Download HOT SURFACE IGNITION: SILICON CARBIDE

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HOT SURFACE IGNITION: SILICON CARBIDE
PURPOSE: The hot surface ignition system uses a hot surface igniter (Fig.1) to
provide the ignition
source to light the
burners. This type of
ignition lights the main
burner flame, and then
proves the flame is
present by flame
rectification. The main
gas valve solenoid will
remain energized as long
as there is a call for heat
and adequate flame
current is being sensed in fig. 1
the burner compartment.
The igniter used with these systems is called a Silicon Carbide igniter.
HOW THE CIRCUIT WORKS.
When a call for heat occurs, the IFC (fig. 2),
energizes the induced draft motor for a prepurge period. After the pre-purge cycle, the
IFC begins an ignition warm up period.
During the ignition warm up period, the IFC
sends 115 volts to the igniter, the igniter
begins to glow. After a set period of time,
the IFC energizes the main solenoid of the
gas valve and attempts a trial for ignition. If
the attempt at ignition is successful, the IFC
will prove the flame by flame rectification.
If the trial for ignition ends without
adequate flame current being detected, the
igniter and gas valve are de-energized and
another trial for ignition sequence begins. If
the IFC keeps failing to establish flame, the
system will lockout.
The flame sensing rod is not part of the
igniter assembly. The independent flame
fig. 2
rod is located on the opposite side of the
burner from the igniter assembly and unless there is a system malfunction, the
flame rod will have an AC potential present during both the ON and OFF cycles.
With the AC potential present at the flame rod and the flame making a conductive
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path between the rod and ground, flame rectification occurs. If at any time during
the call for heat, If flame current drops below the minimum level required by the
ignition control, the control will immediately de-energize the main gas valve. The
flame detection response time of various controls is usually less than one second.
This means that if flame current is lost, the control will detect the loss of flame
and respond VERY RAPIDLY to shut off the gas flow to the main burners by deenergizing the gas valve solenoid.
HOT SURFACE IGNITION SYSTEM SILICON CARBIDE CHECK PROCEDURE
When servicing a hot surface ignition, please use caution to avoid contact with the
silicon carbide element. Oil from your finger tips will create a “hot spot” on the
element and will compromise the integrity of the ignition element. (It will cause
the element to crack and be ruined)
WHAT TOOLS YOU WILL NEED:
MULTIMETER
PROCEDURES
THE FOLLOWING PROCEDURE ASSUMES THAT GAS IS PRESENT AND
THE
IGNITOR IS FAILING TO GLOW
If the igniter element fails to glow, the carbide igniter element may be cracked,
the wires connections between the IFC and the igniter may be bad, or the IFC
board may be defective.
1. Disconnect the power to the furnace.
2. Inspect the igniter for obvious defects such as a cracked or damaged carbide
tip, frayed or damaged wiring, bad or loose wiring connections at terminal plug
connectors. Replace the igniter
assembly and /or repair or replace the
plug connections, if the above
conditions are found.
3. Using an ohmmeter, measure the
resistance through the hot surface
igniter at room temperature. (fig. 3) If
infinite resistance is measured, the
element is open (cracked, broken) and
must be replaced. Depending on the
manufacturer, the igniter should read
fig. 3
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between 20 and 300 ohms (SEE MANUFACTURER SPECIFICATIONS FOR
ACTUAL RANGES)
4. Using an ohmmeter, check for
continuity through the wires through
which the 115 volt signal is sent from the
IFC to the igniter assembly. (fig. 4) You
should read no resistance. Infinite
resistance indicates the wire is open (the
electrical path is broken) and the wire
must be replaced.
5. Restore power to the furnace and
initiate a call for heat. Use a voltmeter to
measure voltage from the IFC to the
fig. 4
igniter assembly at the connector plug
(Fig. 5) Once the pre-purge cycle is completed, the IFC should send 115 volts to
the igniter. If 115 volts is present and the igniter fails to become hot, the igniter
assembly is defective and must be replaced. If 115 volts are not present, the IFC
board is defective and must be replaced.
.
fig. 5