Definition of Terms

The total of the indicated gage pressure plus the atmospheric pressure. Abbreviated "psia" for pounds per square inch absolute.

The number of times that air in a building is completely replaced with outdoor air in one hour.

Act of combining substance with air.

The part of the A/C system that contains the blower, cooling (evaporator) coil, and heater.

The amount of air required to completely replace the air in a room or building; not to be confused with recirculated air

Equipment that conditions air by cleaning, cooling, heating, humidifying, or dehumidifying it. A term often applied to comfort cooling equipment.

A process that maintains comfort conditions in a defined area.

Mechanism designed to lower temperature of air passing through it.

Air distribution outlet or grille designed to direct airflow into desired patterns.

Distribution of the air in a space, called the treated space, by means of devices, called air terminal devices, in a manner so as to meet certain specified conditions, such as air change rate, pressure, cleanliness, temperature, humidity, air velocity and noise level.

The transportation of a specified air flow to or from the treated space or spaces, generally by means of ductwork.

Fan/blower, filter and housing parts of a system.

Leakage of air into rooms through cracks, windows doors and other openings.

A device that registers changes in air conditions such as pressure, velocity, temperature, or moisture content.

Heat pumps or air conditioners that uses the outdoor air to transfer heat to and from the refrigerant in the unit.

A device located in an opening provided at the boundaries of the treated space to ensure a predetermined motion of air in this space.

The amount of air required to completely recirculate the air in a room or building one time would equal an “air turnover rate” of 1.0.  This term is also called the “air change rate” in some cases.

Equipment typically located in the space to be conditioned that uses a high sidewall supply and low sidewall return concept to heat or cool the space.  These units are typically made up of several sections stacked vertically and can include filters, heating sections, cooling coil sections, and air distribution sections as required by the application.

A fitting used to vent air manually or automatically from a system.

Heat of compression, plus the heat of absorption, is transferred from refrigerant within coil to surrounding air, either by convection or fan or blower.

The volume of air moving through a blower or duct. Units of measure are cubic feet per minute (CFM), liters per second (LPS) or cubic meters per hour (m³h).

An instrument used to measure air velocities.

American National Standards Institute is a nonprofit organization that publishes safety standards for the HVAC industry.  These standards are used by other organizations and local code authorities to create test protocols that prove equipment can be operated safely when installed properly.

Air-Conditioning and Refrigeration Institute is a nonprofit, voluntary organization comprised of heating, air conditioning and refrigeration manufacturers. ARI publishes standards for testing and rating heat pumps and air conditioners to provide you with a standardized measure of comparison. So, ARI ensures a level of performance within the industry.

A leading HVAC/R Association - American Society of Heating, Refrigerating and Air Conditioning Engineers - http://www.ashrae.org/

American Society for Testing and Materials.

The pressure exerted upon the earth's surface by the air because of the gravitational attraction of the earth. Standard atmosphere pressure at sea level is 14.7 pounds per square inch (psi). Measured with a barometer.

Controls that react to a change in conditions to cause the condition to stabilize.

A refrigerant control valve that maintains a constant pressure in an evaporator.

A device that propels air in an axial direction.

BacNet is a communication protocol developed under the auspices of ASHRAE that allows open communications between pieces of HVAC equipment on a building.  BacNet is not restricted to any particular manufacturer or line of equipment.

Process of adjusting the flow of air in duct systems, or water flow in hot-water heating systems.

An instrument for measuring atmospheric pressure.

An enclosed air-moving device which redirects the airflow by 90 degrees towards one or more exit points.

Quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit.

A device used to prepare and burn fuel.

A by-product of natural gas combustion that is not harmful.

A colorless, odorless, highly poisonous gas produced when carbon burns without sufficient air nearby.

A device that draws air in axially and discharges it radially.

A class of refrigerants. Generally refers to the Chlorofluorocarbon family of refrigerants. Sometimes called Freon.

Computational Fluid Dynamics is a computer-based mathematical model of fluids in a space.  In the HVAC industry CFD is used to simulate the direction, velocity, and temperature of air in a building.

A standard measurement of airflow that indicates how many cubic feet of air pass by a stationary point in one minute. The higher the number, the more air is being forced through the system.

Amount of refrigerant placed in a refrigerating unit.

Chilled beams are a form of radiant cooling commonly used in europe.  Chilled beams can be "passive" (no external source of air is provided) or "active" (some amount of air is provided from an external source...usually the ventilation air required for a space).  Chilled beams are linear in nature normally measuring about 12" across and extending the length of a room.  Chilled beams are installed in the ceiling.

A device that opens an electric circuit when an overload occurs.

A reaction called rapid oxidation or burning produced with the right combination of a fuel, oxygen, and heat.

The range of temperatures, humidities and air velocities at which the greatest percentage of people feel comfortable.

