Page 16 - Webster Service Manual

11
Checking Fuel Cut-Off
Another error made by some is to say the valve is faulty
or the unit has a poor shut-of f when the real trouble is
air trapped in the nozzle line between the fuel unit valve
and the nozzle itself.
This condition is generally found when a new installa-
tion is made or when air enters the suction line due to
poor joints. This is particularly true on high vacuum
installations. When air is mixed with oil and pumped
through the fuel unit into the combustion chamber , a
good fire can result while the burner is in operation.
However, on shut-down the valve closes and the air
becomes trapped between the valve and the nozzle
itself. This air is no longer under pressure and the bub-
ble expands forcing the oil through the nozzle orifice
into the combustion chamber . This is not poor cut-of f
but trapped air causing the oil to squirt. It is necessary
to have a tight installation and to purge all air in order to
have clean sharp cut-off.
REMEMBER: A squirt out of the nozzle after the burner
stops means that air is trapped. Get rid of the air and
the problem will be gone.
This condition is found less often in a two-pipe system
because air is picked up in the suction line, is dis-
charged into the by-pass chamber of the fuel unit and
returned to the tank if the leak is not too large. However,
air can be trapped in the nozzle line of the two-pipe sys-
tem. Several hours of burner running may be necessary
before the air is absorbed by the oil and passed through
the nozzle.
Fast Cut-Off Required
Unlike heating equipment of the past, compact boilers,
furnaces, domestic water heaters, and burners made
today require substantially faster shut-of f to eliminate
pulsation on starts and stops. The valve must open and
close while the fan speed is high to provide enough
oxygen for complete combustion. This may be accom-