Definitions of Terms of Heating, Ventilation, and Air Conditioning (HVAC)

Adiabatic Process. A thermodynamic process that takes place without any heat being added or subtracted and at constant total heat.
Air, Makeup. New, or fresh, air brought into a building to replace losses due to exfiltration and exhausts, such as those from ventilation and chemical hoods.
Air, Return (Recirculated). Air that leaves a conditioned spaced and is returned to the air conditioning equipment for treatment.
Air, Saturated. Air that is fully saturated with water vapor (100% humidity),
with the air and water vapor at the same temperature.
Air, Standard. Air at 70F (21C) and standard atmospheric pressure [29.92 in (101.3 kPa) of mercury] and weighing about 0.075 lb / ft3 (1.20 kg/m3).
Air Change. The complete replacement of room air volume with new supply air.

Air Conditioning. The process of altering air supply to control simultaneously its humidity, temperature, cleanliness, and distribution to meet specific criteria for a space. Air conditioning may either increase or decrease the space temperature.
Air Conditioning, Comfort. Use of air conditioning solely for human comfort, as compared with conditioning for industrial processes or manufacturing.
Air Conditioning, Industrial. Use of air conditioning in industrial plants where  the prime objective is enhancement of a manufacturing process rather than human comfort.
Baseboard Radiation. A heat-surface device, such as a finned tube with a decorative cover.
Blow. Horizontal distance from a supply-air discharge register to a point at which the supply-air velocity reduces to 50 ft /min.
Boiler. A cast-iron or steel container fired with solid, liquid, or gaseous fuels to generate hot water or steam for use in heating a building through an appropriate distribution system.
Boiler-Burner Unit. A boiler with a matching burner whose heat-release capacity equals the boiler heating capacity less certain losses.
Boiler Heating Surface. The interior heating surface of a boiler subject to heat on one side and transmitting heat to air or hot water on the other side.
Boiler Horsepower. The energy required to evaporate 34.5 lb /hr of water at  212F, equivalent to 33,475 Btu/hr.
Booster Water Pump. In hot-water heating systems, the circulating pump used to move the heating medium through the piping system.
British Thermal Unit (Btu). Quantity of heat required to raise the temperature of 1 lb of water 1F at or near 39.2F, which is its temperature of maximum density.
Central Heating or Cooling Plant. One large heating or cooling unit used to heat or cool many rooms, spaces, or zones or several buildings, as compared to individual room, zone, or building units.
Coefficient of Performance. For machinery and heat pumps, the ratio of the  effect produced to the total power of electrical input consumed.
Comfort Zone. An area plotted on a psychometric chart to indicate a combination of temperatures and humidities at which, in controlled tests, more than 50% of the persons were comfortable.
Condensate. Liquid formed by the condensation of steam or water vapor.
Condensers. Special equipment used in air conditioning to liquefy a gas.
Condensing Unit. A complete refrigerating system in one assembly, including the refrigerant compressor, motor, condenser, receiver, and other necessary accessories.
Conductance, Thermal C. Rate of heat flow across a unit area (usually 1 ft2) from one surface to the opposite surface under steady-state conditions with a unit temperature difference between the two surfaces.
Conduction, Thermal. A process in which heat energy is transferred through matter by transmission of kinetic energy from particle to particle, the heat flowing from hot points to cooler ones.

Conductivity, Thermal. Quantity of heat energy, usually in Btu, that is transmitted through a substance per unit of time (usually 1 hr) from a unit area (usually 1 ft2) of surface to an opposite unit surface per unit of thickness (usually 1 in) under a unit temperature difference (usually 1F) between the surfaces.
Convection. A means of transferring heat in air by natural movement, usually a rotary or circulatory motion caused by warm air rising and cooler air falling.
Cooling. A heat-removal process usually accomplished with air-conditioning equipment.
Cooling, Evaporative. Cooling effect produced by evaporation of water, the required heat for the process being taken from the air. (This method is widely used in dry climates with low wet-bulb temperatures.)
Cooling, Sensible. Cooling of a unit volume of air by a reduction in temperature only.
Cooling Effect, Total. The difference in total heat in an airstream entering and  leaving a refrigerant evaporator or cooling coil.
Cooling Tower. A mechanical device used to cool water by evaporation in the outside air. Towers may be atmospheric or induced- or powered-draft type.
Cooling Unit, Self-Contained. A complete air-conditioning assembly consisting of a compressor, evaporator, condenser, fan motor, and air filter ready for plugin to an electric power supply.
Damper. A plate-type device used to regulate flow of air or gas in a pipe or duct.
Defrosting. A process used for removing ice from a refrigerant coil.
Degree Day. The product of 1 day (24 hr) and the number of degrees Fahrenheit the daily mean temperature is below 65F. It is frequently used to determine heating-load efficiency and fuel consumption.
Dehumidification. In air conditioning, the removal of water vapor from supply  air by condensation of water vapor on the cold surface of a cooling coil.
Diffuser (Register). Outlet for supply air into a space or zone. See also Grille below.
Direct Digital Control (DDC). An electronic control system that uses a computer to analyze HVAC parameters to operate control devices and to start, stop, and optimize mechanical equipment.
Direct Expansion. A means of air conditioning that uses the concept of refrigerant expansion (through a thermostatic expansion valve) in a refrigerant coil to produce a cooling effect.
Ductwork. An arrangement of sheet-metal ducts to distribute supply air, return air, and exhaust air.
Efficiency. Ratio of power output to power input. It does not include considerations of load factor or coefficient of performance.
Emissivity. Ratio of radiant energy that is emitted by a body to that emitted by a perfect black body. An emissivity of 1 is assigned to a perfect black body. A perfect reflector is assigned an emissivity of 0.
Enthalpy. A measure of the total heat (sensible and latent) in a substance and which is equal to its internal energy and its capacity to do work.
Entropy. The ratio of the heat added to a material or substance to the absolute temperature at which the heat is added.

