Laars Mighty Max VW Series, Installation, Operation And Maintenance Instructions

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Page 18
LAARS HEATING SYSTEMS
engine (see Figure 13): a venturi pulls the gas into
the combustion air stream. In this system, changes in
combustion air flow automatically change the gas
flow.
The flow of air through the venturi creates a
pressure difference. At the narrowest point of the
venturi, the throat, high velocity creates a low
pressure condition which pulls gas in through an
orifice.
For a correct gas/air ratio, the gas pressure must
be the same as the air pressure, but with a slight
negative offset. A special gas regulator (called a
“negative pressure regulator”) which has an equalizer
tube connected to the venturi inlet, maintains the
required gas pressure.
When the system is operating, a combustion fan
forces air into the venturi, creating pressure at the
inlet. The gas regulator sets gas pressure, and gas is
pulled through the orifice. The sizes of the venturi
throat and gas orifice are factory set to provide the
correct air/gas ratio.
3C-2. Venturi Adjustment
The field checkout involves measuring gas and
venturi pressures, and observing the flame through the
sight glass. If necessary, the gas input rate can be
measured by timing the gas meter.
Use a single, inclined manometer or digital
manometer with a 4.0 inch water column range. Install
shutoff valves at the gas orifice (regulator outlet) tap
(red), at the venturi inlet tap (blue) and at the venturi
throat tap (yellow). After installing the shutoff valves,
be certain they are closed.
“Low” Side of
Manometer
“High” Side of
Manometer
Venturi Inlet
Tap (Blue)
Venturi Throat
Tap (Yellow)
Gas Orifice
Tap (Red)
Figure 14. Measurement of Venturi Throat Pressure
Differential.
1. With the heater off, connect the positive side of
the manometer to the shutoff valve on the
venturi inlet tap (blue). Open the shutoff valve.
2. Loosen the nut on the blower damper to allow
for adjustment. Turn the boiler on so that the
blower is running and the heater is not firing. In
this unfired condition, adjust the damper until
the venturi inlet pressure (blue tap) is 1.2 inches
water column.
3. Approximately 40 seconds after the blower starts
the gas valves will open. The heater is now
firing. If the heater is not running, check all
manual gas valves and heater safety devices.
Ensure proper gas supply pressures according to
the table in Section 2.
4. Now that the heater is firing, use the blower
damper to readjust the venturi inlet pressure
according to the installation’s altitude in Table
10 (+1.6"w.c. at sea level).
Elevation Venturi Inlet Gas Throat
Ft. Pressure Pressure Differential
(Blue Tap) Offset Pressure
“WC H
2
0” “WC H
2
0” “WC H
2
0”
SEA LEVEL +1.6 +0.4 +2.6
1000 +1.5 +0.4 +2.5
2000 +1.5 +0.4 +2.4
3000 +1.4 +0.4 +2.3
4000 +1.4 +0.3 +2.2
5000 +1.3 +0.3 +2.2
6000 +1.3 +0.3 +2.1
7000 +1.2 +0.3 +2.0
8000 +1.2 +0.3 +1.9
9000 +1.1 +0.3 +1.9
10000 +1.1 +0.3 +1.8
Table 10. Venturi Pressure Settings.
Air
Equalizer
Tube
Negative
Pressure Regulator
Gas
Gas
Orifice
Air/Gas
Mixture
Figure 13. Typical Venturi System.