Definitions of
Steam Heating Terms
Absolute Pressure - the atmospheric pressure added to the gauge pressure and
is typically expressed as a unit of as pounds per square inch absolute.
Accumulator Tank – a tank installed when the returns in a vacuum pump heating
system are below the inlet connection of a vacuum pump receiver. Its location should
be such that condensate flows by gravity to the accumulator, and is lifted to the
vacuum pump receiver.
Atmospheric Pressure - the weight of a column of air, one square inch in cross
section and extends from the earth to the upper level of the blanket of air surrounding
the earth. This air is a pressure of 14.7 pounds per square inch at sea level, where
water can be boiled at 212°F.
Blow Down Valve (aka blowoff valve) - A valve that permits a boiler control to
be flushed out and the proper operation of the control to be checked.
Boiler - A closed vessel in which steam is generated by heating water with a fuel.
Boiler Crown - the part of the boiler that is at the top of the fire chamber.
Boiler Feed Pump - a pump that is operated by a control that monitors the actual
water level in a boiler.
Boiler-Header Drip Connection – a piping arrangement that drips a steam
header to a return header and equalizes pressure between the two.
Boiler Heating Surface - the area of the boiler’s heat transmitting surface that is
in contact with water or steam in the boiler.
Boiler Horse Power - the equivalent evaporation of 34.5 lbs of water per hour at
212°F to steam at 212°F. This is the equal to a heat output of 33,475 BTU per hour,
which is equal to approximately 140 sq. ft. of steam radiation (EDR).
British Thermal Unit - The quantity of heat required to raise the temperature of
1 lb. of water 1°F.
Check Valve – a valve that allows water to flow in one direction only. In a steam
system it controls water flow between a pump and the boiler inlet, so that water
cannot flow back from the boiler to pump.
52 National Oilheat Research Alliance