Definition of some
common HVAC industry terms
ABSOLUTE HUMIDITY - The weight of water vapor in a
given amount of air. Grains per cubic foot
ABSOLUTE PRESSURE - Pressure measured with a base of
ABSOLUTE TEMPERATURE - A temperature scale expressed
in degrees °F or °F using absolute zero as a base. Referred
to as the Rankin or Kelvin scale.
ABSOLUTE ZERO - The temperature at which molecular
activity theoretically ceases. -456.69 °F or -273.16
AIR CONDITIONING - The process of controlling the temperature,
humidity, cleanliness and distribution of the air.
AIR, STANDARD CONDITIONS - Conditions at which BTU
ratings for summer air conditioning equipment is rated.
95 °F dry bulb, 75 °Fwet bulb at the condenser inlet
and 80 °F dry bulb, 67 °F wet bulb at the evaporator
AMBIENT - Refers to the temperature surrounding a body
or unit under test.
ATMOSPHERIC PRESSURE - The weight of a 1 square inch
column of the earth's atmosphere. At sea level this
pressure is 14.696 pounds per square inch.
BIMETAL - Two metals with different rates of expansion
fastened together. When heated or cooled they will warp
and can be made to open or close a switch or valve.
BOILING POINT - The temperature at which the addition
of any heat will begin a change of state from a liquid
to a vapor.
BRITISH THERMAL UNIT (BTU) - The amount of heat necessary
to change the temperature of 1 pound of pure water 1
degree Fahrenheit (oF).
CAPILLARY TUBE - A refrigerant control consisting of
a small diameter tube which controls flow by restriction.
They are carefully sized by inside diameter and length
for each particular application.
CENTIGRADE - A temperature scale with the freezing
point of water 0° and the boiling point 100° at sea
CHECK VALVE - A valve designed to permit flow in one
COMPRESSION - The reduction of volume of a vapor or
gas by mechanical means.
COMPRESSION RATIO - The ratio determined by dividing
the discharge pressure, in PSI, by the suction pressure
COMPRESSOR - A mechanical device used to compress gases.
Three main types are: reciprocating, centrifugal and
CONDENSATION POINT - The temperature at which the removal
of any heat will begin a change of state from a vapor
to a liquid.
CONDENSING MEDIUM - The substance, usually air or water,
to which the heat in a condenser is transferred.
CONDENSING UNIT - The portion of a refrigeration system
where the compression and condensation of refrigerant
is accomplished. Sometimes referred to as the 'high
CONDUCTION - The transfer of heat from molecule to
molecule within a substance.
CONTACTOR - An electromagnetic actuated relay. Usually
used to refer to the relay which closes the circuit
to a compressor.
CONVECTION - The transfer of heat by a moving fluid.
COOLING ANTICIPATOR - A resistance heater (usually
not adjustable) in parallel with the cooling circuit.
It is 'on' when the current is 'off", adding heat
to shorten the off cycle.
COP - Ratio of work performed or accomplished as compared
to the energy used.
CRANKCASE HEATER - Electric heating element that is
used to heat the compressor crankcase to prevent migrating
refrigerant from condensing and diluting the crankcase
oil during the off-cycle.
CUBIC FEET PER MINUTE - A common means of assigning
quantitative values to volumes of air in transit, usually
CYCLE - The complete course of operation of a refrigerant
back to a selected starting point in a system. Also
used to describe alternating current through 360 space
DENSITY - Mass or weight per unit of volume. For example,
standard air = .075 pounds per cubic foot.
DISCHARGE LINE - A tube used to convey the compressed
refrigerant vapor from the compressor to the condenser
DISCHARGE PRESSURE - The pressure read at the compressor
outlet. Also called head pressure or high side pressure.
DRY AIR - Air which contains no moisture vapor.
DRY BULB TEMPERATURE - Temperature read with an ordinary
EFFECTIVE TEMPERATURE - An arbitrary concept which
combines into a single value the effect of temperature,
humidity, and air movement as sensed by the human body.
ENTHALPY - Total amount of heat in one pound of a substance
calculated from accepted temperature base, expressed
in BTU's per pound mass.
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EQUIVALENT LENGTH - That length of straight tubing
which has the same pressure drop as the fitting, valve
or accessory (of the same nominal size) being considered.
EVAPORATIVE COOLING - The cooling effect of vaporization
of a liquid in a moving air stream.
EVAPORATOR - A device in which a liquid refrigerant
is vaporized. Some superheating usually takes place.
EVAPORATOR SUPERHEAT - The actual temperature of the
refrigerant vapor at the evaporator exit as compared
to the saturated vapor temperature indicated by the
EXTERNAL STATIC PRESSURE - The sum of the static and
velocity pressures of a moving air system at the point
FAHRENHEIT - A temperature scale with the freezing
point of water 320 F and the boiling point 2120 F at
FEET PER MINUTE - A term assigned to a velocity of
a moving air stream, usually express FPM.
FILTER-DRIER - A device that removes moisture, acid
and foreign matter from the refrigerant.
