SECTION 6 AIR CONDITIONING HEATING AND HUMIDIFICATION UNIT

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

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.

UNIT OBJECTIVES 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.

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.

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.

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.

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.

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.

INTRODUCTION TO GASFIRED 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

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

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

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

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

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

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

Manufactured Gas • 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

Propane • • 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

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

Open to atmosphere 1. 75 3. 5 in. W. C. 0 Connected to outlet of the gas valve Outlet 1. 75 Gas Valve Inlet WATER MANOMETER

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

OXYGEN FUEL HEAT COMPLETE COMBUSTION HEAT CO 2 H 2 O VAPOR

OXYGEN FUEL HEAT INCOMPLETE COMBUSTION HEAT SOOT CO 2 CO H 2 O VAPOR TOXINS

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

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

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

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

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

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

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

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

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

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

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

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 thermocouple – Signal is sent to the valve’s power unit – Pilot valve remains in the open position

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

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

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

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

Orifice Spud Heat Exchanger Burners Valve Manifold

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, 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

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

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

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

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

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

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

COMBUSTION EFFICIENCY • • Incomplete combustion produces carbon monoxide Correct air adjustments are essential CO 2 levels aid in secondary air adjustments CO 2 level increases as secondary air decreases CO 2 level for natural gas is 11. 7% to 12. 2% CO 2 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

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

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

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

• • • 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

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