1. Unit 7 Gas Heat: Natural and LP

2. OBJECTIVES Describe the major components of a gas furnace.
After studying this Unit, the student should be able to:
Describe the major components of a gas furnace.
List three types of fuel burned in a gas furnace and their characteristics.
Describe the heat exchange for air and water for gas-burning appliances.
List the types of safety systems used to prove gas combustion.
Name three types of ignition systems for gas heat.

3. 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

4. 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

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

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

7. 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

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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. Heat to space
Flue gases
Orifice
Spud
Heat Exchanger
Heat from burner
Burners
Manifold
Valve
Cool air from space

19. 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

20. Unit Summary 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
GAS REGULATORS Drop the gas pressure to the proper level
Many regulators can be adjusted
Most regulators are built into the gas valve
LP regulators are located at the tank

21. Unit Summary 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
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

22. Unit Summary 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
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

23. Unit Summary
Heat exchangers – Provide heat transfer between the air from the conditioned space and the combustion gases
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