Pump of a refrigerating mechanism which draws a low pressure on cooling side of refrigerant cycle and squeezes or compresses the gas into the high pressure or condensing side of the cycle.

The moisture collected on an evaporator coil.

A small pump used to pump condensate to a higher level.

Part of the outdoor portion of a split-system air conditioner or heat pump. By converting refrigerant that is in a gas form back to a liquid, the coil transfers heat carried by the refrigerant to the outside air.

The temperature at which a vapor changes to a liquid.

Part of a refrigerating mechanism which pumps vaporized refrigerant from the evaporator, compresses it, liquefies it in the condenser and returns it to the refrigerant control.

The transfer of heat through a solid material.

The movement of heat by fluid flow(e.g. air, water).

A device that is located in ductwork to adjust air flow.

Dry Bulb Temperature

A decibel describes the relative loudness of a sound on a logarithmic scale.

Direct Digital Control is the method of activating, deactivating, and monitoring the control points in an HVAC system using computer-logic and microprocessors.  This method of control includes the software that provides the instructions to the HVAC unit components and directs their action based upon the input from a variety of sensor types.  DDC controls can be embedded in individual pieces of equipment, can be a total unit controller, or can be a remote computer that communicates unit operational commands via a network.

The reduction of water vapor in air by cooling the air below the dew point; removal of water vapor from air by chemical means, refrigeration, etc.

The exact temperature at which moisture begins to form.

The burner fires directly in the air stream being heated, rather than through a heat exchanger. 100% of available BTUs are delivered to the heated space because no flue or heat exchanger is required. This results in no wasted energy.

DOAS stands for Dedicated Outdoor Air System.  A DOAS piece of HVAC equipment is designed specifically to filter, temper, and control the amount of outside air that enters a building.  DOAS equipment is typically used to provide the amount of ventilation air required by the building codes to satisfy the fresh air requirements for the building occupants.  A DOAS unit is only a component of a much larger system.  The remainder of the system is designed to satisfy the sensible cooling load of the building without concern for the outside air requirement.

The Department of Energy is a federal agency in charge of setting industry efficiency standards and monitoring the consumption of energy sources.

A pipe or closed conduit made of sheet metal, fiberglass board, or other suitable material used for conducting air to and from an air handling unit.

A ratio calculated by dividing the cooling capacity in Btu's per hour (Btuh) by the power input in watts at any given set of rating conditions, expressed in Btuh per watt (Btuh/watt). EER & SEER can not be compared equally.  EER changes with the inside and outside conditions, falling as the temperature difference between inside and outside gets larger.

A rating on comfort equipment is similar to the miles per gallon rating on your car.

This device preheats incoming outside air during the winter and pre-cools incoming air during the summer to reduce the impact of heating and or cooling the indoor air.

Environmental Protection Agency - http://www.epa.gov/

The condition that occurs when heat is absorbed by liquid and it changes to vapor.

Part of a split-system air conditioner or heat pump located indoors. The evaporator coil cools and dehumidifies the air by converting liquid refrigerant into a gas, which absorbs the heat from the air. The warmed refrigerant is then carried through a tube to the outdoor unit (condenser coil).

Uncontrolled air leakage out of a building.

The air flow leaving the treated space.

The temperature scale that places the boiling point of water at 212°F and the freezing point at 32°F.

A device that produces a pressure difference in air to move it.

The use of a pressure control to turn a condenser fan on and off to maintain a correct pressure within the system.

A family of mathematical relationships that allows the calculation of new operating characteristics from known system conditions

A device for removing dust particles from air or unwanted elements from liquids.

A dimensionless parameter relation air velocity to fan tip velocity. It's used to determine approximate fan width for centrifugal impellers.

An instrument for measuring pressure.

A class of refrigerants. Generally refers to Halogenated Chlorofluorocarbon family of refrigerants.

This is a device that enables furnaces to transfer heat from combustion safely into breathable air. The primary heat exchanger transfers heat from combustion gases to the air blowing through the ductwork.

The amount of heat gained, measured in BTU's, from a space to be conditioned, at the local summer outdoor design temperature and a specified indoor design condition.

The amount of heat lost, measured in BTU's from a space to be conditioned, at the local winter outdoor design temperature and a specified indoor design condition.

This device bring fresh, outside air into a building while simultaneously exhausting stale indoor air outside. In the process of doing this, an HRV removes heat from the exhaust air and transfers it to the incoming air, pre-heating it.

A class of refrigerants. Generally refers to Hydrofluorocarbon family of refrigerants

A device that adds moisture to warm air being circulated or directed into a space.

The amount of moisture in the air. Air conditioners remove moisture for added comfort.

A device designed to regulate humidity input by reacting to changes in the moisture content of the air.

Heating, Ventilating and Air Conditioning

The resistance or opposition to airflow in a system. Same as back pressure or static pressure.

