10 OM-NGB
Sequence of Operation
CAUTION
WAIT AT LEAST 60 SECONDS BEFORE
ATTEMPTING TO RE-LIGHT THE MAIN
BURNER AFTER IT SHUTS OFF.
When electrical power is turned on to the unit, the following happens:
• The drain valve closes
• The water valve opens
• The unit fills with water
As the boiler fills, the water is detected by two probes. The first of these is the “mid”
probe, which activates the RESET light. The second (“hi” probe) is reached when the
boiler is full, and shuts off the water supply. As the water supply drops below this
probe, the water supply opens until it is again reached.
The gas valve has a step-opening feature. When the control calls for the main burner
to light, the outlet pressure of the valve is maintained at a preset (non-adjustable) rate
for several seconds, before full rated pressure is allowed to develop.
Once the main burner shuts down, step-opening gas valves need at least 60 seconds
to reset. If an attempt to re-light the burner is made before these valves reset, it may
bypass or shorten the length of the low pressure step, and could re-light the main
burner under full flow rate.
A thermostatically-controlled air vent remains open while the boiler fills. As steam
begins to develop, this vent will close. Some steam may escape from this vent before
it is fully closed (at approximately 200ºF).
Once the pressure has reached 9-1/2 PSI, the main burner will be shut off by an
operating pressure switch. Residual heat stored in the boiler’s heat exchanger can,
however, cause the pressure to continue to build, even after the burner has shut down.
This is especially true when the unit is heated for the first time.
If/when the pressure reaches 12 PSI, a relief valve will open to prevent pressure from
increasing past 12 PSI. As pressure decreases, the main burner will automatically re-
light to maintain 9-1/2 PSI.
Even if something causes the pressure to pass 12PSI, a high-limit safety switch will
shut down the boiler electrically when it reaches 14-1/2 PSI. If this happens, the unit
should not be re-started until the problem which caused the shut-down has been
corrected.
As an additional safety measure, the unit is equipped with an A.S.M.E.-certified safety
valve which will open to relieve excess pressure at 15 PSI. The ability of this valve to
discharge steam pressure is greater than the boiler’s ability to generate steam.
When electrical power is turned off, the gas valve automatically shuts off flow to the
main gas burner.
A thermostatic switch (mounted on the boiler shell) keeps the drain valve closed until
the temperature drops to approximately 170ºF.
At that point, the thermostatic switch opens and water drains from the boiler. A
vacuum breaker allows air to enter the boiler for this purpose.
CAUTION
ESCAPING STEAM MAY CAUSE SEVER
BURNS. STAY AWAY FROM THERMOSTATIC
AIR VENT AND PRESSURE RELIEF VALVES.
