Transcript Slide 1

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HVACR114 – Electrical for Gas
Heat
Sequence of Operation
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Sequence 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.
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Standing 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.
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Standing Pilot
• 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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Standing Pilot
• 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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Standing Pilot
• 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 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.
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Spark Ignition
• 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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Spark Ignition
• 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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Spark Ignition
• Atmospheric (continued)
– Once the call for heat is complete the thermostat opens the RW terminals and the control module is shut off.
– This in turn de-energizes the MV and the PV terminals on the
gas valve.
Induced 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.
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Induced Draft – Spark Ignition
• 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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Induced Draft – Spark Ignition
• 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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Induced Draft – Spark Ignition
– 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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Induced Draft – Spark Ignition
• 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.
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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.
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Induced Draft – Direct (hot surface
ignition)
• 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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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.
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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.
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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.
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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.
• The pressure switch opens and breaks current flow to
the 24 volt control module and gas valve
• This in turn de-energizes the MV terminals on the gas
valve.
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Induced Draft – Direct (hot surface
ignition)
• Hot surface ignition systems do not need to have a prepurge 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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