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combined in the throat and mix thoroughly as they
proceed through the venturi tailpipe to the burner.
Service ports are provided at three places. On the
right side of the venturi an inlet service port is
provided just above the air filter box. Also on the right
side, a throat service port is provided - at the narrowest
section of the venturi casting. A gas pressure port is
provided on the outlet face (right side) of the gas valve.
To evaluate system operation requires accurate
measurement at these ports. A digital manometer,
known to be accurately calibrated, is ideal. Other
instruments may be used, but the “positive/negative”
nature of the readings must be very clear. Also, gas
pressure offset measurements are at very low levels
(0.2” WC) and the instrumentation must be capable of
determining it accurately.
4F-2. Venturi System Checkout
4F-2a. Unfired Venturi Differential
Pressure
Before firing the unit, determine if air flow
through the system is normal 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 33). 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 "unfired venturi
differential" pressure. This is the pressure difference
created by airflow through the venturi. It should be
approximately 4.7" ± 0.3" at sea level. (See Section
4F-5 for other elevations.)
If the reading is normal, proceed to gas pressure
offset measurement per the following section. If not,
investigate air flow per section 4F-3.
Note that when the heater is operated with the gas
valve off, it will eventually "lock out" because the
ignition control does not sense flame. Normally, there
will be enough time to obtain the measurement.
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 regulator to track air
pressure even when abnormal things occur, such as
blockage of the combustion air. Before firing, confirm
that this tube and the venturi pressure switch tubes are
in place and firmly connected.
Turn the heater off at the Flex Temp control.
Turn the gas valve knob back to "On". With the
manometer connections remaining as they were (See
Figure 34) , start the heater at the Flex Temp control.
When the burner fires, the manometer should read 0.2”
± 0.1” WC — i.e., gas pressure is 0.2" WC less than
venturi inlet pressure. If the offset is incorrect, adjust
the gas pressure regulator to the correct pressure. This
can usually be done while the heater is firing, 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 firmly 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
unqualified persons, the gas valve regulator
cap requires a special tool for removal.
Contact your Teledyne Laars representative
about a special tool for this cap.
4F-2c. Gas Orifice Differential
Turn off the heater, disconnect the manometer
tubes and replace the plug at the venturi inlet.
Reconnect the tubes with the positive side of the
manometer connected to the gas valve outlet and the
negative side to the venturi throat as shown in
Figure 35.
Turn on the heater. The manometer will read zero
until the gas valve opens. Thereafter it should read 4.0”
± 0.3” WC. See the sections on Air Flow Investigation
and High Elevation Operation if this is not the case.
Figure 32. Venturi Flow System Components.
VENTURI
THROAT
SERVICE
PORT
VENTURI
INLET
SERVICE
PORT
GAS VALVE
SERVICE
PORT