HVAC Terms and Definitions

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.

Decibel (db)

A decibel describes the relative loudness of a sound.

Degree Day

A degree-day is a computation that gauges the amount of heating or cooling needed for a building. A degree-day is equal to 65 degrees Fahrenheit minus the mean outdoor temperature.

Air Terminal Device

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

Dehumidifier

A device that removes excess moisture from the air.

Condenser

A device that transfers unwanted heat out of a refrigeration system to a medium (either air, water, or a combination of air and water) that absorbs the heat and transfers it to a disposal point. There are three types of condensers: air-cooled condensers, water-cooled condensers, and evaporative condensers. The evaporative condenser uses a combination of air and water as its condensing medium. Most residential systems have an air-cooled condenser.

Burner

A device that uses fuel to support combustion.

A-Coil

A heat exchanger consisting of two diagonal coils that are joined together in a manner that look like the letter A.

Coil

A heating or cooling element made of pipe or tubing, usually with plates or fins.

ASHRAE

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

Cooling Capacity

A measure of the ability of a unit to remove heat from an enclosed space. COP - Coefficient of Performance of a heat pump means the ratio of the rate of useful heat output delivered by the complete heat pump unit (exclusive of supplementary heating) to the corresponding rate of energy input, in consistent units and under operating conditions.

CAE (Combined Annual Efficiency)

A measure of the amount of heat produced for every dollar of fuel consumed for both home heating and water heating.

COP (Coefficient of Performance)

A measurement of the instantaneous efficiency of heating or cooling equipment. It represents the steady-state rate of energy output of the equipment divided by the steady-state rate of energy input to the equipment, expressed in consistent units(i.e. watts-out per watts-in or Btu/h-out per Btu-in). Thus, the resultant value of COP is unitless. Most vapor-compression heating and cooling equipment has COPs greater than unity. That means it delivers more heat energy than it consumes.

AFUE (Annual Fuel Utilization Efficiency)

A measurement of the seasonal energy efficiency of gas heating equipment. It is the annual output energy of the equipment divided by its annual input energy, expressed in consistent units (i.e. Btu-out per Btu-in). Thus, the resultant value of AFUE is unitless. AFUE includes any input energy required by the pilot light but does not include any electrical energy or fans or pumps. A higher rating indicates more efficient equipment. This rating is calculated in accordance with the Department of Energy test procedures.

Accurator

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

Cooling Anticipator

A resistance heater (usually not adjustable) in parallel with the cooling circuit. It is 'on' when the current is 'off", adding heat to shorten the off cycle.

Condenser Coil

A series or network of tubes filled with refrigerant, normally located outside the home, that removes heat from the hot, gaseous refrigerant so that the refrigerant becomes liquid again.

CFM (Cubic Feet Per Minute)

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. A typical system produces 400 CFM per ton of air conditioning.

Contractor

A switch that can repeatedly cycle, making and breaking an electrical circuit. When sufficient current flows through the A-coil built into the contactor, the resulting magnetic field causes the contacts to be pulled in or closed.

Centigrade (Measure of Temperature)

A temperature scale with the freezing point of water 00 and the boiling point 1000 at sea level.

Chilled Water System

A 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.

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.

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.

AC

Abbreviation for alternating current, a type of electric current in which the polarity is constantly reversing causing the electron flow to reverse.

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.

Aeration

Act of combining substance with air.

ACCA

Air Conditioning Contractors of America

Air Diffuser

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

ARI (Air-Conditioning and Refrigeration Institute)

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.

ASTM

American Society for Testing and Materials.

Charge

Amount of refrigerant in a system.

Blower (fan)

An air handling device for moving air in a distribution 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.

BTU/H

British Thermal Unit/per hour. One of the two (watts-hours is the other) standard units of measure ( IP System) for the amount of energy consumed by a process, the amount of energy transferred from one location to another, or the amount of embodied energy (such as the heat contents of fuels). Specifically, it is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. The term 'kbty' stands for "kiloBtu" or 1,000 Btu. The term of 'Mbtu' stands for "MegaBtu" or 1,000,000 Btu. One Btu/his equal to 0.293 watt-hours (Wh).

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.

Carbonyxhemoboblin Saturation

Carbon monoxide poisoning.

CFC

Chlorofluorocarbons, used as a refrigerant in air conditioners and heat pumps, linked to the depletion of the ozone layer. Sometimes called Freon.

CADR

Clean Air Delivery Rate (CADR) provides the number of cubic feet of clean air a unit delivers each minute. Efficiency of the system times the air flow of the system.

Air Coil

Coil on some types of heat pumps used either as an evaporator or condenser.

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. CONDENSING UNIT:

Adiabatic Compression

Compressing refrigerant gas without removing or adding heat.

Air Conditioning

Control of the temperature, humidity, air movement and cleaning of air in a confined space.