Evaporator. A cooling coil in a refrigeration system in which the refrigerant is evaporated and absorbs heat from the surrounding fluid (airstream).
Exfiltration. Unintentional loss of conditioned supply air by leakage from ductwork, rooms, spaces, etc., that is to be considered a load on the air-conditioning system.
Film Coefficient (Surface Coefficient). Heat transferred from a surface to air or other fluid by convection per unit area of surface per degree temperature difference between the surface and the fluid.
Furnace, Warm-Air. Heating system that uses a direct- or indirect-fired boiler to produce warm air for heating.
Grille. A metal covering, usually decorative, with openings through which supply or return air passes.
Head. Pressure expressed in inches or feet of water. A head of 12 in, or 1 ft, of water is the pressure equivalent to a column of water 12 in, or 1 ft, high. See also Inch of Water below.
Heat, Latent. Heat associated with the change of state (phase) of a substance, for example, from a solid to a liquid (ice to water) or from a liquid to a gas (water to steam vapor).
Heat, Sensible. Heat associated with a change in temperature of a substance.
Heat, Specific. Ratio of the thermal capacity of a substance to the thermal capacity of water.
Heat, Total. Sum of the sensible and latent heat in a substance above an arbitrary datum, usually 32F or 0C.
Heat Capacity. Heat energy required to change the temperature of a specific quantity of material 1.
Heat Pump. A refrigerant system used for heating and cooling purposes.
Heat Transmission Coefficient. Quantity of heat (usually Btu in the United States) transmitted from one substance to another per unit of time (usually 1 hr) through one unit of surface (usually 1 ft2) of building material per unit of temperature difference (usually 1F).
Heater, Direct-Fired. A heater that utilizes a flame within a combustion chamber to heat the walls of the chamber and transfers the heat from the walls to air for space heating, as in a warm-air heater.
Heater, Unit. A steam or hot-water heating coil, with a blower or fan and motor, used for space heating.
Heating. The process of transferring heat from a heat source to a space in a building.
Heating, District. A large, central heating facility that provides heat from steam or hot water to a large number of buildings often under different ownership.
Heating, Radiant. Heating by ceiling or wall panels, or both, with surface temperatures higher than that of the human body in such a manner that the heat loss from occupants of the space by radiation is controlled.
Heating, Warm-Air. A heating system that uses warm air, rather than steam or hot water, as the heating medium.
Heating Surface. Actual surface used for transferring heat in a boiler, furnace, or heat exchanger.

Heating System, Automatic. A complete heating system with automatic controls to permit operation without manual controls or human attention.
Heating System, Hot-Water. A heating system that utilizes water at temperatures of about 200F.
Humidity. Water vapor mixed with dry air.
Humidity, Absolute. Weight of water vapor per unit volume of a vapor-air mixture.
It is usually expressed in grains / ft3 or lb / ft3.
Humidity, Percent. Ratio of humidity in a volume of air to the maximum amount of water vapor that the air can hold at a given temperature, expressed as a percentage.
Humidity, Relative (RH). Ratio of the vapor pressure in a mixture of air and water vapor to the vapor pressure of the air when saturated at the same temperature.
Humidity, Specific (Humidity Ratio). Ratio of the weight of water vapor, grains, or pounds, per pound of dry air, at a specific temperature.
Hygrometer. A mechanical device used to measure the moisture content of air.
Hygroscopic. Denoting any material that readily absorbs moisture and retains it.
Hygrostat. A mechanical device that is sensitive to changes in humidity and used to actuate other mechanical devices when predetermined limits of humidity are reached.
Inch of Water. A unit of pressure intensity applied to low-pressure systems, such as air-conditioning ducts. It is equivalent to 0.036136 psi.
Infiltration. Leakage into a(n air)-conditioned area of outside air (usually unwanted), which becomes a load on the (air-)conditioning system.
Insulation, Thermal. Any material that slows down the rate of heat transfer (offers thermal resistance) and effects a reduction of heat loss.
Louvers. An arrangement of blades to provide air slots that will permit passage of air and exclude rain or snow.
MBH. 1000 Btu/hr (Btu/ h).
Micron. 0.001 mm. It is frequently used to designate particle sizes of dust and??the efficiency of filtration by air-conditioning filters.
Modulating. Process of making incremental adjustments, usually by an automatic device operating a valve or damper motor.
Pressure, Absolute. Pressure above an absolute vacuum. Absolute pressure equals the sum of gage and barometric pressures.
Pressure, Atmospheric. Air pressure indicated by a barometer. The standard atmospheric pressure is 29.92 in of mercury, or 14.696 psi (101.3 kPa).
Pressure, Head. Condensing pressure, often considered as the refrigerant compressor-discharge pressure.
Pressure, Saturation. The pressure that corresponds to a specific temperature that will permit simultaneous condensation and evaporation.
Pressure, Suction. The pressure in the suction line of a refrigeration system.
Pressure, Head. See Head above.
Psychrometer. A mechanical device utilizing a wet-bulb and dry-bulb thermometer and used to determine the humidity in an air-water vapor mixture, such as room air.