FLASH GAS - Instantaneous evaporation of some liquid
refrigerant at the metering device due to pressure drop
which cools the remaining liquid refrigerant to desired
FREEZING POINT - The temperature at which the removal
of any heat will begin a change of state from a liquid
to a solid.
GAUGE PRESSURE - Pressure measured with atmospheric
pressure as a base.
HEAT - A form of energy causing the agitation of molecules
within a substance.
HEAT EXCHANGER - A device for the transfer of heat
energy from the source to the conveying medium.
HEAT FLOW - Heat flows from a warmer to a cooler substance.
The rate depends upon the temperature difference, the
area exposed and the type of material.
HEAT OF COMPRESSION - The heat added to a vapor by
the work done on it during compression.
HEAT OF THE LIQUID - The increase in total heat (Enthalpy)
per pound of a saturated liquid as its temperature is
increased above a chosen base temperature. (Usually
- 400F for refrigerants). It is expressed in BTU's.
HEAT TRANSFER - The three methods of heat transfer
are conduction, convection and radiation.
INCHES OF MERCURY - Atmospheric pressure is equal to
29.92 inches of mercury.
LATENT HEAT - Heat that produces a change of state
without a change in temperature; i.e., ice to water
at 32 °F or water to steam at 212 °F .
LATENT HEAT OF CONDENSATION - The amount of heat energy
in BTU's that must be removed to change the state of
one pound of a vapor to one pound of liquid at the same
LATENT HEAT OF FUSION - The amount of heat energy,
in BTU's required to change the state of one pound of
a liquid to one pound of solid at the same temperature.
LATENT HEAT OF MELTING - The amount of heat energy,
in BTU'S, that must be removed to change the state of
one pound of solid to one pound of vapor at the same
LATENT HEAT OF VAPORIZATION - The amount of heat energy
in BTU's required to change the state of one pound of
a liquid to one pound of vapor at the same temperature.
LIFT - To elevate a fluid from one level to a higher
LIQUID LINE - A tube used to convey the liquid refrigerant
from the condenser outlet to the refrigerant control
device of the evaporator.
MANOMETER - A tube filled with a liquid used to measure
MBH - One MBH is equivalent to 1,000 BTU's per hour.
MEAN TEMPERATURE DIFFERENCES - The mean of difference
between the temperature of a fluid receiving and a fluid
MELTING POINT - The temperature at which the addition
of any heat will begin a change of state from a solid
to a liquid.
MERCURY MANOMETER - Used to measure vacuum in inches
Meter - See http://www.flowmeterdirectory.com/meter.html
MICRON - A unit used to measure high vacuums. One micron
equals 1/25,400 of one inch mercury.
MUFFLER - Device installed in hot gas line to silence
OIL SEPARATOR - A device for separating out oil entrained
in the discharge gas from the compressor and returning
it to the crankcase.
PARTIAL PRESSURE - The pressure exerted by any individual
gas in a mixture.
PITCH - The slope of a pipe line for the purpose of
PITOT TUBE - A device comprising a small diameter orifice
projecting directly into an air stream measuring total
pressure and surrounded by an annular section with small
diameter entrances normal to the flow, measuring static
pressure; both sections are usually connected to a manometer
to indicate velocity pressure.
PRECHARGED LINES - Refrigerant line's which are filled
with refrigerant and are sealed at both ends. The seals
are broken when the lines are installed and the line
charge becomes part of the total system charge.
PRESSURE DROP - The decrease in pressure due to friction
of a fluid or vapor as it passes through a tube or duct
PRESSURE - TEMPERATURE RELATIONSHIP - The change effected
in temperature when pressure is changed or vice versa.
Only used at saturated conditions. An increase in pressure
results in a temperature increase. A decrease in temperature
results in a pressure decrease.
PUMPDOWN - Process of pumping refrigerant out of the
evaporator and suction line at the end of the on- cycle
by closing a solenoid valve in the liquid line and letting
the compressor shut-off by the low pressure control.
PSYCHROMETER - A devices having both a dry and wet
bulb thermometer. It is used to determine the relative
humidity in a conditioned space. Most have an indexed
scale to allow direct conversion from the temperature
readings to the percentage of relative humidity.
PSYCHROMETRIC CHART - A chart on which can be found
the properties of air under varying conditions of temperature,
water vapor content, volume, etc.
QUICK CONNECT - Name given to the end connections on
precharged lines which screw on to mated fittings of
the outdoor and indoor sections. Tightening the quick
connections ruptures the seals on the fittings and the
line charge becomes part of the total system charge.
RADIATION - The transfer of heat without an intervening
medium. It is absorbed on contact with a solid surface.
RECEIVER - A vessel for holding refrigerant liquefied
by the condenser.
REFRIGERANT - A substance which produces a refrigerating
effect while expanding or vaporizing.
REFRIGERANT CONTROL - A device used to meter the amount
of refrigerant to an evaporator. It also serves as a
dividing point between the high and low pressure sides
of the system.
