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Construction Glossaries and Definitions

Heating & Air Conditioning/HVAC Glossary

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Air Change - The amount of air required to completely replace the air in a room or building; not to be confused with re-circulated air.

Air Conditioner - Assembly of equipment for the simultaneous control of air temperature, relative humidity, purity, and motion.

Absolute Humidity - The weight of water vapor in a given amount of air space. Absolute humidity is measured by grains per cubic foot.

Absorption Cycle - Absorption chillers differ from mechanical vapor compression chillers in that they utilize a thermal or chemical process to produce the refrigeration effect necessary to provide chilled water. There is no mechanical compression of the refrigerant taking place within the machine as occurs within more traditional vapor compression type chillers. Most commercial absorption chillers utilize lithium bromide (a salt) and water as the fluid pair.

Accumeter - The metering (flow control) device inside a Carrier centrifugal chiller. Its unique design always feeds the cooler with liquid refrigerant, which has a much greater cooling capability than a gaseous refrigerant.

Accurator - A piston type metering device that feeds the proper amount of liquid refrigerant into the evaporator.

Air Cooled - Uses a fan to discharge heat from the condenser coil to the outdoors.

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

Air Flow - The distribution or movement of air.

Air Handler - The portion of the central air conditioning or heat pump system that moves heated or cooled air throughout a home's ductwork. In some systems a furnace handles this function.

Air-Cooled System - A type of air conditioning system that uses freon as a refrigerant and air as a condensing medium. Typically, the air-cooled condenser is located outside and refrigerant lines are piped to it from the indoor unit.

Annual Fuel Utilization Efficiency (AFUE) - A rating that denotes the efficiency of gas heating equipment. It is the amount of heating your equipment delivers for every dollar spent on fuel. A higher rating indicates more efficient equipment. This rating is calculated in accordance with the Department of Energy test procedures.

ARI - Air-Conditioning and Refrigeration Institute is a non-profit, 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 quality within the industry.

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.

Auto Changeover - A control package that provides for automatic switching from a primary air conditioning system to a backup in the event of a failure of the primary system.

Balance Point - An outdoor temperature, usually between 30° F and 45° F, at which a heat pump's output exactly equals the heating needs of the home. Below the balance point, supplementary electric resistance heat is needed to maintain indoor comfort.

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.

Blower - An air handling device for moving air in a distribution system.

Boiling Point - The temperature at which the addition of any heat will begin a change of state from a liquid to a vapor.

BTU (British Thermal Unit) - The standard of measurement used for measuring the amount of heat required to raise the temperature of one pound of water by one degree (Fahrenheit). BTUH - The number of BTUs in an hour.

BTU/Hour - The abbreviation for British thermal units per hour. The amount of heat required to raise one pound of water one degree fahrenheit per hour, a common measure of heat transfer rate.

Capacity - The ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in BTU's. For cooling, it is usually given in tons.

Carbon Monoxide - Carbon Monoxide (CO) is a colorless, odorless gas produced by burning any fuel. CO is poisonous and symptoms of CO poisoning are similar to those of the flu - headaches, fatigue, shortness of breath, nausea and dizziness. The Consumer Product Safety Commission (CPSC) recommends a yearly, professional inspection.

Centigrade (Measure of Temperature) - A temperature scale with the freezing point of water 00 and the boiling point 1000 at sea level.

Central Air Conditioner System - System in which air is treated at a central location and carried to and from the rooms by one or more fans and a system of ducts.

Centrifugal Compressor - A type of compressor used in vapor compression refrigeration cycles where a rotating impeller is the device which compresses the refrigerant vapor. The vapor is drawn into the impeller axially, and is discharged radially after energy is added to the vapor within the impeller.

CFM (Cubic Feet per Minute) - The abbreviation for cubic feet per minute commonly used to measure the rate of air flow in an air conditioning system.

Charge - Amount of refrigerant in a system.

Chilled Water System - Type of air conditioning system that has no refrigerant in the unit itself. The refrigerant is contained in a chiller, which is located remotely. The chiller cools water, which is piped to the air conditioner to cool the space.

Comfort Air Conditioning - Comfort air conditioning systems are designed for the comfort of people, not the protection of computer-based electrical systems. Unlike people, computers generate dry (sensible) heat, but not humidity. Only about 60-70% of a comfort system's total capacity is dedicated to the removal of sensible heat, while 30-40% is for dehumidification. With a large percentage of their total capacity devoted to the removal of moisture, comfort systems can lower room humidity far below acceptable standards. A larger comfort system is required to obtain the same sensible capacity as a precision cooling system.


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