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LAARS HEATING SYSTEMS
pre-ignition purge, while the fan clears the
combustion chamber, the igniter is turned
on. The igniter takes about 25 seconds to
heat up. You can see a glow through the
view port (see Figure 13).
NOTE: The manual gas valve must be ON for the
burner to ignite. This valve is turned ON as follows:
•
Size 775/1000
Valve is ON when handle is
parallel to as pipe.
•
Sizes 320/400
Turn counterclockwise 520/625
to ON.
c.
When the igniter is hot, the ignition control
turns on the gas valve and the burner
ignites. You can see the burner flame
through the view port (see Figure 13).
d.
The boiler operates until the aquastat
senses that the water is hot enough, and the
burner shuts off. The combustion fan runs
for about one minute to blow all
combustion products out of the boiler.
If the igniter fails to ignite the burner in step 3
(for example, if there is air in the gas line), the
ignition control shuts off the gas valve after a few
seconds of operation. The purge and ignition sequence
is automatically repeated. If there is no ignition in
three tries, the ignition control “locks out” until the
problem is corrected. Contact a qualified service
technician.
3B. Hi-Limit Checkout
After running the boiler for a long enough period
to bring the water temperature within the range of the
hi-limit, slowly back off the high limit setting until the
boiler shuts off. The main burners should re-ignite
when the hi-limit is turned back up to its original
setting and the hi-limit is reset.
3C. Venturi and Gas Pressure Regulator
System
3C-1. Overall Operation
The gas control system of the Mighty Max boiler
is similar to that of a carburetor of a gasoline engine: a
venturi pulls the gas into the combustion air stream
(see Figure 14). In this system, changes in combustion
air flow automatically change the gas flow.
Overall
Operation
Figure 14. Typical Venturi System.
Air
Equalizer
Tube
Negative
Pressure Regulator
Gas
Gas
Orifice
Air/Gas
Mixture
Junction
Box
Front View of Boiler
NOTE: Sight glass
location may vary.
Figure 13. Periodic Flame Observation.
Igniter
Sight Glass
For Flame Observation
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.
