Industry Related Glossaries

HVAC Terminology

Absolute Humidity:
Amount of moisture in the air, indicated in grains per cubic foot.

Absolute Pressure:
Gauge pressure plus atmospheric pressure (14.7 lbs. per sq. in.).

Absolute Temperature:

Temperature measured from absolute zero.

Absolute Zero Temperature:

Temperature at which molecular motion ceases.


Substance with ability to take-up, or absorb another substance.


A solution or surface that is capable of soaking up (taking in) another substance or energy form.

Activated Alumina:

Chemical used as a drier or desiccant.

Activated Carbon:
Specially processed carbon used as a filter-drier; commonly used to clean air.

Active Solar Heating System:

A system in which solar energy is absorbed in a collector, stored, and distributed by an auxiliary circulating system.


That portion of a regulating valve that converts mechanical fluid, thermal energy, or electrical energy into mechanical motion to open or close valve seats.


Substance which has property to hold molecules of fluids without causing a chemical or physical change.


Act of combining a substance with air. Agitator-Device used to cause motion in confined fluid.


Fuel to energy. A rating of 90 means that approximately 90% of the fuel is utilized to provide warmth to your home, while the remaining 10% escapes as exhaust.

An acronym for the American Society of Heating, Refrigerating and Air Conditioning Engineers 
An acronym for the American Society of Heating, Refrigerating and Air Conditioning Engineers 

Back Seating:
Fluid opening/closing such as a gauge opening; to seat the joint where the valve stem goes through the valve body.


Plate or vane used to direct or control movement of fluid or air within confined area.

Balance Point:

The point at which the heating capacity of a heat pump is equal to the heat losses of the structure it is heating.


Instrument to measuring atmospheric pressure. It may be calibrated in pounds per square inch or in inches of mercury in column.


British Thermal Unit. In scientific terms, it represents the amount of energy required to raise one pound of water one degree Fahrenheit. One BTU is the approximate equivalent to the heat given off by a single wooden kitchen match. For your home, it represents the measure of heat given off when fuel is burned for heating or the measure of heat extracted from your home for cooling. There is generally 1060 BTU of heat realized from burning one cubic foot of natural gas at sea level.

Calcium Sulfate:

Chemical compound (CaSO4) which is used as a drying agent or desiccant in liquid line (high side) filter-dryers.


Heat required to raise temperature of one gram of water one degree centigrade.


Device used to measure quantities of heat or determine specific heats.


A standard of airflow measurement. Cubic feet per minute. A typical system produces 400 CFM per ton of air conditioning.

Coil, Evaporator or Indoor:

The coil where heat is absorbed by warm air passiong across. Liquid refrigerant boils as it is metered into coil, and changes from liquid to vapor. Role of indoor coil is reversed in HEAT PUMPS, to that of CONDENSER COIL when placed in heating mode.


The heart or "pump" within an air conditioning or heat pump system. The compressor maintains adequate pressure to cause refrigerant to condense and flow in sufficient quantities to meet the cooling requirements of the system.


Valve for controlling airflow. Found in duct work, movable plate opens and closes to control airflow. Can be manually or automatically controlled to regulate airflow to desired system zones.

Decibel (dB)
Unit used for measuring relative loudness of sounds. One decibel is equal to approximate difference of loudness ordinarily detectable by human ear, the range of which is about 130 decibels on scale beginning with one for faintest audible sound.  

Downflow Furnace:
A furnace that pulls cold return air from its top and expels warm air at its base. Also referred to as counterflow (Flue gas travels up, circulated air travels down.).


Round or rectangular pipes or controlled paths acting as conduit for return, mixed, makeup, supply or exhaust air. Supply air is downstream of high pressure side of fan. Return air is upstream of low pressure inlet of fan.

Effective Area:
Actual flow area of an air inlet or outlet. Gross area minus area of vanes or grille bars

Fahrenheit Scale:
On a Fahrenheit thermometer, under standard atmospheric pressure, boiling point of water is 212 deg. and freezing point is 32 deg. above zero on its scale.

Fail Safe Control:
Device which opens circuit when sensing element fails to operate.


A radial or axial flow device used for moving or producing artificial currents of air.


Vapor phase or state of a substance.

Gauge Manifold:

A device constructed to hold compound and high pressure gauges containing hand valves to control flow.

Gauge, Vacuum:

Instrument used to measure pressures below atmospheric pressure.

Head Pressure Control:

Pressure operated control which opens electrical circuit if high side pressure becomes excessive.

Head, Static:
Pressure of fluid expressed in terms of height of column of the fluid, such as water or mercury.

Head, Velocity:
In flowing fluid, height of fluid equivalent to its velocity pressure.

Heat Exchanger:

A device for the transfer of heat energy from the source to the conveying medium, with the latter often being air or water. Most common combinations are: Refrigerant to air or Refrigerant to water (DX), Water to air (hydronic), Steam to air, Steam to water.


A device introducing water vapor to conditioned space as a means to raise relative humidity and overall human comfort.


