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Page 28
4F-3. Air Flow Investigation
If gas pressure offset is correctly set but gas
orifi ce differential is abnormal, check air fl ow through
the system per the following sections.
4F-3a. Unfi red Venturi Differential Pres-
sure.
Determine if air fl ow through the system is in
a normal range by operating the system with the gas
valve turned off. 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 37). Turn the gas valve knob to
"Off". Turn the heater on by selecting "Pool" or "Spa"
at the Flex Temp control and adjusting the temperature
setting high. The blower will start immediately and
the manometer will indicate the "unfi red venturi
differential" pressure. This is the pressure difference
created by airfl ow through the venturi. It should be
approximately 4.7" ± 0.3" at sea level. (See Section
4F-5 for other elevations.)
Note that when the heater is operated with the
gas valve off, it will eventually "lock out" because the
ignition control does not sense fl ame. Normally there
will be enough time to obtain the measurement, but if
necessary, additional blower operation can be induced
by turning the Flex-Temp control off and then back on
again.
If the reading is not normal, attempt to correct it
by blower speed adjustment. Blower speed is adjusted
by turning a special screw on the right side of the
motor - at about 4 O'clock. (See Figure 38.) A small
countersunk hole is provided there for access to the
screw, but is covered by a thin silver-colored label.
Remove or puncture the label.
A small fl at-blade screw driver is required. It
must have a 3/32" or 2.5 mm (.098") blade and must
be about 4-1/2" long. A jeweler's screw driver or an
electronics pot adjustment screw driver (with handle
-
+
Figure 37. Unfi red venturi differential measurement.
cut short if necessary) can be used. Insert the screw
driver through the hole on the motor housing. Turn it
clockwise to increase the fl ow and counter-clockwise
to decrease.
If blower speed adjustment results in acceptable
venturi differential, shut off the system at the Flex-
Temp control and return to "Gas orifi ce differential"
measurement per Section 4F-2b. Make a fi nal speed
adjustment as necessary to obtain correct gas orifi ce
differential.
If the system cannot be made to operate normally
by blower speed adjustment, investigate possible fl ow
problems per the following sections.
4F-3b. 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 39. In a
normally operating heater, the pressure will be 0.2 to
0.6" WC less than the ambient pressure. For a model
350 with clean fi lter 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), fl ow is probably restricted by a dirty fi lter or
blockage in the combustion air supply system. Correct
any such problems and verify normal operation by
returning to the venturi system checkout procedures.
Pressure readings will be less if the heater is
located at an elevation signifi cantly above sea level.
4F-3c. Flow in Heater and Vent
If the venturi inlet pressure reading is normal
(or less than normal), fl ow may be blocked within the
heater or in the exhaust venting, or the combustion
blower may be operating below standard. To evaluate
this, connect the negative side of the manometer
Figure 38. Blower Speed Adjustment.
SPEED
ADJUSTMENT
SCREW
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