Make-Up-Water - fresh water added to a system to compensate for water lost through
evaporation and leakage.
Mechanical Return - a method of returning condensate to a boiler when the height
between the end of the main and the boiler water line (drip connection) is insufficient.
A mechanical return system is typically comprised of a condensate receiver and a
condensate pump.
Minimum Water Level - is the lowest boiler water level where the burner can be
safely operated.
No Pressure Return - a dry return below the boiler water line. Condensate flows by
gravity through this return and enters the condensate pump above the high water line of
the pump’s receiver. Air in this return is at atmospheric pressure because the receiver has
an open air vent that extends above the boiler water line.
Open Air Vent - an open length of piping extending up from a condensate pump
receiver to above the boiler water line. The vent keeps air pressure in both the pump
receiver tank and the no pressure return equal to atmospheric pressure outside the
system.
Over-firing - a situation where the firing rate of the boiler is above the boiler’s rating.
This can cause short cycling, the system pressure to rise and the relief valve to open.
Parallel-Flow System - Piping arrangement in which both steam and condensate flow
together in the same direction in the supply main and return.
Pitch of Mains - is the amount of slope given to a horizontal main pipe. Correct pitch
for horizontal supply mains and dry returns must be 1/4" min. in 10' in the direction of
steam and condensate flow.
Pressure Drop - the difference between steam pressure at top of boiler and pressure
at the end of the steam supply main. It is caused from friction within piping and the
condensing process in mains and heating units.
Safety Factor - additional static head needed to compensate for higher pressures and
greater steam consumption during warm-up periods.
Sensible Heat - is the amount of thermal energy that is required to raise an objects
temperature.
Skimming - a procedure for cleaning the surface of the water in a boiler. This process
should be done on ALL new boiler installations, and when foaming occurs.
National Oilheat Research Alliance 55