The total amount of moisture in air. Relative humidity (RH), is the amount of moisture in air, relative to its total capability based upon its temperature (dewpoint). Moisture will condense on surfaces which are below this dewpoint.

Heating Seasonal Performance Factor, a rating used in measuring the heating efficiency of a heat pump. The higher the number, the more efficient the unit.


Heating, ventilation and air conditioning.

Ignition Transformer:

A transformer designed to provide a high voltage current. Used in many heating systems to ignite fuel.


Thomson Effect-Change in temperature of a gas on expansion through a porous plug fro m a high pressure to a lower pressure.

Kata Thermometer:

Large bulb alcohol thermometer used to measure air velocities or atmospheric conditions by means of cooling effect.

Kelvin Scale (K):

Thermometer scale on which unit of measurement equals the centigrade degree and according to which absolute zero is 0 deg., the equivalent of -271.16 deg. C. Water freezes at 273.16 deg. and boils at 373.16 deg

Latent Heat:

Heat energy absorbed in process of changing form of substance (melting, vaporization, fusion) without change in temperature or pressure. Also referred to as "hidden" heat.

Leak Detector:
Device or instrument such as a halide torch, an electronic sniffer; or soap solution used to detect leaks.

Limit Control:
Control used to open or close electrical circuits as temperature or pressure limits are reached.

Magnetic Gasket:
A sealing material which adheres due to small magnets inserted in gasket.

Instrument to measuring pressure of gases and vapors. Gas pressure is balanced against column of liquid such as mercury, in U-shaped tube.

Natural Convection:

Movement of a fluid caused by temperature differences (density changes).

Package System:
A self-contained air conditioning and/or heating unit, whereas all components are located in one cabinet. Used primarily for commercial, and occasionally in residential applications, and installed either adjacent to structure or on roof.

Partial Pressures:
Condition where two or more gases occupy a space and each one creates part of the total pressure.

Pascal's Law:

A pressure imposed upon a fluid is transmitted equally in all directions.

PSC Motor:

High-efficiency design motor used on virtually all of today's HVAC & R equipment requiring motors over 1/10hp. 

Either a sling type, or electronic. Instrument used to determine wet bulb temperatures and relative humidity. Combining RH with dry bulb temperature will yield total heat.

Radiant Heating:

Heating system in which warm or hot surfaces are used to radiate heat objects and thereby condition space. Universaly, heat radiated from our sun traveling through Earth's atmosphere and is absorbed by its surface (except that small amount absorbed by CO2 or other atmospheric "greenhouse" gases). Absorbed radiant heat then spreads via conduction through surface objects, and then heats the surrounding air via convection. This means (Radiant/Conduction/Convection) is the same principal which also applies to modern radiant heating methods in homes today.


Transfer of heat by heat waves. Same type of heat as what is felt by the sun's rays, even though the outside temperature may be cool.

Receiver Heating Element:

Electrical resistance mounted in or around liquid receiver, used to maintain head pressures when ambient temperature is at freezing or below freezing.

Safety Motor Control:

Electrical device used to open circuit if the temperature, pressure, and/or the current flow exceed safe conditions.

Seasonal Energy Efficiency Ratio, a rating that measures the cooling efficiency of a heat pump or air conditioner (BTU output / Watt of energy consumption). The higher the number, the more efficient the unit.

Sensible Heat:
That heat that which be measured using a thermometer or thermocouple probe (as opposed to latent heat).

Split System:

A comfort system consisting of components in two locations. Common examples include an outside unit, such as an air conditioning condenser, and an indoor unit, such as a furnace and mounted evaporator coil or air handler.

Super Heat:
The temperature rise within an evaporator/suction line assembly from the evaporator's saturation temperature.

Temperature Humidity Index:

Actual temperature and humidity of sample of air, compared to air at standard conditions.

A temperature control device. In HVAC, typically wall mounted in conditioned space. Programmable thermostats allow varying levels of comfort during different times of day.


Quantity of heat equivalent to 100,000 Btu.


A unit of measurement used for determining cooling capacity. One ton is the equivalent of 12,000 BTUs per hour.


Thermostatic Expansion Valve. A metering valve which acts as a superheat controller. Most are mechanically operated, and utilize a remote sensing bulb attached to the outlet of the evaporator assembly (via a sealed capillary tube) to regulate flow of sub-cooled liquid refrigerant at the evaporator inlet. 


Reduction in pressure below atmospheric pressure.

Vacuum Control Systems:

In many automobile air conditioning systems, intake manifold vacuum is used to operate dampers and controls in system.

Vacuum Pump:
Special high efficiency device (pump) used create deep vacuum within an HVAC/R systems, for the purpose of moisture removal (dehydration), removal of non-condensibles and can also be used, as well, for leak checking.

Valve, Solenoid:

Valve actuated by magnetic action by means of an electrically energized coil.

Wet Bulb:
Device used in measurement of relative humidity. Evaporation of moisture lowers temperature of wet bulb compared to dry bulb temperature in same area.


A method of dividing a home or buildig into separate zones and enabling individual control and savings by limiting HVAC resources to only occupied zones, or those calling for such.