2 User Manual

3
Troubleshooting
28
Service Manual
Gas manifold pressure adjustment
procedure
Figure 3-1_Front Chamber Pressure Measurement
5. Turn the power switch to the “ON” position.
6. Set the thermostat to call for heat. The fan will start.
7. While the combustion air fan is running for the prepurge
cycle and the pressure in the front chamber stabilizes,
record the pressure in the front chamber. This pressure will
be used to calculate the net manifold pressure.
8. Turn the power switch to the “OFF” position.
9. Remove the manometer or magnahelic hose from the front
chamber barb and reinstall the hose from the air pressure
switch to the barb.
Figure 3-2_Manifold Gas Pressure Measurement
NOTICE
The gas valve is referenced to the fan
pressurized chamber by a hose connected
from the vent of the gas valve regulator to
the chamber pressure tap located on the left
side of the jacket. Reference the figures in
this section for component location and
connection points for pressure
measurement. The referenced chamber
pressure must be subtracted from the
manifold pressure to obtain actual net
manifold pressure for normal operation. A
manometer or magnahelic gauge legible in
0.1" increments up to 10" water column is
required to check and adjust the manifold
pressure. The regulator cover screw on the
gas valve must be in place and tight for the
appliance to operate properly.
1. Turn the power switch to the “OFF” position.
2. Remove the top front jacket panel.
3. Turn all gas valve knobs and the manual gas cock to the
“OFF” position.
4. Locate the air pressure switch located on the left side
panel. Trace the hose from the air pressure switch to the
chamber pressure barb located in the top of the upper
chamber. Remove the hose from the chamber pressure
barb and connect the hose from the manometer or
magnahelic to this barb to measure the air pressure in
the front chamber.
10. Remove the 1/8" hex plug, located on the “outlet” side of
the gas valve and install a fitting suitable to connect to a
manometer or magnahelic gauge (see FIG. 3-2). Minimum
range of scale should be up to 5" w.c. for natural gas and
10" w.c. for propane gas.
11. A reference pressure hose is factory installed from the
barbed connection on the gas valve regulator to the fan
pressurized front chamber. All manifold pressure
measurements made with the reference hose in place will
be the sum of the net manifold pressure and the air
pressure in the front chamber.
12. Turn the power switch to the “ON” position.
13. Turn all gas valve knobs to the “ON” position.
14. Set the thermostat to call for heat.
15. Before the valve opens, the manometer or magnahelic
gauge will read a positive air pressure of 1.4" w.c. to
1.8" w.c. in the jacket as supplied by the combustion air fan
in the pressure cycle. There is no gas flow at this point.
This air pressure reading is normal.