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HVAC121 – Gas Heat
Flame Rectification
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Flame Rectification
• Every automatic (intermittent) ignition system needs to
be able to sense when flame is present or not.
• Flame rectification is this sensing of the flame.
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Flame Rectification
• Once the ignition process starts the spark or hot surface
igniter ignites the flame.
• Once the flame is ignited the flame is a conductor.
• A little AC voltage is sent through the flame sensor rod.
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Flame Rectification
• The flame then converts this AC voltage to DC and
sends it to ground.
• The control senses this current flow and knows that
there is a flame.
• This is called flame rectification.
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Flame Rectification
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Flame Rectification
• The flow of current is sensed by the ignition
control.
– This verifies that the flame is lit
– When the pilot flame is verified the main burner is
allowed to open.
– The flame goes out, the DC current stops flowing to
ground.
– The control senses the stop of the current flow and
closes the main burner.
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Flame Rectification
• When the flame is verified the spark in the spark
ignition system is shut down.
• When the flame is verified the HSI in the Hot Surface
Ignition System is shut down.
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Troubleshooting
• When the ignition process fails on a spark ignition
system there are two parts to check.
– The sensor / Spark wire
– The ground wire
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Troubleshooting
• Spark Wire:
– Just like a automotive spark wire make sure all contact points
are good and make sure that there are no frayed or missing
pieces on the insulation.
– Make sure all connections are tight.
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Troubleshooting
• Ground wire:
– Replace the ground wire.
– Run a new ground wire from the control board, to the burner
assembly and back to a true frame ground of the equipment.
– Make sure the equipment is properly grounded.
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Troubleshooting
• Hot surface Ignition:
– Check the wire between the control and the flame sensor.
– Replace the ground wire, again to burner assembly and
control and frame ground.
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Troubleshooting
• Put a meter in SERIES with the ground terminal on the
control and the burner ground wire.
• The meter should be set to Amps and will show around
2 micro amps when the flame is present.
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Troubleshooting
• If the amperage does not show try cleaning or replacing
the flame sensor.
• If flame sensor is clean or replaced and the ground wire
and terminals are good then you have a bad control.
• Controls can not be repaired, they must be replaced.
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Thermocouples
• The thermocouple is constructed of two dissimilar
metals joined at one end.
• When heat is applied to the joint a voltage is created.
• A thermocouple will produce 30mv
• Thermopiles or Power piles can produce either 250mv
or 750mv
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Thermocouples
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Thermocouples
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Thermocouples
• A thermocouple is used in some furnaces to sense pilot
flame.
• The voltage created by the thermocouple is enough to keep
the pilot coil energized.
• The pilot coil is like a solenoid that will stay pulled in after
being moved and voltage is applied to the coil by the
thermocouple.
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Thermocouple
• If the pilot flame goes out the thermocouple cools and
the voltage drops to 0.
• The pilot coil can not stay energized and the gas valve
closes.
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Thermopiles
• What is a thermopile?
– Twenty-Five thermocouples put in series.
– This greatly increases the DC voltage output increasing
current available.
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Thermopiles
• Voltage Output:
– Single Thermocouple
• 25 to 30 Millivolts
– 10 Thermocouples in series
• 250 to 300 Millivolts
– 25 Thermocouples in series
• 625 to 750 Millivolts
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Thermopiles
• Pilot-stat (thermopile) generates enough voltage to:
– Hold open the pilot valve
– Energize the main gas valve
• The holding circuit operates the same as the
thermocouple circuit.
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Thermopiles
• The thermostat is wired in series with the main gas
valve.
• When the space temperature drops, the thermostat
closes and energizes the main gas valve.
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Thermopiles
• The thermostat with a millivolt system differs from the
24 volt system.
– The heat anticipator operates on a lower voltage.
– A milli-volt anticipator is not adjustable
• The 24 volt and the milli-volt thermostat will look the
same but is not interchangeable.
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