HVAC Terms Reference Guide

This comprehensive reference provides definitions of common HVAC industry terms and concepts.

Temperature & Pressure Terms

ABSOLUTE HUMIDITY - The weight of water vapor in a given amount of air.
ABSOLUTE PRESSURE - Pressure measured with the base of zero.
ABSOLUTE TEMPERATURE - A temperature scale expressed in degrees °F or °C 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 °C.
ATMOSPHERIC PRESSURE - The weight of a 1 unit column of the earth's atmosphere.
GAUGE PRESSURE - Pressure measured with atmospheric pressure as a base.

CENTIGRADE - A temperature scale with the freezing point of water 0 °C and the boiling point 100 °C at sea level.
FAHRENHEIT - A temperature scale with the freezing point of water 32 °F and the boiling point 212 °F at sea level.
DRY BULB TEMPERATURE - Temperature read with an ordinary thermometer.
WET BULB TEMPERATURE - Temperature read with a thermometer whose bulb is encased in a wetted wick.

BRITISH THERMAL UNIT (BTU) - The amount of heat necessary to change the temperature of 1 pound of pure water 1 degree Fahrenheit (°F).
MBH - One MBH is equivalent to 1,000 BTU's per hour.
TON OF REFRIGERATION - The amount of heat of fusion absorbed by melting 1 short ton of pure ice at 0 °C (32 °F) in 24 hours.
ENTHALPY - Total amount of heat in one pound (kg) of a substance calculated from accepted temperature base, expressed in BTU's per pound mass (J/kg).
SENSIBLE HEAT - Heat that can be measured or felt. Sensible heat always causes a temperature rise.
LATENT HEAT - Heat that produces a change of state without a change in temperature.
SPECIFIC HEAT - The amount of heat necessary to change the temperature of one pound of a substance 10 °F.

REFRIGERANT - A substance which produces a refrigerating effect while expanding or vaporizing.
COMPRESSOR - A mechanical device used to compress gases. Main types: screw, scroll, reciprocating, centrifugal and rotary.
CONDENSER - A device that removes heat from a substance.
EVAPORATOR - A device in which a liquid refrigerant is vaporized. Some super-heating usually takes place.
CAPILLARY TUBE - A refrigerant control consisting of a small diameter tube which controls flow by restriction.
CHECK VALVE - A valve designed to permit flow in one direction only.
FILTER-DRIER - A device that removes moisture, acid and foreign matter from the refrigerant.
OIL SEPARATOR - A device for separating out oil entrained in the discharge gas from the compressor and returning it to the crankcase.

DISCHARGE PRESSURE - The pressure read at the compressor outlet. Also called head pressure or high side pressure.
COMPRESSION RATIO - The ratio determined by dividing the discharge pressure, in PSI (Pa), by the suction pressure in PSI (Pa).
PRESSURE DROP - The decrease in pressure due to friction of a fluid or vapor as it passes through a tube or duct.
STATIC PRESSURE - The normal force per unit area at a small hole in the wall of a duct.
VELOCITY PRESSURE - In a moving fluid, the pressure capable of causing an equivalent velocity.
MANOMETER - A tube filled with a liquid used to measure pressures.
MICRON - A unit used to measure high vacuums. One micron equals 1/25,400 of one inch mercury.

AIR CONDITIONING - The process of controlling the temperature, humidity, cleanliness and distribution of the air.
RELATIVE HUMIDITY - The percentage of water vapor present in a given quantity air compared to the amount it can hold at its temperature.
CUBIC FEET PER MINUTE (CFM) - A common means of assigning quantitative values to volumes of air in transit.
FEET PER MINUTE (FPM) - A term assigned to a velocity of a moving air stream.
DRY AIR - Air which contains no moisture vapor.
AMBIENT - Refers to the temperature surrounding a body or unit under test.

HEAT TRANSFER - The three methods of heat transfer are conduction, convection and radiation.
CONDUCTION - The transfer of heat from molecule to molecule within a substance.
CONVECTION - The transfer of heat by a moving fluid.
RADIATION - The transfer of heat without an intervening medium.
DENSITY - Mass or weight per unit of volume.
VACUUM - Any pressure below atmospheric pressure.
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.
THERMOSTAT - A bimetal actuated switch to close and open a circuit to indicate or terminate operation of a heating or air conditioning system.