Design Conditons

Cooling loads vary with inside and outside conditions. A set of conditions specific to the local climate are necessary to calculate the expected cooling load for a home. Inside conditions of 75 degrees Fahrenheit and 50% relative humidity are usually recommended as a guideline. Outside conditions are selected for the 2.5% design point.

Air Cleaner

Device used for removal of airborne impurities.

Agitator

Device used to cause motion in confined fluid.

Air Conditioner

Device used to control temperature, humidity, cleanliness and movement of air in a confined space.

DC

Direct current electricity. This type of electricity (as opposed to Alternating Current, or AC) flows in one direction only, without reversing polarity.

Air Diffusion

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.

DB

Dry Bulb Temperature

Damper

Found in ductwork, this movable plate opens and closes to control airflow. Dampers are used effectively in zoning to regulate airflow to certain rooms.

Absolute Pressure

Gauge pressure plus atmospheric pressure.

Air Source Equipment

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

Cooling Load

Heat which flows into a space from outdoors and/or indoors.

Add on Heat Pump

Installing a heat pump in conjunction with an existing fossil fuel furnace.

Air Infiltration

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

Air Cooler

Mechanism designed to lower temperature of air passing through it.

Direct Expansion System

One of two types of basic cooling media (the other is chilled water). Direct expansion systems utilize freon for cooling and dehumidification. The three most common methods of heat rejection are air cooled, water cooled and glycol cooled.

Condensing Unit

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 outdoor portion of a split system air conditioner contains the compressor and outdoor coil ignoring the reverse cycle operation, also the outdoor in a heat pump system.

Balancing or Air Balancing

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

Direct Vent

Pulls outside air for combustion and vents combustion gases directly outside.

AK Value (of an air terminal device)

Quotient obtained by dividing a measured air flow rate by a measured air velocity according to a specific process and a specific instrument.

Absorption Refrigeration

Refrigerator which creates low temperature by using the cooling effect formed when a refrigerant is absorbed by chemical substance.

Backdrafting

Reverse flow of combustion gases down the chimney of a vented combustion appliance, which is often caused by depressurization of the room where the appliance is located.

Drier

Sometimes called filter/drier, it removes moisture and keeps the refrigerant clean.

Accumulator

Storage tank which receives liquid refrigerant from evaporator and prevents it from flowing into suction line before vaporizing.

Absorbent

Substance with the ability to take up or absorb another substance without causing a chemical or physical damage.

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.

Absolute Zero Temperature

Temperature at which all molecular motion ceases. (-460F and -273C)

Absolute Temperature

Temperature measured from absolute zero.

Actuator

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

DOE (Department of Energy)

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

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.

Direct Gas-Fired Heater

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.

Airflow

The distribution or movement of air.

Commissioning

The final step in installing a heating or air conditioning system. Every component is checked and tested for compliance with codes, ACCA manuals, manufacturer requirements, and occupant needs. After commissioning, the technician will: provide documentation of testing, provide all equipment manuals, and show the homeowner/owner how to operate the system.

AHU (Air Handler Unit)

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

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 capacity than a gaseous refrigerant.

Celsius

The metric temperature scale in which water freezes at zero degrees and boils at 100 degrees, designated by the symbol "C". To convert to Fahrenheit, multiply a Celsius temperature by 9, divide by 5 and add 32 (25 x 9 equals 225, divided by 5 equals 45, plus 32 equals 77 degrees Fahrenheit).

Convection

The movement of heat by air flow.

ACH (Air Changes per Hour)

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

Burner Orifice

The opening through which gas flows to the air/gas mixing chamber of the burner.

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.

Condensation

The process by which a gas is changed into a liquid at constant temperature by heat removal.

Defrost Cycle

The process of removing ice or frost buildup from the outdoor coil during the heating season.

Compressor

The pump that moves the refrigerant from the indoor evaporator to the outdoor condenser and back to the evaporator again. The compressor is often called "the heart of the system" because it circulates the refrigerant through the loop.

Comfort Zone

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

Capacity

The rate at which the heating or cooling load can be satisfied by a given equipment system designed to heat or cool and dehumidify a conditioned space or heat service hot water. Heating and cooling capacity is normally given on equipment nameplates in units of Btu/h. The air conditioning industry often uses units of "tons" to refer to equipment capacity. One ton of capacity equals 12,000 Btu/h.

Compression Ratio

The ratio determined by dividing the discharge pressure, in PSI, by the suction pressure in PSI.

Compression

The reduction of volume of a vapor or gas by mechanical means.

Dehumidification

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.

Air Gap

The space between magnetic poles or between rotating and stationary assemblies in a motor or generator.

Boiling Point

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

Condensation Point

The temperature at which the removal of any heat will begin a change of state from a vapor to a liquid.

Conduction

The transfer of heat through a solid material.

Air Distribution

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

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.

Absolute Humidity

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

Crankcase Heater

This is the electric resistance heater installed on compressor crankcases to boil off liquid refrigerant that may have combined with compressor oil. Many newer cooling systems do not require crankcase heaters, however heat pumps do require crankcase heaters.

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.

Air Cooled

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