=The Blow Off Valve= is an angle or globe valve attached to the
lower part of a boiler for the purpose of blowing off the sediment
accumulated by the use of impure water, and should be used more or less
frequently according to the condition of the water used, but never less
than once a week. There is also a surface blow off which is attached
to the boiler at about the safe water level for the purpose of blowing
off the scum which accumulates on top of the water. This scum should be
blown off once or twice a day.