SERVICING
39
.
ID Blower Pressure Switch Negative Pressure Measurement
(80% Upflow Single-Stage Furnace Shown, Counterflow Similar)
HIGH ALTITUDE APPLICATION (USA)
Check your furnace rating plate to determine its rated al-
titude (either 4500 or 5500 ft). Do not attempt to increase
the firing rate by changing orifices or increasing the mani
-
fold pressure below 4500/5500 feet. This can cause poor
combustion and equipment failure. High altitude installations
above 4500/5500 feet may require both a pressure switch
and an orifice change. These changes are necessary to
compensate for the natural reduction in the density of both
the gas fuel and the combustion air at higher altitude.
For installations above 4500/5500 feet, please refer to your
distributor for required kit(s). Contact the distributor for a
tabular listing of appropriate manufacturer’s kits for propane
gas and/or high altitude installations. The indicated kits must
be used to insure safe and proper furnace operation. All
conversions must be performed by a qualified installer, or
service agency.
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 orifice size must
be determined based on the BTU/ft
3
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.
Furnace design makes this extremely unlikely unless safety
controls have been by-passed or tampered with. Never by-
pass or alter furnace controls.
If delayed ignition should occur, the following should be
checked:
1. Improper gas pressure - Adjust to proper pressure
(See
CHECKING GAS PRESSURE
).
2. Improper burner positioning - burners should be in lo-
cating slots, level front to rear and left to right.
3. Carry over (lighter tube or cross lighter) obstructed -
clean.
4.
Main burner orifice(s) deformed, or out of alignment to
burner - replace.
CHECKING INTEGRATED IGNITION
CONTROL BOARDS
NOTE:
Failure to earth ground the furnace, reversing the
neutral and hot wire connection to the line (polarity), or a
high resistance connection in the neutral line may cause the
control to lockout due to failure to sense flame.
WARNING
To avoid the risk of electrical shock, wiring to the unit must
be properly polarized and grounded. Disconnect power be
-
fore servicing listed below.
The ground wire must run from the furnace all the way back
to the electrical panel. Proper grounding can be confirmed
by disconnecting the electrical power and measuring resis-
tance between the neutral (white) connection and the burner
closest to the flame sensor. Resistance should be less than
10 ohms.
The ignition control is a combination electronic and electro-
mechanical device and is not field repairable. Complete unit
must be replaced.
