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A detailed list of heating,
air conditioning, and HVAC terms
Glossary
Absolute humidity: The mass of water vapor per
unit volume of air.
Absolute pressure: Pressure above a perfect
vacuum. Absolute pressure is the sum of gage
pressure plus atmospheric pressure.
Air exchange rate: Used in two ways: 1. the
number of times the outdoor air replaces the
volume of air in a building per unit time, typically
expressed as air changes per hour; 2. the number
of times that the ventilation system replaces the
air within a room or area within the building.
Air flow: The movement of air from one location
to another, commonly measured in cubic feet per
minute (CFM).
Air velocity: The distance traveled per unit of time,
commonly measured in feet per minute (FPM).
Air handling unit (AHU): Refers to equipment that
includes a blower or fan, heating and/or cooling
coils, and related equipment such as controls,
condensate drain pans, and air filters. Does not
include ductwork, registers or grilles, or boilers
and chillers.
Allergen: A substance capable of causing an aller-
gic reaction because of an individual’s sensitivity
to that substance.
Ambient air: Unconditioned atmospheric air.
Atmospheric pressure: Standard atmosphere is
the pressure equivalent of 14.696 psi or 29.921 in.
Hg at sea level. Measured with a barometer, it is
an indication of the “weight” of the air.
Boiler: A pressure vessel that safely and efficiently
transfers heat to water.
BTU: British Thermal Unit. A measure of the heat
given off when fuel is combusted. One BTU is
equal to the heat necessary to raise one pound of
water 1 °F.
Building envelope: Elements of the building,
including all external building materials, win-
dows, and walls, that enclose the internal space.
Building-related illness (BRI): Diagnosable
illness whose symptoms can be identified and
whose cause can be directly attributed to airborne
building pollutants (e.g., Legionnaire’s disease,
hypersensitivity pneumonitis).
Capacity: The output of a heating or cooling system
for a given amount of space. For heating, this is
usually expressed in BTU’s. For cooling, it is usu-
ally expressed in tons.
Carbon dioxide: (CO2), a clear, odorless gas found
in nature (around 380 ppm), most commonly
associated with respired air. The gas is largely
considered non-poisonous, but can result in suf-
focation in sufficient quantities.
Carbon monoxide: A clear, odorless gas made
when carbon or other fuel is burned during the
combustion process. The gas is poisonous, with
exposure resulting in symptoms such as head-
aches, nausea, or even death.
Ceiling plenum: Space below the flooring and
above the suspended ceiling that accommodates
the mechanical and electrical equipment and that
is used as part of the air distribution system. The
space is kept under negative pressure.
Central air handling unit (Central AHU): This is
the same as an Air Handling Unit, but serves more
than one area.
Chiller: Hydronic air conditioning system which
cools water, which cools air.
Compressor: Mechanical device that compresses
refrigerant or other fluid.
Condensate: Liquid formed when a vapor has
cooled below its dewpoint.
Condenser: Heat exchanger that removes heat from
high-pressure refrigerant vapor.
Conditioned air: Air that has been heated, cooled,
humidified, or dehumidified to maintain an inte-
rior space within the “comfort zone.” (Sometimes
referred to as “tempered” air.)
Constant air volume systems: Air handling
system that provides a constant air flow while
varying the temperature to meet heating, and
cooling needs.
Cooling coil: Typically copper or aluminum
tubing arranged to transfer the heat from air
to a refrigerant.
Dampers: Controls that vary airflow through an
air outlet, inlet, or duct. A damper position may
be immovable, manually adjustable or part of an
automated control system.
Demand controlled ventilation: Any system used
to vary air intake rates based upon the loads
placed upon the system, given a predefined space.
Dewpoint temperature (dp): Temperature below
which moisture in the air begins to condense.
Differential pressure: The difference in static
pressure measured between two locations.
Diffusers: Components of the ventilation system
that distribute and diffuse air to promote air circu-
lation in the occupied space. Diffusers supply air,
and grilles return air.
Draft: Air movement resulting in a local cooling
effect on the body. The intensity of the draft is
dependent upon a variety of factors which include
air velocity, temperature, occupant activity, and
occupant clothing.
Drain trap: A dip in the drain pipe of sinks, toilets,
floor drains, etc., which is designed to stay filled
with water, thereby preventing sewer gases from
escaping into the room.