow tips indicate that the flame is air-starved and that carbon monoxide is being produced

22. HEAT
OXYGEN
FUEL
COMPLETE COMBUSTION
HEAT
H2O VAPOR
CO2

23. INCOMPLETE COMBUSTION
HEAT
OXYGEN
FUEL
INCOMPLETE COMBUSTION
HEAT
CO2
H2O VAPOR
SOOT CO
TOXINS

24. BURNER FLAMES SHOULD BE BLUE WITH ORANGE TIPS
The orange in the flame is the result of dust particles drawn in with the primary air

25. GAS COMBUSTION
Gas pressure and primary air are the only adjustments that can be made in the field
Gas/air mixture is important
0 to 4% natural gas in the mixture will not burn
4 to 15% natural gas will burn and can explode
16 to 100% natural gas will not burn or explode
Limits of flammability vary for different gases
Excess primary air (about 50%) is supplied for better combustion

26. GAS REGULATORS Drop the gas pressure to the proper level
Maintain constant pressure at the outlet of the regulator where gas is fed to the gas valve
Many regulators can be adjusted
Most regulators are built into the gas valve
LP regulators are located at the tank
Always consult manufacturer’s specifications when setting/adjusting gas regulators

27. GAS PRESSURE REGULATOR
Vent Cap
Regulator Adjusting Screw
Spring
Diaphragm
Outlet
Inlet
Stem
Seat

28. GAS PRESSURE REGULATOR
No flow when the regulator is in the closed position

29. DIAPHRAGM-TYPE GAS VALVE
Uses gas pressure to open the valve
Valve opens when the coil is energized
Gas above the diaphragm is vented to the atmosphere
Gas pressure pushes up on the diaphragm
Diaphragm is pushed up to open the valve
Valve is closed when the coil is de-energized
Gas pressure pushes down on the diaphragm
Atmospheric pressure pushes up on the diaphragm

30. THE DIAPHRAGM-TYPE GAS VALVE
Spring
Coil
Diaphragm
Bleeder Valve
Gas Outlet
Gas Inlet

31. THE DIAPHRAGM-TYPE GAS VALVE Valve is in the closed position
The pressure pushing up on the diaphragm is equal to the pressure pushing down on the diaphragm

32. THE DIAPHRAGM-TYPE GAS VALVE Valve is in the open position
When the coil is energized, the gas above the diaphragm is vented and the main gas pressure pushes up on the diaphragm to open the valve

33. HEAT MOTOR-CONTROLLED VALVE
On a call for heat, a rod attached to the valve is heated, causing the rod to expand or elongate
The expanding rod causes the valve to open
The rod is heated by a resistance wire that is energized by the heating circuit
When the call for heat is satisfied, the rod contracts and the spring closes the valve
20 seconds to open the valve, 40 seconds to close

34. AUTOMATIC COMBINATION GAS VALVE
Used in most modern gas furnaces
Equipped with manual control, pilot supply, pilot adjustment, safety shutoff, pressure regulator, main gas valve controls and programmed safe-lighting features
Valves with dual shutoffs are called redundant gas valves

35. STANDING PILOT VALVES Pilot light is lit all the time
Have safety shutoff valves
Pilot light must be lit manually
Lighting the pilot
Push and hold button in “pilot” position
Manually light the pilot
Pilot flame engulfs the thermocouple
Signal is sent to the valve’s power unit
Pilot valve remains in the open position

36. INTERMITTENT PILOT VALVES
Pilot light is lit when there is a call for heat
When thermostat is satisfied, the pilot goes out
Do not utilize thermocouples or power units
Can use two automatic valves
First valve is solenoid controlled and opens to permit passage of pilot gas
Second valve is servo controlled and opens to permit passage of gas for main burner operation

37. DIRECT BURNER VALVES
Electronic module or integrated furnace controller lights the main burner directly
There is no pilot flame
Ignition is accomplished by hot surface igniter, spark or glow coil
Do not utilize power units or reset buttons
Can use two automatic valves, one controlled by solenoid, the other by servo
Both valves are energized on a call for heat

38. SLOW-OPENING VALVES
Used when there are two automatic gas valves being used
The first valve opens completely
The second valve opens slowly
Minimal amount of fuel is ignited at the beginning of the heating cycle
Slow and controlled ignition
Main burner flame grows slowly

39. OTHER FURNACE PARTS Manifold – Attached to the outlet of gas valve
Gas flows through the manifold to the burner
Orifice – Precisely drilled hole in the spud
Gas flows to the burners through the orifice
Allows correct amount of gas flow to the burner
Burners – Where combustion takes place
Primary and secondary air
Heat exchangers – Provide heat transfer between the air from the conditioned space and the combustion gases

40. Orifice
Spud
Heat Exchanger
Burners
Manifold
Valve

41. THE FAN SWITCH Turns the blower on and off automatically
Can be controlled by time or temperature
Blower operation is delayed at both the beginning and the end of the heating cycle
Gives the heat exchanger time to heat up at the beginning of the cycle
Allows the heat exchanger to cool off at the end of the heating cycle
Electronic modules can be used to control the blower

42. THE FAN SWITCH Cams Set lever Dial Shaft Switch
Bimetal element (located near the heat exchanger)

43. THE LIMIT SWITCH
Used as a safety device to open the gas valve circuit if the heat exchanger overheats
Normally opens between 200°F and 220°F
Fan switch and limit switch can be combined into a single unit
Can be line voltage, low voltage or a combination
Fan and limit contacts controlled by the same bimetal
Internal jumper removed to separate the controls
Can be automatic or manually reset devices