REFRIGERANT DISTRIBUTOR - A device which meters equal
quantities of refrigerant to independent circuits in
the evaporator coil.
REFRIGERANT MIGRATION - The movement of refrigerant
through the system to the compressor crankcase during
the off-cycle, caused by its attraction to oil.
REFRIGERANT OPERATING CHARGE - The total amount of
refrigerant required by a a system for correct operation.
REFRIGERANT VELOCITY - The rate at which refrigerant
is moving at a given point in a system, usually given
in feet per minute (FPM).
REFRIGERATION - The transfer of heat from a place where
it is not wanted to a place where its presence is not
REFRIGERATION EFFECT - The amount of heat a given quantity
of refrigerant will absorb in changing from a liquid
to a vapor at a given evaporating pressure.
RELATIVE HUMIDITY - The percentage of water vapor present
in a given quantity air compared to the amount it can
hold at its temperature.
RELAY - A device used to open and close an electrical
circuit. The relay may may be actuated by a bimetal
electrically heated strip, a rod wrapped with a fine
resistance wire causing expansion when energized, a
bellows actuated by expansion of a fluid or gas or an
REVERSING VALVE - A device in a heat pump that is electrically
controlled to reverse the flow of refrigerant as the
system is switched from cooling to heating; also called
a four-way valve.
RISER - A vertical tube or pipe which carries refrigerant
in any form from a lower to a higher level.
SATURATED VAPOR - Vapor in contact with a liquid.
SATURATION - A condition of stable equilibrium of a
vapor and a liquid. SENSIBLE HEAT
Heat that can be measured or felt. Sensible heat always
causes a temperature rise.
SIGHT GLASS - A glass installed in the liquid line
permitting visual inspection of the liquid refrigerant
for the purpose of detecting vapor in the liquid. They
also generally have a device included to monitor moisture
content of the refrigerant.
SLUGGING - A condition in which a quantity of liquid
enters the compressor causing hammering and possible
SPECIFIC HEAT - The amount of heat necessary to change
the temperature of one pound of a substance 10 F.
SPECIFIC VOLUME - The volume of a substance per unit
of mass; i.e., standard air 13.33 cubic feet per pound.
The reciprocal of density.
STANDARD AIR DENSITY - .075 pounds per cubic foot.
Equivalent to dry air at 700 F and at sea level pressure.
STATE CONDITION - Substances can exist in three states
- solid, liquid or vapor.
STATIC PRESSURE - The normal force per unit area at
a small hole in the wall of a duct.
STATIC TAP - A means by which static pressures of a
duct system may be read directly, usually consisting
of a small diameter hole in the side of the duct connected
to a manometer.
SUB COOLING - Cooling of a liquid, at a constant pressure,
below the point at which it was condensed.
SUBLIMATION - A condition where a substance changes
from a solid to a gas without becoming a liquid.
SUCTION LINE - A tube used to convey the refrigerant
vapor from the evaporator outlet to the suction inlet
SUCTION LINE ACCUMULATOR - A device located in the
suction line that intercepts quantities of a liquid
refrigerant and thereby prevents damage to the compressor.
SUPERHEAT - Heat added to a vapor after all liquid
has been vaporized.
TEMPERATURE - A measurement of heat intensity.
THERMISTOR - Basically a semiconductor which has electrical
resistance that varies inversely with temperature.
THERMOSTAT - A bimetal actuated switch to close and
open a circuit to indicate or terminate operation of
a heating or air conditioning system.
THERMOSTATIC EXPANSION VALVE - Refrigerant control
which monitors the flow rate according to the superheat
at the evaporator outlet.
THERMOSTAT SUBBASE - When installed with a thermostat
it permits selection of function for heating, cooling,
automatic changeover and blower cycling or continuous
TON OF REFRIGERATION - The amount of heat necessary
to completely melt one ton of 320F ice in 24 hours.
200 BTUs per minute, 12,000 BTUs per hour, 288,000 BTUs
in 24 hours. This is based on the latent heat of fusion
for ice which is 144 BTUs per pound.
TOTAL HEAT (Enthalpy) - Total heat energy in a substance.
The sum of sensible and latent heat.
TOTAL PRESSURE - The sum of all partial pressures in
a mixture of gases.
TRAP - A depression or dip in refrigerant piping in
which oil will collect. A trap may be placed at the
base of a suction or hot gas riser to improve oil return
up the riser.
VACUUM - Any pressure below atmospheric pressure.
VAPOR BARRIER - The term applied to an impervious layer
of material superimposed upon a layer of insulation.
Vapor barriers are always applied on the warm side of
the insulation layer.
VAPOR PRESSURE - The pressure exerted by vapor.
VELOCITY PRESSURE - In a moving fluid, the pressure
capable of causing an equivalent velocity as applied
to move the same fluid through an orifice such that
all pressure energy expanded is converted into kinetic
WATER MANOMETER - Used to measure pressure in inches
WET BULB TEMPERATURE - Temperature read with a thermometer
whose bulb is encased in a wetted wick