The ultimate guide to Heating Ventilation Air Conditioning and Refrigeration (HVAC/R) terms & definitions

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Annualized Fuel Utilization Efficiency is a measure of your furnace’s heating efficiency. The higher the AFUE percentage, the more efficient the furnace. The minimum percentage established by the Depart Of Energy for furnaces is 78%.

The distribution or movement of air.

Air Filtration System
A device that removes allergens, pollutants and other undesirable particles from air that is heated or cooled.

Air changes per hour (Also called air change rate or air exchange rate. Abbreviated ACH or ac/hr.)
The hourly ventilation rate divided by the volume of a space. For perfectly mixed air or laminar flow spaces, this is equal to the number of times per hour that the volume the space is exchanged by mechanical and natural ventilation.

Air Cleaner
A device that removes allergens, pollutants and other undesirable particles from air that is heated or cooled.

Air conditioner
An appliance, system, or mechanism designed to dehumidify and extract heat from an area. Usually this term is reserved for smaller self contained units such as a residential system.

Air handler/air handling unit (AH or AHU)
A central unit consisting of a blower, heating and cooling elements, filter racks or chamber, dampers, humidifier, and other central equipment in direct contact with the airflow. This does not include the ductwork through the building.

Annual Operating Hours (AOH)
The total ration of full- and part-load operating hours in a geographical area

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communications protocol for building automation and control networks. BACnet was designed to allow communication of building automation and control systems for applications such as heating, ventilating, and air-conditioning control, lighting control, access control, and fire detection systems and their associated equipment. The BACnet protocol provides mechanisms for computerized building automation devices to exchange information, regardless of the particular building service they perform. Proper communication between building automation devices is critical for maximizing building energy efficiency, indoor air quality, and other aspects of “green” buildings.

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.

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.

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

Microscopic living organisms that grow and multiply in warm, humid places.

Btu (British Thermal Unit) 
British Thermal Unit represents the amount of energy required to raise one pound of water one degree Fahrenheit. One BTU is the equivalent of 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.

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The output or producing capability of a cooling or heating system. For heating, this is usually expressed in BTU’s. For cooling, it is usually given in tons.

Capillary Tube
A refrigerant control consisting of a small diameter tube which controls flow by restriction. They are carefully sized by inside diameter and length for each particular application.

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 degrees Celsius and the boiling point 100 degrees Celsius at sea level.

Central Air Conditioner System
A 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. It’s a means of  assigning quantitative values to volumes of air in transit.

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.

A device that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This cooled liquid flows through pipes in a building and passes through coils in air handlers, fan-coil units, or other systems, cooling and usually dehumidifying the air in the building. Chillers are of two types; air-cooled or water-cooled. Air-cooled chillers are usually outside and consist of condenser coils cooled by fan-driven air. Water-cooled chillers are usually inside a building, and heat from these chillers is carried by recirculating water to a heat sink such as an outdoor cooling tower.

Combined Annual Efficiency (CAE)
A measure of the amount of heat produced for every dollar of fuel consumed for both home and water heating.

The part of the outdoor air conditioner or heat pump that compresses and pumps refrigerant to meet cooling requirements. The compressor maintains adequate pressure to cause refrigerant to flow in sufficient quantities in order to meet the cooling requirements of the system and your home. It is the heart of an air conditioning or heat pump system.

Condenser Coil
The outdoor portion of an air conditioner or heat pump that either releases or collects heat, depending on the time of the year.

Condensing Medium
The substance, usually air or water, to which the heat in a condenser is transferred.

Condensing Unit
The portion of a refrigeration system where the compression and condensation of refrigerant is accomplished. Sometimes referred to as the ‘high side’.

The transfer of heat from molecule to molecule within a substance.

An electromagnetic actuated relay. Usually used to refer to the relay which closes the circuit to a compressor.

The transfer of heat by a moving fluid.

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.

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

COP – (Coefficient of Performance) 
Ratio of work performed or accomplished as compared to the energy used.

The complete course of operation of a refrigerant back to a selected starting point in a system.

Refers to the process of an HVAC system turning on and off. Some systems require less cycling than others, leading to higher energy efficiency and less wear on the system.

