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Gas Furnaces

Gas Furnaces. North Seattle College HVAC Program Instructor – Mark T. Weber, M.Ed. Gas Furnaces - 1. Introduction to Gas-Fired, Forced-Hot-Air Furnaces. Heat-producing system Consists of manifold, burners, ignition, controls, heat exchanger, and venting system Heated air distribution system

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Gas Furnaces

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  1. Gas Furnaces North Seattle College HVAC Program Instructor – Mark T. Weber, M.Ed. Gas Furnaces - 1

  2. Introduction to Gas-Fired, Forced-Hot-Air Furnaces • Heat-producing system • Consists of manifold, burners, ignition, controls, heat exchanger, and venting system • Heated air distribution system • Blower moves air throughout ductwork and ductwork assembly

  3. Types of Furnaces • Include: • Upflow: stands vertically, top air discharge • “Low-boy”: used for little headroom • Downflow: stands vertically; bottom air discharge • Horizontal: positioned on its side • Multipoise or multipositional: can be installed in any position

  4. Types of Furnaces (cont'd.)‏ The airflow for an upflow gas furnace

  5. Types of Furnaces (cont'd.)‏ The airflow for a low-boy for the more common upflow applications but can be changed furnace. The low profile of the low-boy is accomplished by placing the blower behind the furnace in a separate cabinet rather than in line with the heat exchanger

  6. Types of Furnaces (cont'd.)‏ The airflow for a downflow, or counterflow, gas furnace.

  7. Types of Furnaces (cont'd.)‏ Figure 31-5 (B) The airflow for a horizontal gas furnace The airflow for a horizontal gas furnace

  8. Gas Fuels • Natural gas • 90-95% methane and other hydrocarbons • Lighter than air (specific gravity = 0.60)‏ • Colorless, odorless, and not poisonous • Displaces oxygen and can lead to suffocation • Odorants are added for detection purposes • Produces about 1050 Btu per cubic foot when burned with air

  9. Gas Fuels (cont'd.)‏ • Liquefied petroleum • Liquefied propane, butane, or a combination of both: keep under pressure until used • Propane produces 2500 Btu/ft³ but requires 24ft³ of air • Heavier than air (specific gravity = 1.52)‏ • Displaces oxygen; can explode when accumulated • Butane produces 3200 Btu/ft³ • Difficult to work with, not very popular

  10. Gas Fuels (cont'd.)‏ • Manifold pressures • Expressed in inches of WC‏ • 1 psi = 27.7 in. WC • Some common manifold pressures: • Natural gas and propane/air mixture: 3 to 3.5 in. WC • Natural gas: 3.5 in. WC • LP gas: 11 in. WC A manometer used for measuring pressure in inches of water column. Courtesy Ferris State University. Photo by John Tomczyk.

  11. Gas Fuels (cont'd.)‏ A digital manometer being used to measure gas pressure in inches of water column Courtesy Ferris State University. Photo by John Tomczyk

  12. Gas Combustion • Requires fuel, oxygen, and heat • Ignition temperature for natural gas is 1100-1200ºF • Perfect combustion produces carbon dioxide, water vapor, and heat • Poor combustion produces carbon monoxide, soot, and other products • Flame should be blue with orange tips • Yellow tips indicate carbon monoxide

  13. Gas Combustion (cont’d.) (A) Primary air is induced into the air shutter by the velocity of the gas stream from the orifice. (B) Ignition of the gas is on top of the burner. (C) Incomplete combustion yields yellow “lazy” flame. Any orange color indicates dust particles drawn in with the primary air

  14. Gas Combustion (cont’d.) • Only gas pressure and primary air can be adjusted in the field • Gas/air mixture is important • 0-4% natural gas will not burn; 4-15% natural gas will burn but can explode; 15-100% natural gas will not burn or explode • Limits of flammability vary for gases • Extra primary air supplies better combustion

  15. Draft Inducer Fan • The Draft Inducer Fan mechanically draws a supply of combustion air through the burners, heat exchanger(s) and out through the flue. • Always positioned to create a negative pressure in the burner and heat exchanger and a positive pressure in the flue. Here is an 80% furnace. The draft Inducer Fan is here Air is drawn in here at the burners

  16. Draft Proving switch(es) • Before the furnace will go any further, draft must be proven. • This is to ensure there is enough secondary air for complete combustion. • There can be 1 or 2 pressure switches • Switches are in the “normally open” position. • The switch(es) are placed on the positive or negative side of the fan.

  17. Gas Valves • Drop gas pressure to proper level; maintain constant pressure at the outlet where gas is fed to the burners • Most valves can be adjusted • Always consult manufacturer's specifications when setting or adjusting gas valves and regulators

  18. Gas Valve • Gas is piped from regulator to gas valve at manifold • Several types exist Natural gas installation where the gas passes through a separate regulator to the gas valve and then to the Manifold. Courtesy Cengage Learning 2013

  19. Automatic Combination Gas Valve • Used in many modern gas furnaces • Include manual control, and safety shutoff, pressure regulator, main gas valve controls • Valves with dual shutoffs: redundant gas valves

  20. Automatic Combination Gas Valve (cont'd.)‏ • Intermittent pilot automatic gas valves • Pilot light is lit when there is a call for heat; when the call ends, the pilot goes out • Do not use power units or thermocouples; just two automatic valves: • Solenoid operated valve opens to permit passage of pilot gas • Servo operated valve opens to permit passage of gas for main burner operation

  21. Automatic Combination Gas Valve (cont'd.)‏ • Direct burner automatic gas valves • Electronic module or IFC lights the main burner directly, without a pilot flame • Ignition is accomplished by a spark, hot surface igniter, or glow coil • No power units or reset buttons • Two automatic valves are energized on a call for heat • Slow-opening is used when two automatic gas valves are used

