1 HVACR114 – Electrical for Gas Heat Sequence of Operation

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HVACR114 – Electrical for Gas Heat

HVACR114 – Electrical for Gas Heat

Sequence of OperationSequence of Operation

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Sequence of OperationSequence of Operation

• The sequence of operation is the order in which the call for heat, the ignition, the heating (or blower operation) and the shutdown work.

• Depends on the type of the furnace.• Every manufacture may have different

timings, but still the same concept.

• The sequence of operation is the order in which the call for heat, the ignition, the heating (or blower operation) and the shutdown work.

• Depends on the type of the furnace.• Every manufacture may have different

timings, but still the same concept.

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Standing PilotStanding Pilot

• A standing pilot system is one that has a constant pilot light.

• If the pilot light goes out it must be manually reset.

• The heat from the pilot light creates voltage on a thermocouple which keeps the gas valve operational.

• A standing pilot system is one that has a constant pilot light.

• If the pilot light goes out it must be manually reset.

• The heat from the pilot light creates voltage on a thermocouple which keeps the gas valve operational.

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• Sequence:– Thermostat calls for heat.– Sends 24 Volts to the gas valve.– The gas valve opens and allows gas into the

burner manifold.– The gas ignites and starts heating up the

furnace.

• Sequence:– Thermostat calls for heat.– Sends 24 Volts to the gas valve.– The gas valve opens and allows gas into the

burner manifold.– The gas ignites and starts heating up the

furnace.

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• Sequence:– The furnace heats up to the fan cut-in point

and the fan control turns on the blower. – This provides heat to the space.– Once the thermostat opens the R-W circuit

the gas valve closes.– The blower continues to run and cools the

furnace.

• Sequence:– The furnace heats up to the fan cut-in point

and the fan control turns on the blower. – This provides heat to the space.– Once the thermostat opens the R-W circuit

the gas valve closes.– The blower continues to run and cools the

furnace.

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• Sequence:– Once the temperature of the furnace has

dropped the fan control opens and the blower shuts down.

• High Limit:– If the high limit opens the gas valve shuts down

and the blower continues to run.

• Sequence:– Once the temperature of the furnace has

dropped the fan control opens and the blower shuts down.

• High Limit:– If the high limit opens the gas valve shuts down

and the blower continues to run.

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Spark IgnitionSpark Ignition

• Spark Ignition, or intermitent pilot systems are when the pilot light is shut off after every call for heat.

• On most intermitent pilot systems there is also a draft inducer, but not all.

• Spark Ignition, or intermitent pilot systems are when the pilot light is shut off after every call for heat.

• On most intermitent pilot systems there is also a draft inducer, but not all.

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• Atmospheric Burner:– Thermostat Calls for heat– Sends 24 Volts to the control module– Control Module sends a 10000 volt spark to pilot

assembly– Control Module sends 24 Volts to the PV

terminals of the gas valve, opening the valve.

• Atmospheric Burner:– Thermostat Calls for heat– Sends 24 Volts to the control module– Control Module sends a 10000 volt spark to pilot

assembly– Control Module sends 24 Volts to the PV

terminals of the gas valve, opening the valve.

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• Atmospheric Burner (cont)– Pilot ignites and heats up the pilot sensor,

creating DC current for flame rectification.– Once control module senses flame it sends 24

volts to the MV terminals of the gas valve, opening the main valve.

• Atmospheric Burner (cont)– Pilot ignites and heats up the pilot sensor,

creating DC current for flame rectification.– Once control module senses flame it sends 24

volts to the MV terminals of the gas valve, opening the main valve.

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• Atmospheric (continued)– Once the call for heat is complete the thermostat

opens the R-W terminals and the control module is shut off.

– This in turn de-energizes the MV and the PV terminals on the gas valve.

• Atmospheric (continued)– Once the call for heat is complete the thermostat

opens the R-W terminals and the control module is shut off.

– This in turn de-energizes the MV and the PV terminals on the gas valve.

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Induced Draft – Spark IgnitionInduced Draft – Spark Ignition

• The induced draft type of intermittent pilot uses a draft inducer motor to move the combustion air through the furnace.

• This draft inducer must be running first.

• The induced draft type of intermittent pilot uses a draft inducer motor to move the combustion air through the furnace.

• This draft inducer must be running first.

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• Thermostat calls for heat, sends 24 volt to a Draft Inducer Relay which turns on draft inducer motor.

