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

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LAARS HEATING SYSTEMS
3F-1. Pressure Measurement Ports
Air flow enters the venturi through the filter box
and blower assembly. It is pushed through a
converging section and into the throat, where pressure
is reduced substantially. Gas flow is pulled into the
throat through an orifice. The orifice is located
between the throat and the regulator. Air and gas are
combined in the throat and mix thoroughly as they
proceed through the venturi tailpipe to the burner.
Service taps are provided at three places. One is
located on the chamber with the gas connection, this
tap is called the gas plenum tap. The other is located
above the gas plenum tap, this port is called the
venturi inlet tap. The third tap, gas orifice tap, is
located on the red orifice holder directly before the
gas connects to the venturi. These taps have service
plugs in them. Do not remove any of the plastic
fittings or plastic tubing. To evaluate system operation
requires accurate measurement at these taps. An
inclined manometer with a zero to 6 inches water
column range is ideal. Other instruments may be used,
but the “positive/negative” nature of the readings must
be well understood. Gas pressure offset measurements
are at very low levels (0.4" WC), the instrumentation
must be capable of determining it accurately.
3F-2. Venturi Adjustment
Note that an equalizer tube is connected from a
port on the side of the venturi inlet to the port of the
regulator. This is a very important component which
allows the regulator to track air pressure even when
abnormal conditions 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.
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.
Install shutoff valves at the gas orifice (regulator
outlet) tap (red), at the venturi inlet tap and at the gas
plenum tap. Do not remove any of the plastic fittings
or plastic tubing. After installing the shutoff valves,
be certain they are closed.
a. Unfired Venturi Differential Pressure
NOTE: Turn off the main manual gas valve.
The difference in pressure between the venturi
inlet tap and the gas plenum tap (see Figure 15). This
measurement is taken by connecting the positive side
of the manometer to the venturi inlet tap and
connecting the negative side of the manometer to the
gas plenum tap. This measurement is taken with the
boiler not firing. It is a temporary setting used to start
the boiler and check for air flow problems.
b. Gas Offset Pressure
The difference in pressure between the venturi
inlet tap and the outlet of the gas regulator (see
Figure 16). This measurement is taken by connecting
the positive side of the manometer to the venturi inlet
tap and connecting the negative side of the manometer
to the gas orifice tap. This measurement is an
indication of the gas to air ratio and must be
performed while the unit is firing.
Figure 15. Unfired Venturi Differential Pressure.
5.8" WC
Gas
Plenum Tap
Gas
Orifice Tap
Venturi
Inlet Tap
Figure 16. Gas Offset Pressure.
0.4" WC