  22. Burner Parts • Manifold • Pipe through which gas flows to burners • Attached to the outlet of the gas valve • Orifice • Precisely sized hole in the spud • Allows correct amount of gas into burner • Burners: where combustion takes place • Require primary/secondary air in correct quantities

  23. Heat Exchangers • Provide rapid heat transfer from the combustion products to the air that will be distributed in the space to be heated • Must have correct airflow • Many sizes, shapes, and materials A primary heat exchanger with four sections or clamshells. Courtesy Cengage Learning 2013

  24. The Blower Fan Switch • Automatically turns the blower on/off • Can be temperature controlled or on a time delay • Blower activation and shutoff are delayed • Gives heat exchanger time to heat up at the beginning of the cycle and cool off at the end of the heating cycle

  25. The Limit Switch • Safety device that closes the gas valve if the heat exchanger overheats • High-limit cut-out requirements vary, but are often between 200°F and 220°F • Fan switch and limit switch can be combined into a single unit • Can be high-voltage, low-voltage, or combo • Can controlled by the same bimetal helix • May be automatic or manual reset devices

  26. Pilots • Small burners used to ignite the gas burner on older conventional gas furnaces • May be aerated or nonaerated • Standing pilots burn continuously; other pilots are ignited by an electric spark or other ignition device when the thermostat calls for heat • Pilot flame provides heat for safety devices that shut off gas flow if the pilot goes out

  27. Ignition Systems • Can ignite the pilot or main burner • Intermittent pilot ignition • Spark from electronic module ignites pilot, which ignites main burner • Pilot burns only when thermostat calls for heat • Natural gas: pilot valve remains open (not 100% shutoff)‏ • LP gas: pilot valve will close if pilot does not light

  28. Ignition Systems (cont'd.)‏ • Direct spark ignition (DSI)‏ • No pilot is used • Sensor rod sends signal through flame rectification to DSI module to confirm firing • Hot surface ignition (Most common now) • Uses silicon carbide placed in gas stream • Igniter is allowed to get very hot before gas valve opens: immediate ignition

  29. Flame Rectification • Rectification circuit produces DC power in the milliamp range, which tells the furnace to open the gas valve. Flame proving device. • Is place in front of burner. Only produces DC electricity with burner is on.

  30. 80 % Gas Furnaces • Draws in combustion air from inside the building • Has a single heat exchanger. • Combustion air is drawn in pass the burner. • Flue gases pass through the heat exchangers, through the draft inducer fan and out through the flue.

  31. High-Efficiency Gas Furnaces 90 % plus • Draws in combustion air from outside thus reduces outdoor air infiltration (through windows, doors and walls) and room depressurization. • Have multiple heat exchangers. A primary just like a 80% or less furnace and a secondary that operates below the dew point temperature of the combustion gases and thus draws out the latent heat of the flue gases.

  32. High-Efficiency Gas Furnaces 90 % plus • Flue gases flow from top to bottom in primary heat exchanger. • Then enter the secondary heat exchanger. Flue gases condense releasing latent heat. Secondary must be made of stainless steel and hi-temp plastic.

  33. High-Efficiency Gas Furnaces (cont'd.)‏ • Annual fuel utilization efficiency rating (AFUE) allows consumer to compare furnace performance before buying • Furnace efficiency ratings are determined by amount of heat transferred to the heated medium • Conventional furnace: 78-80% AFUE • Mid-efficiency furnace: 78-83% AFUE • High-efficiency furnace: 87-98% AFUE

  34. Electronic Ignition Modules and Integrated Furnace Controllers • Control the ignition and sequence of operations in most modern gas furnaces • 100% shutoff system • Both pilot/main gas valve shut down in failure • Non-100% shutoff system • Main gas valve shuts down; pilot continues to bleed gas • Continuous retry with 100% shutoff • 90-sec trial for ignition

  35. Electronic Ignition Modules and IFCs (cont'd.)‏ • Lockout: time period to light or relight the pilot or main burner • Soft: semi-shutdown but module will keep trying to relight the system • Hard: full shutdown; module must be reset • Prepurge, interpurge, and post-purge periods allow blower motor to clear heat exchanger • IFC provides sequence schemes

  36. Two-Stage Gas Furnaces • Use a two-stage gas valve and a two-speed draft inducer fan with two pressure switches • First stage puts out manifold pressure of 1.75 in. WC and operates at 50-70% of total heating output • Second stage puts out manifold pressure of 3.5 in. WC and 100% of total furnace output

  37. Venting • In conventional gas furnaces • Use convection to vent flue gases but lose heat to prevent corrosive condensation • Use Type B vent; approved masonry materials • In high efficiency gas furnaces • Recirculate gases to retain heat • Use small fans and plastic pipe • Category specifies correct amount of excess air

  38. Gas Piping • Observe all national and local codes • Piping size varies with furnace ratings and gas type • Should be steel, cast iron, plastic • Use pipe dope or Teflon tape on joints • Piping at furnace should provide for a drip tap, a shutoff valve, and a union • Assembly must be leak tested and purged

  39. Combustion Efficiency • Incomplete combustion produces carbon monoxide • Enough secondary air must be provided to prevent this • Flames should be blue with orange tips • CO test gets information from flue gases needed to make air adjustments • Less secondary air means more CO

  40. Summary Remember – You are mixing natural gas and electricity in a metal box in someone’s basement or garage. What could possibly go wrong???

  41. For more information please contact Mark T. Weber, M.Ed., CMHE At North Seattle Community College WWW.NorthSeattle.edu Mark.weber@seattlecolleges.edu

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