Psychrometric Chart. A chart used in air-conditioning design and analysis that indicates various properties of an air-water vapor mixture along with various relevant mathematical values.
Psychrometry. A branch of physics that concerns itself with the measurement and determination of atmospheric conditions, with particular emphasis on moisture mixed in the air.
Radiation. Transfer of energy in wave form, from a hot body to a colder body, independent of any matter between the two bodies.
Radiation, Equivalent Direct. Rate of steam condensation at 240 Btu/ (hr)(ft2) of radiator surface.
Refrigerant. A substance that will accept large quantities of heat, that will cause boiling and vaporization at certain temperatures, and that can be utilized in airconditioning systems.
Register. See Diffuser.
Resistance, Thermal. The thermal quality of a material that resists passage of heat. Also, the opposite of conductance.
Resistivity, Thermal. The reciprocal of conductivity.
Split System. A separation of air-conditioning components, such as location of an air-blower-evaporator coil far from the compressor-condenser unit.
Steam. Water in gas or vapor form.
Steam Trap. A mechanical device that allows water and air to pass but prevents passage of steam.
Subcooling. Cooling at constant pressure of a refrigerant liquid to below its condensing temperature.
Suction Line. The low-temperature, low-pressure refrigerant pipe from an evaporator to a refrigerant compressor.
Sun Effect. Heat from the sun that tends to increase the internal temperature of a space or building.
Temperature, Absolute. Temperature measured on a scale for which zero is set at -273.16C, or -459.69F (presumably the temperature at which all molecular motion stops in a gas under constant pressure). The scale is called Kelvin, and 1K = 1C = 9/5F.
Temperature, Design. An arbitrary design criterion used to determine equipment size to produce air conditioning, heating, or cooling capable of maintaining the designated temperature.
Temperature, Dew Point. Temperature of air at which its wet-bulb temperature and dry-bulb temperature are identical and the air is fully saturated with moisture.
Condensation of water vapor begins at this temperature and will continue if the temperature is reduced further.
Temperature, Dry-Bulb. Temperature measured by a conventional thermometer.
It is used to determine the sensible heat in air.
Temperature, Effective. A single or arbitrary index that combines into a single value the effects of temperature, humidity, and air motion on the sensation of comfort. This value is that of the temperature of still, saturated air that will induce an identical feeling of comfort.

Temperature, Wet-Bulb. Air temperature as indicated by a thermometer with a wet bulb. This temperature is less than the dry-bulb temperature, except when the air is fully saturated with water vapor, or at 100% relative humidity, when wet-bulb and dry-bulb temperatures will be equal.
Ton, Refrigeration. Refrigeration effect equivalent to 200 Btu/min, or 12,000 Btu/hr.
Vapor. The gaseous state of water and other liquid substances.
Vapor Barrier. An impervious material used to prevent the passage of water vapor and to prevent condensation.
Velocity Pressure. The pressure caused by a moving airstream, composed of both velocity pressure and static pressure.
Ventilation. The process of supplying air to any space within a building without noticeable odors and without objectionable levels of contaminants, such as dusts and harmful gases, and of removing stale, polluted air from the space. Outside air is generally used as an acceptable source of ventilation air.
Ventilator, Unit. A type of unit heater with various modes of operation and degrees or percentages of outside air (frequently used for heating classrooms).
Volume, Specific. Volume, ft3 / lb, occupied by a unit weight of air.
Water, Makeup. Generally the water supplied to a cooling tower to replace the cooling water lost by evaporation or bleedoff.
Water Vapor. A psychrometric term used to denote the water in air (actually lowpressure, superheated steam) that has been evaporated into the air at a temperature corresponding to the boiling temperature of water at that very low pressure.

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