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Page 27
WARNING
When servicing is complete be sure to replace
pressure service port plugs and assure that
pressure switch and equalizer tube
connections are unaltered and firmly in place.
4F-3. Air Flow Investigation
If unfired venturi differential or gas orifice
differential are abnormal, check air flow through the
system per the following sections.
4F-3a. Combustion Air Flow
Begin by connecting the negative side of the
manometer to the venturi inlet and leaving the positive
side open to ambient, as shown in Figure 37. In a
normally operating heater, the pressure will be 0.2 to
0.6" WC less than the ambient pressure. The reading
for a model 220 with clean filter and no combustion air
duct will be near the low end of this range. For a model
350 with clean filter and no combustion air duct, the
reading will be near the middle. If combustion air is
ducted to the heater, the reading will be near the high
end. If an excessive reading is observed (i.e. more
negative), flow is probably restricted by a dirty filter
or blockage in the combustion air supply system.
Figure 33. Unfired Venturi Differential Measurement.
+
-
Figure 35. Gas Orifice Differential Measurement.
Figure 34. Gas Pressure Offset Measurement.
Figure 36. Gas Pressure Offset Measurement with
U-Tube Manometer
To Venturi
Inlet Service
Port
To Gas Valve
Service Port
0.2" WC
-
+
4.70
+
-
Figure 37. Venturi Inlet Pressure Measurement.