• Draft inducer motor starts and closes a low pressure switch (proves draft).

• Low pressure switch sends 24 volts to the control module.




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• Control Module sends a 10000 volt spark to pilot assembly

• Control Module sends 24 Volts to the PV terminals of the gas valve, opening the valve.

• Control module uses flame rectification to turn sense the presence of the pilot

• Control Module sends a 10000 volt spark to pilot assembly

• Control Module sends 24 Volts to the PV terminals of the gas valve, opening the valve.

• Control module uses flame rectification to turn sense the presence of the pilot

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– Pilot ignites and heats up the pilot sensor, creating DC current for flame rectification.

– Once control module senses flame it sends 24 volts to the MV terminals of the gas valve, opening the main valve.

– Pilot ignites and heats up the pilot sensor, creating DC current for flame rectification.

– Once control module senses flame it sends 24 volts to the MV terminals of the gas valve, opening the main valve.

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• Once the call for heat is complete the thermostat opens the R-W terminals and the draft inducer motor is shut off.

• The pressure switch opens and breaks current flow to the 24 volt control module and gas valve

• This in turn de-energizes the MV and the PV terminals on the gas valve.



• This in turn de-energizes the MV and the PV terminals on the gas valve.

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Induced Draft – Direct (hot surface ignition)Induced Draft – Direct (hot surface ignition)• Direct or Hot Surface Ignition types use a

glowing, red, hot surface in the main gas stream to ignite the burners.

• There is no pilot light.• The ignition sequence is very much like the

spark ignition, minus pilot.• There is ALWAYS a draft inducer on these

models.

• Direct or Hot Surface Ignition types use a glowing, red, hot surface in the main gas stream to ignite the burners.

• There is no pilot light.• The ignition sequence is very much like the

spark ignition, minus pilot.• There is ALWAYS a draft inducer on these

models.

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Induced Draft – Direct (hot surface ignition)Induced Draft – Direct (hot surface ignition)• Thermostat calls for heat, sends 24 volt to a

Draft Inducer Relay which turns on draft inducer motor.






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Induced Draft – Direct (hot surface ignition)Induced Draft – Direct (hot surface ignition)• The control modules sends 120 Volts to the

hot surface igniter causing it to heat up and glow.

• This takes around 90 to 120 seconds.

• The control modules sends 120 Volts to the hot surface igniter causing it to heat up and glow.

• This takes around 90 to 120 seconds.

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Induced Draft – Direct (hot surface ignition)Induced Draft – Direct (hot surface ignition)• The control module sends 24 volt to the MV

terminals of the gas valve and gas flows into the burner manifold.

• The control module has the hot surface igniter on one side of the burners, and a sensor on the furthest burner tube.

• The control module sends 24 volt to the MV terminals of the gas valve and gas flows into the burner manifold.

• The control module has the hot surface igniter on one side of the burners, and a sensor on the furthest burner tube.

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Induced Draft – Direct (hot surface ignition)Induced Draft – Direct (hot surface ignition)• Once the gas ignites the flame sensor on

the other side “sees” this flame and shuts down the HSI and continues to keep the MV open.

• If no flame is sensed in 3-5 seconds the ignition sequence terminates, closing the gas valve and either locking out or trying again.

• Once the gas ignites the flame sensor on the other side “sees” this flame and shuts down the HSI and continues to keep the MV open.

• If no flame is sensed in 3-5 seconds the ignition sequence terminates, closing the gas valve and either locking out or trying again.

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Induced Draft – Direct (hot surface ignition)Induced Draft – Direct (hot surface ignition)• Once the call for heat is complete the

thermostat opens the R-W terminals and the draft inducer motor is shut off.


• This in turn de-energizes the MV terminals on the gas valve.



• This in turn de-energizes the MV terminals on the gas valve.

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Induced Draft – Direct (hot surface ignition)Induced Draft – Direct (hot surface ignition)• Hot surface ignition systems do not need to

have a pre-purge as the 90 to 120 seconds of igniter warm up solves this.

• Some systems will have a pre-purge and then stop the inducer motor and during the warm up. We will talk more about those in the future.

• Hot surface ignition systems do not need to have a pre-purge as the 90 to 120 seconds of igniter warm up solves this.

• Some systems will have a pre-purge and then stop the inducer motor and during the warm up. We will talk more about those in the future.

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1 HVACR114 – Electrical for Gas Heat Sequence of Operation