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Unit 8
Gas-Burning Equipment
OBJECTIVES
After studying this Unit, the student should be able to:
• State the difference between a natural gas and an LP gas
system.
• Describe several control sequences for gas heat and warm-air
furnaces.
• Describe the difference between a standard-efficiency
furnaces and a high-efficiency furnace.
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 – Multi-positional
Return Air
Supply Air
Floor
THE UPLOW FURNACE
Heat Producing System
Blower
Return Air
Supply Air
Floor
Blower
THE LOWBOY FURNACE
Heat Producing System
Return Air
THE DOWNFLOW FURNACE
Supply Air
Floor
Return Air
Supply Air
Floor
THE HORIZONTAL FURNACE INSTALLED IN A BASEMENT
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
MODULATING FURNACES
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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
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
THE FAN SWITCH
Cams
Set lever
Dial
Switch
Shaft
Bimetal element
(located near the heat
exchanger)
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
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
Pilot burner
Thermocouple
Power unit coil
Valve
FLAME RECTIFICATION
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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)
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
ELECTRONIC IGNITION MODULES &
INTEGRATED FURNACE CONTROLLERS (IFC)
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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
Unit Summary
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The gas furnace consists of a heat producing system and an air distribution system
Furnace configurations include upflow, downflow, lowboy, horizontal and multipoise
The fan switch controls blower operation
The limit switch is a safety that opens its contacts if the heat exchanger temperature
rises too high
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)
Two-stage furnaces use two-stage gas valves
High efficiency furnaces use small blowers to remove the flue gases