SERVICING
51
Two stage gas valves always require 24 volts between com-
mon and low fire (main coil) to open. Also, the furnace front
cover pressure switch is wired in series with the low (main)
solenoid of the gas valve. In the event of a non functioning
gas valve, always check the front cover pressure switch.
Also see section S-307 on Checking Gas Pressure and
section S-310 on Checking Pressure Switches
S-305 CHECKING MAIN BURNERS
B
URNERS
Burners have been redesigned for 34.5" chassis furnaces.
Overall length and width dimensions remain the same as
40" model burners. The burners used 34.5" models have
burner head insert with larger diameter center hole and a
larger number of surrounding holes.
The main burners are used to provide complete combustion
of various fuels in a limited space, and transfer this heat of
the burning process to the heat exchanger.
Proper ignition, combustion, and extinction are primarily
due to burner design, orifice sizing, gas pressure, primary
and secondary air, vent and proper seating of burners.
34.5" Burner
WARNING
D
ISCONNECT
ALL
G
AS AND
E
LECTRICAL
P
OWER
S
UPPLY.
In checking main burners, look for signs of rust, oversized
and undersized carry over ports restricted with foreign ma-
terial, etc, burner cross-over slots should not be altered in
size.
S-306 CHECKING ORIFICES
*MEC96 model furnaces have factory installed #45 natural
gas orifices (except *MEC96030 which has #50).
The only time resizing is required is when a reduction in
firing rate is required for an increase in altitude or a furnace
is being converted for use with LP gas.
Orifices should be treated with care in order to prevent dam-
age. They should be removed and installed with a box-end
wrench in order to prevent distortion. In no instance should
an orifice be peened over and redrilled. This will change the
angle or deflection of the vacuum effect or entraining of pri-
mary air, which will make it difficult to adjust the flame prop-
erly. This same problem can occur if an orifice spud of a
different length is substituted.
WARNING
D
ISCONNECT
ALL
G
AS AND
E
LECTRICAL
P
OWER
S
UPPLY.
1. Check orifice visually for distortion and/or burrs.
2. Check orifice size with orifice sizing drills.
A
GAS
STREAM B
The length of Dimension "A" determines the angle of Gas
Stream "B".
DENT OR
BURR
GAS
STREAM B
A dent or burr will cause a severe deflection of the gas stream.
S-307 CHECKING GAS PRESSURE
Gas Supply Pressure Measurement
G
AS
P
RESSURE
T
EST
The line pressure supplied to the gas valve must be within the
range specified below. The supply pressure can be measured
at the gas valve inlet pressure tap or at a hose fitting installed in
the gas piping drip leg. The supply pressure must be measured
with the burners operating. To measure the gas supply pres-
sure, use the following procedure.
