LAARS Mighty Max VW (Sizes 320M - 400M) - Installation, Operation and Maintenance Instructions - Manual (Page 21)

Mighty Max Volume Water Heater Page 21
By setting the gas offset pressure and gas orifice
differential pressure according to Table 10, the correct
input rate and gas to air ratio is achieved.
3.6.3 Venturi Setup Procedure -
Natural Gas
1. Loosen the nut on the blower shutter to allow for
adjustment. Turn the heater on so that the blower
is running and the heater is not firing. Measure
the unfired venturi differential pressure. In this
unfired condition, adjust the shutter until the
unfired venturi differential pressure is according
to Table 10, “Unfired Venturi Differential” (5.8
± .3 inches wc at sea level). If this pressure range
can not be achieved, check for blockage in the
combustion air inlet, boiler and venting system.
If there is no obvious cause contact a qualified
Laars service technician.
2. 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
Table 4 in Section 2.
GAS GAS ORIFICE UNFIRED
ELEVATION, OFFSET DIFFERENTIAL VENTURI
FT PRESSURE PRESSURE DIFFERENTIAL
inch W.C. inch W.C. inch W.C.
SEA LEVEL +0.4 +4.0 +5.8
2000 +0.4 +3.7 +5.3
4000 +0.4 +3.4 +4.9
6000 +0.4 +3.2 +4.6
8000 +0.4 +2.9 +4.2
10000 +0.4 +2.7 +3.9
Table 10. Venturi Pressure Settings - Natural Gas.
3. Measure the gas offset pressure. Using the
regulator only, adjust the gas offset pressure
according to the installation’s altitude in Table 10
(+0.4 inches wc. at sea level). REPLACE THE
REGULATOR CAP BEFORE TAKING GAS
PRESSURE READINGS. Turn the regulator
screw clockwise to decrease the gas offset
pressure, turn the regulator screw
counterclockwise to increase the offset.
4. Using the toggle switch, turn the heater off. Turn
the heater back on and check the gas offset
pressure while the heater is firing. If the gas
offset pressure is not according to Table 10,
adjust the regulator as needed.
5. Measure the gas orifice differential pressure.
This pressure must be adjusted according to
Table 10 (4.0 ± .2 inches wc at sea level). Use
the blower shutter to adjust the gas orifice
differential.
6. By adjusting the gas orifice differential, the gas
offset pressure will change. Therefore you must
repeat steps 3-5 until the gas offset and gas
orifice differential pressures are according to
Table 10.
7. After setting all pressures, turn the heater off and
replace each shutoff valve with the factory
installed threaded plugs. The venturi has now
been adjusted for proper operation.
3.7 Venturi Adjustment - Propane Gas
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 (see Figure 17).
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.
“Low” Side of
Manometer
“High” Side of
Manometer
Venturi Inlet
Tap (Blue)
Venturi Throat
Tap (Yellow)
Gas Orifice
Tap (Red)
Figure 17. Measurement of Venturi Throat Pressure
Differential - Propane Gas.
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