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A movable plate or gate placed in a duct to control air flow by increasing friction in the duct. Dampers are used effectively in zoning to regulate airflow to certain rooms.

Deep water source cooling (DWCS)
Also called deep water air cooling  or Deep lake water cooling is a form of air cooling for process and comfort space cooling which uses a renewable, large body of naturally cold water as a heat sink. It uses water at 4 to 10 degrees Celsius drawn from deep areas within lakes, oceans, aquifers or rivers, which is pumped through the one side of a heat exchanger. On the other side of the heat exchanger, cooled water is produced.

Delta T (ΔT)
A reference to a temperature difference. It is used to describe the difference in temperature of a heating or cooling medium as it enters and as it leaves a system.

Direct Expansion Systems
Direct expansion systems utilize freon for cooling and dehumidification.

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

Discharge Line
A tube used to convey the compressed refrigerant vapor from the compressor to the condenser inlet.

Discharge Pressure
The pressure read at the compressor outlet. Also called head pressure or high side pressure.

Refers to a type of precision air conditioning system that discharges air downward, directly beneath a raised floor, commonly found in computer rooms and modern office spaces.

Downflow Furnace
A furnace that pulls in return air from the top and expels warm air at the bottom.

Dry-charged unit
An air-conditioner or heat pump that is shipped dry and charged with refrigerant at the place of installation. Dry-charged units are appropriate for homeowners who need a replacement unit compatible with R-22 refrigerant.

Dual Fuel
A HVAC system that pairs an electric heat pump with a gas furnace, providing an energy-efficient alternative to the conventional furnace/air conditioner combination.

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 series of Pipes or channels that carry air throughout a building.

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Economizers (Economisers)
An HVAC component that uses outside air, under suitable climate conditions, to reduce required mechanical cooling. They are mechanical devices intended to reduce energy consumption, or to perform useful function such as preheating a fluid. When the outside airs enthalpy (see below) is less than the required supply air during a call for cooling, an economizer allows a building’s mechanical ventilation system to use up to the maximum amount of outside air.

Emergency Heat (Supplemental or Auxillary Heat)
The back-up heat built into a heat pump system.

Energy Efficiency Ratio (EER)
The ratio of the cooling capacity of the air conditioner in British Thermal Units per hour, to the total electrical input in watts under ARI-specified test conditions.

Energy Star®
A program developed by the Environmental Protection Agency (EPA) in order to reduce the nation’s energy consumption. ENERGY STAR® -qualified heating equipment can be up to 15 percent more efficient than standard models. ENERGY STAR® -qualified cooling equipment can be up to 7 percent more efficient than minimum-standard equipment.

Total amount of heat in one pound (kg) of a substance calculated from accepted temperature base, expressed in BTU’s per pound mass (J/kg). For a given sample of air, a measure of the total heat content (the sum of the heat energy of the dry air and heat energy of the water vapor within it). It is typically used to determine the amount of fresh outside air that can be added to recirculated air for the lowest cooling cost.

A device in which a liquid refrigerant is vaporized. It’s a A component in the basic refrigeration cycle that absorbs or adds heat to the system. Evaporators can be used to absorb heat from air or from a liquid. The evaporator is the cold side of an air conditioner or heat pump.

Evaporator Coil
A network of tubes filled with refrigerant located inside the home that takes heat and moisture out of indoor air as liquid refrigerant evaporates.

Evaporative Cooling
The cooling effect of vaporization of a liquid in a moving air stream.

Evaporator Superheat
The actual temperature of the refrigerant vapor at the evaporator exit as compared to the saturated vapor temperature indicated by the suction pressure.

External Static Pressure
The sum of the static and velocity pressures of a moving air system at the point of measurement.

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Fan Coil Unit (FCU)
A small terminal unit that is often composed of only a blower and a heating and/or cooling coil, as is often used in hotels, condominiums, or apartments.

A temperature scale with the freezing point of water 32 oF and the boiling point 212 oF at sea level.

A device that removes moisture, acid and foreign matter from the refrigerant.

Flash Gas
Instantaneous evaporation of some liquid refrigerant at the metering device due to pressure drop which cools the remaining liquid refrigerant to desired evaporation temperature.