The component of a fan assembly that is composed of a number of contoured blades.

A unit of air pressure measurement equal to the pressure exerted by a column of water 1 inch high.

Process by which the primary air sets into motion an air volume, called secondary air, in the room.

Kilowatt, equals 1000 Watts.

Kilowatt hour is the amount of kilowatts of electricity used in one hour of operation of any equipment.

The net amount of moisture added to the inside air by plants, people, cooking, infiltration, and any other moisture source. The amount of moisture in the air can be calculated from a combination of dry-bulb and wet-bulb temperature measurements.

Heat, that when added or removed, causes a change in state - but no change in temperature.

The Leadership in Energy and Environmental Design green building rating system is a nationally recognized benchmark for the design, construction, and operation of high performance buildings.

A unit of measurement. The velocity of the air.

LonTalk is a communication protocol developed by Echelon Corporation that allows various pieces of HVAC equipment in a building to share information and control signals.  LonTalk is open to manufacturers who choose to pay Echelon Corporation a royalty for the use of the protocol.

Make-up air equipment is designed to draw sufficient outside air into a building to wholly, or partially, offset the amount of air that leaves the building via the exhaust system.  Make-up air may, or may not, be tempered or filtered before being introduced into the building.

An instrument that measures air pressure differences between locations. Tubes are usually attached to a manometer and run to the spaces where pressures are measured. Essentially a U-tube partly filled with a liquid, usually water, mercury or a light oil. The pressure exerted on the liquid is indicated by the liquid displaced. A manometer can be used as a differential pressure gage.

A device that converts electrical energy into mechanical energy. A motor can operate on direct current (DC) or alternating current (AC) voltage supply, not both.

A small unit of air pressure.

Printed Circuit Board.

A sensing device used to measure total pressures in a fluid stream. It was invented by a French physicist, Henri Pitot, in the 1700's.

An air flow passage.

A dimensionless parameter relating the static pressure potential of a fan to its tip velocity pressure equivalent. It's used to determine approximate fan diameter.

The transfer of heat directly from one surface to another. (No intermediate air acting as a transfer mechanism required).

Substance used in refrigerating mechanism. It absorbs heat in evaporator by change of state from a liquid to a gas, and releases its heat in a condenser as the substance returns from the gaseous state back to a liquid state.

Relative Humidity

The rotating part of the motor which includes the propeller assembly.

Revolutions per minute, the measurement of fan blade speed.

Also referred to as the boiling point or the condensing temperature. This is the temperature at which a refrigerant will change state from a liquid to a vapor or visa versa.

e heat gain of the building due to conduction, solar radiation, infiltration, appliances, people, and pets. Burning a light bulb, for example, adds only sensible load to the building. This sensible load raises the dry-bulb temperature.

Heat, that when added or removed, causes a change in temperature but not in state.

Components which are inserted into the air distribution system and designed to reduce airborne noise which is propagated along the ducts.

A dimensionless parameter based on fan rotational speed, flow rating and pressure rating. It's a tool used to select the type of fan for a specific application. Each Different fan type achieves peak efficiency at a unique specific speed range.

Refrigeration or air conditioning installation, which places condensing unit outside or away from evaporator. These unit are connected together by a supply and return refrigerant lines.

The volumetric rate of airflow at standard air conditions.

A measure of an air mover's efficiency based on its air horsepower in terms of flow and static pressure vs. required shaft input power.

The difference in air pressure between the suction side and pressure side of the blower. Unit of measure is inches of water column (in. wc) or Pascals (Pa).

The part of the motor that is fixed, typically found in the hub of the fan.

Liquid refrigerant which is cooled below its saturation temperature.

Refrigerant vapor which is heated above its saturation temperature. If a refrigerant is superheated, there is no liquid present.

The resistance to air flow when moving air through an airflow system. Examples: air filters, air grilles and abrupt changes in flow direction.

The point of operation of the air mover on its air performance curve. It is described by an airflow and static pressure point. The operating point is that point on the air mover performance curve where the system resistance curve crosses the air performance curve.

An instrument used to detect differences in the level of heat.

A device that senses temperature change and changes some dimension or condition within to control an operating device.

A unit of measure for cooling capacity. One ton = 12,000 BTUs per hour.

A measure of an air movers efficiency based on its air horsepower in terms of flow and total pressure vs. required shaft input power.

Captures heating or cooling energy from stale indoor air and transfers it to fresh incoming air.

The housing or frame of the fan.

Watt, the SI unit of power. In electricity it is Volts x Amps.

Wet Bulb

Common measure of air pressure used in HVAC systems.

The difference between the dry-bulb and wet-bulb temperatures. 

When a wet wick is placed over a standard thermometer and air is blown across the surface, the water evaporates and cools the thermometer below the dry-bulb temperature. This cooler temperature (called the wet-bulb temperature) depends on how much moisture is in the air.