44. SAFETY DEVICES – STANDING PILOT
Flame-proving devices prevent gas from flowing through the valve if the pilot is out
Thermocouples and thermopiles – As long as the pilot is lit, the thermocouple will stay hot and the gas valve will remain open
Bimetallic safety device – Heated by the pilot
Liquid-filled remote bulb – Heated by the pilot
If the pilot goes out, the device will initiate the closing of the gas valve

45. Pilot burner
Thermocouple
Power unit coil
Valve

46. IGNITION SYSTEMS Can be used to ignite the pilot or main burner
Intermittent pilot – Spark lights pilot
Natural gas – pilot valve remains open
LP gas – pilot valve will close if pilot does not light
Direct spark ignition (DSI) – No pilot used
Uses a DSI module and an igniter/sensor assembly
Hot surface ignition – Uses silicon carbide/nitride
Immediate ignition upon opening of the gas valve

47. FLAME RECTIFICATION Uses the flame as a switch
Flame is located between electrodes
The flame completes the rectification circuit
Rectification circuit converts ac power to dc
The furnace recognizes the dc power and opens the gas valve
Can be used on spark to pilot or DSI systems
Integrated Furnace Controllers (IFC)

48. HIGH EFFICIENCY GAS FURNACES
Annual Fuel Utilization Efficiency (AFUE)
Efficiency increased by keeping excessive heat from being vented to the atmosphere
Have multiple heat exchangers
Range from 87% to 97% AFUE ratings
35°F to 65°F rise across the heat exchanger
110°F to 120°F stack temperature
Condensing furnaces

49. ELECTRONIC IGNITION MODULES & INTEGRATED FURNACE CONTROLLERS (IFC)
Control the ignition and sequence of operations
100% shutoff and non 100% shutoff systems
Soft lockout – allows time to light or relight pilot
Hard lockout – power must be interrupted and restored after allotted time has elapsed
Pre-purge, inter-purge and post-purge
IFCs provide sequence of operation for the system

50. TWO-STAGE FURNACES Two-stage gas valves and combustion blowers
First stage provides 50% to 70% of heating output
First stage gas pressure is 1.75 in. W.C.
Second stage provides 100% of total heating output
Second stage gas pressure is 3.5 in. W.C.
Two pressure switches to prove flame
Low pressure switch for low blower speed
High pressure switch for high blower speed
Provide better control of space temperature

51. MODULATING FURNACES Follows the heat loss of the structure
Discharge air temperature is modulated for even heating of the structure
Adjusts to changes in gas heating values and air density
Determines exact heating requirements for the space
Optimizes furnace efficiency and performance
Utilize variable speed blowers

52. VENTING Conventional gas furnaces
Flue gases should be vented as quickly as possible
Cooling flue gases can condense
Must use “B” vent or approved masonry materials
Chimneys must be properly sized and lined
High efficiency gas furnaces
Use small fans to remove flue gases
Flue gases can and do condense (corrosive)
Plastic (PVC) pipe is used as venting material

53. GAS PIPING Observe all national and local codes
Piping size varies with furnace ratings and gas type
Should be installed with as few fittings as possible
Should be sized properly to avoid pressure drops
Steel or wrought iron piping should be used
Use pipe dope or teflon tape on pipe joints
Drip trap, manual shutoff and union
Piping assembly must be leak tested and inspected

54. Manual gas shutoff valve
TYPICAL GAS PIPING LAYOUT
Manual gas shutoff valve
Tee
Union
Drip trap
Gas valve
Cap

55. COMBUSTION EFFICIENCY
Incomplete combustion produces carbon monoxide
Correct air adjustments are essential
CO2 levels aid in secondary air adjustments
CO2 level increases as secondary air decreases
CO2 level for natural gas is 11.7% to 12.2%
CO2 of 14% for butane and 13.7% for propane
Correct flame should be blue with orange tips
Yellow flame indicates the production of carbon monoxide

56. UNIT SUMMARY - 1
The gas furnace consists of a heat producing system and an air distribution system
Furnace configurations include upflow, downflow, lowboy, horizontal and multipoise
Common fuels on gas furnaces include natural gas, manufactured gas and liquefied petroleum
Normal manifold pressures are 3.5 in. W.C. for natural gas, 11 in. W.C. for LP and 2.5 in. W.C. for manufactured gas
27.7 in. W.C. = 1 psig

57. UNIT SUMMARY - 2 Manifold pressures are measured with manometers
Combustion requires heat, oxygen and fuel
Burner flames should be blue with orange tips
Regulators maintain the proper gas pressure
Types of gas valves include the diaphragm, solenoid, heat motor-controlled, automatic combination, standing pilot, intermittent pilot, direct burner and the slow opening
Other furnace parts include the spud, burners, manifold and heat exchanger

58. UNIT SUMMARY - 3 The fan switch controls blower operation
The limit switch is a safety that opens its contacts if the heat exchanger temperature rises too high
Safety devices on the standing pilot system include thermocouples, thermopiles, bimetal devices and liquid-filled controls
Ignition systems are used to light the pilot or main gas and can be intermittent, direct spark ignition (DSI), and hot surface ignition

59. UNIT SUMMARY - 4
Flame rectification devices convert ac to dc and are used on spark to pilot or DSI systems
High efficiency furnaces have 87% to 97% AFUE ratings (Annual Fuel Utilization Efficiency)
Integrated Furnace controllers (IFC) control ignition and the system’s sequence of operations
Two-stage furnaces use two-stage gas valves
Modulating furnaces adjust to match the load
The venting of flue gases must be done in accordance with all federal and local codes

60. UNIT SUMMARY - 5
Flue gases on conventional furnaces must be removed quickly to prevent condensing of the gases
High efficiency furnaces use small blowers to remove the flue gases
Piping must be done according to all building codes
Piping circuit must be inspected and leak tested prior to being put into operation
Incomplete combustion produces carbon monoxide
Correct air adjustments are important to combustion