Unit 8Gas-Burning Equipment

OBJECTIVES

• 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.

After studying this Unit, the student should be able to:

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

THE UPLOW FURNACE

Return Air Supply Air

Floor

Blower

Heat Producing System

THE LOWBOY FURNACE

Return Air Supply Air

Floor

Blower

Heat Producing System

THE DOWNFLOW FURNACE

Return Air

Supply Air

Floor

THE HORIZONTAL FURNACE INSTALLED IN A BASEMENT

Return Air Supply Air

Floor

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• 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

Shaft

Bimetal element (located near the heat exchanger)

CamsSet lever

Dial

Switch

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

Thermocouple

Pilot burner

Power unit coil

Valve

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)

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)

• 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

• 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