SERVICING
39
WARNING
Line voltage now present
6. Place unit in heating cycle, measure current draw of
ignitor during preheat cycle. The steady state current
at 120V is 0.37 to 0.68 amps.
7. After checking and/or replacing of hot surface ignitor,
reinstall burner compartment door and verify proper
unit operation.
CHECKING FOR FLASHBACK
Flashback will also cause burning in the burner venturi, but
is caused by the burning speed being greater than the gas-
air flow velocity coming from a burner port.
Flashback may occur at the moment of ignition, after a
burner heats up or when the burner turns off. The latter is
known as extinction pop.
Since the end results of flashback and delayed ignition
can be the same (burning in the burner venturi) a definite
attempt should be made to determine which has occurred.
If flashback should occur, check for the following:
1. Improper gas pressure - adjust to proper pressure.
2. Check burner for proper alignment and/or replace burner.
3.
Improper orifice size - check orifice for obstruction.
CHECKING PRESSURE SWITCH
The pressure control is a safety device to prevent the
combustion cycle from occurring with inadequate venting
caused by a restricted or blocked vent pipe.
WARNING
High Voltage!
Disconnect all power before servicing
or installing this unit. Multiple power
sources may be present. Failure to do
so may cause property damage, personal
injury or death.
1. Remove burner compartment door to gain access to
pressure switch(es).
2. Remove wires from the pressure switch(es) electrical
terminals.
3. Remove the pressure control hose from the control and
interconnect with an inclined manometer as shown in
the following figure.
4. With an ohm meter connected across the pressure
switch terminals and with the inducer running, the switch
should close and the ohm meter should show a complete
circuit across the pressure switch. If the switch is not
closed, compare the negative pressure to the closing
point specified for the particular switch. Either the switch
is defective or the inducer/venting system is inadequate.
-.
OPEN TO
ATMOSPHERE
HOSE TO INDUCED
DRAFT BLOWER TAP
1/4" TEE
INDUCED DRAFT BLOWER
PRESSURE SWITCH
PRESSURE SWITCH
HOSE
INDUCED
DRAFT
BLOWER
*
*
*
*
*
*
*
M
1
2
3
P
C
ON
O
F
F
COIL COVER
PRESSURE SWITCH
Blower Pressure Switch
Negative Pressure Measurement
HIGH ALTITUDE APPLICATION (USA)
When furnaces are installed at high altitude, the
appropriate High Altitude Orifice or Pressure Switch
Kit must be applied. This is required due to the natural
reduction in the density of both the gas fuel and
combustion air as altitude increases. The High Altitude
Orifice Kit will provide the proper design certified input rate
within the specified altitude range.
High Altitude Orifice or Pressure Switch Kits are purchased
according to the installation altitude and usage of
either natural or propane gas. Refer to the Technical
Manual or product Specification Sheet for a tabular
listing of appropriate altitude ranges and corresponding
manufacturer’s high altitude (Natural or Propane Gas)
orifice or pressure switch kits.
Do not derate the furnace by adjusting the manifold
pressure to a lower pressure than specified on the furnace
rating plate. The combination of the lower air density and
a lower manifold pressure will prohibit the burner orifice
from drawing the proper amount of air into the burner. This
may cause incomplete combustion, flashback, and possible
yellow tipping.
In some areas the gas supplier may artificially derate the
gas in an effort to compensate for the effects of altitude.
If the gas is artificially derated the appropriate orfice size
must be determined based on the BTU/ft3 content of
the derated gas and the altitude. Refer to the National
Fuel Gas Code, NFPA 54/ANSI Z223.1, and information
provided by the gas supplier to determine the proper orifice
size.
CHECKING FOR DELAYED IGNITION
Delayed ignition is a delay in lighting a combustible mixture
of gas and air which has accumulated in the combustion
chamber.
