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Hi-E2
4F-2a. Gas Orifi ce Differential
This measurement indicates how much "pull"
is available to move gas through the orifi ce. Turn off
the heater and connect the manometer tubes with the
positive side connected to the gas valve outlet and the
negative side to the venturi throat as shown in Figure
34. Turn on heater. The manometer will read zero
until the gas valve opens, and then it will indicate the
gas orifi ce differential. The gas orifi ce differential
should be 4.0" ± 0.3" WC for a unit at sea level.
See section on High Elevation Operation for other
elevations.
Note the reading and turn off the heater at
the Flex Temp control. Then go on to the next
measurement - gas pressure offset.
4F-2b. Gas Pressure Offset
Note that an equalizer tube is connected from
a port on the front of the venturi inlet to the vent
of the gas valve regulator. This is a very important
component which allows the reg u la tor to track air
pressure even when ab nor mal things occur, such as
blockage of the combustion air. Before fi ring, confi rm
that this tube and the venturi pressure switch tubes are
in place and fi rmly connected.
Connect the positive side of the manometer to the
venturi inlet port, which is at the bottom on the right
side. Connect the negative side of the manometer to
the outlet pressure port of the gas valve. (See Figure
35.) Turn the heater on. Until the heater fi res, the
manometer will indicate several inches of water
pressure, but after the gas valve opens, it will indicate
the gas pressure offset, which should be 0.2" ± 0.1"
WC - i.e. the gas pressure should be 0.2" WC less than
the pressure at the venturi inlet.
If the offset is incorrect, adjust the gas pressure
regulator to the correct pressure. This can usually be
done while the heater is fi ring, but the burner may go
off when the regulator seal cap is removed. In any
case, the offset pressure measurement must be read
when the heater is operating and the seal cap is fi rmly
in place. Note that a special tool is required to remove
the regulator cap.
Figure 36 shows the gas pressure offset reading
when a U-Tube manometer is used.
WARNING
In order to discourage incorrect adjustment by
unqualifi ed persons, the gas valve regulator cap
requires a special tool for removal. Contact your
representative about a special tool for this cap.
WARNING
When servicing is complete be sure to replace pres-
sure service port plugs and assure that pressure
switch and equalizer tube connections are unaltered
and fi rmly in place.
-
+
Figure 34. Gas orifi ce differential measurement.
-
+
Figure 35. Gas pressure offset measurement.
To Venturi
Inlet Service
Port
0.2" WC
Figure 36. Gas pressure offset measurement with
U-tube manometer.
Page 27
To Gas Valve
Service Port
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