A transfer of fluid volume per unit time.

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

Fresh Air Intake (FAI)
An opening through which outside air is drawn into the building. This may be to replace air in the building that has been exhausted by the ventilation system, or to provide fresh air for combustion of fuel.

An indoor heating unit that works in conjunction with an air conditioner or heat pump. It adds heat to air or an intermediate fluid by burning fuel (natural gas, oil,  propane, butane, or other flammable substances) in a heat exchanger.

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A facing across a duct opening, often rectangular in shape, containing multiple parallel slots through which air may be delivered or withdrawn from a ventilated space. The grille directs the air flow in a particular direction and prevents the passage of large items.

A grid-tied system is a residential solar systems that allows you to use your own solar-generated electricity to save energy and reduce costs. At times when the solar system is not producing electricity, such as at night, electricity is provided by the utility company’s grid, or network of power stations.

Gauge Pressure
Pressure measured with atmospheric pressure as a base.

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Heat Exchanger
Transfers heat from one fluid to another without the fluids coming into direct contact with each other.

Heat Flow
Heat flows from a warmer to a cooler substance. The rate depends upon the temperature difference, the area exposed and the type of material.

Heat of Compression
The heat added to a vapor by the work done on it during compression.

Heat Pump
An air-conditioning system that can reverse the direction of refrigerant flow to provide either cooling or heating to the indoor space.

Heat of the Liquid
The increase in total heat (Enthalpy) per pound of a saturated liquid as its temperature is increased above a chosen base temperature. (Usually – 40 oF for refrigerants).

Heating Season Performance Factor (HSPF)
A rating of the seasonal efficiency of a heat pump unit when operating in the heating mode.

Heat Transfer
The three methods of heat transfer are conduction, convection and radiation.

Horizontal Flow
Air enters at the end or any side of the unit and is discharged horizontally out the other end or any side of the unit.

An indoor air quality device that introduces moisture to heated air as it passes from the furnace into the ductwork for distribution throughout the building.

An automatic device used to maintain humidity at a fixed or adjustable set point.

A term which stands for Heating, Ventilation, and Air Conditioning.

Hybrid Comfort System
A home comfort system that combines a heat pump with a gas furnace (also available in packaged systems). For areas with colder temperatures, combining electric heating (heat pump) with gas heating (furnace) lets you choose from two fuel sources in order to respond to fluctuations in utility costs.

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Inches of Mercury
Atmospheric pressure is equal to 29.92 inches of mercury.

Indoor Coil
Indoor coils are attached to your furnace or air handler. As indoor air flows across it, heat and moisture are drawn out, leaving air that is cool, comfortable and conditioned.

Indoor Unit
The air handler of an air-conditioning system, which contains a heat exchange coil, filters, remote signal receiver and fan and provides conditioned air into the space.

Integrated Energy Efficiency Ratio (IEER)
Expresses cooling part-load EER efficiency for commercial unitary air conditioning and heat pump equipment on the basis of weighted operation at various load capacities.

Integrated Part Load Capacity (IPLC)
The cooling capacity of the system operating at part-load conditions.

Integrated Part Load Value (IPLV)
is the efficiency performance factor at part-load cooling capacity. This performance is critical due to the higher quantity of operating hours under part-load conditions than at full load.

Intermediate Fluid
A liquid or gas used to transfer heat between two heat exchangers.  An intermediate fluid is used when the hot and cold fluids are too bulky (such as air) or difficult to handle (such as halocarbon refrigerant) to directly transfer the heat.

Inverter (Technology)
An inverter in an air conditioner is used to control the speed of the compressor motor to drive variable refrigerant flow in an air conditioning system to regulate the conditioned-space temperature. By contrast, traditional air conditioners regulate temperature by using a compressor that is periodically either working at maximum capacity or switched off entirely. Inverter-equipped air conditioners have a variable-frequency drive that incorporates an adjustable electrical inverter to control the speed of the motor and thus the compressor and cooling output.

ISO 9000
A family of international standards for quality management and assurance.

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The joule (symbol J), is a derived unit of energy, work, or amount of heat in the International System of Units. It is equal to the energy transferred (or work done) when applying a force of one newton through a distance of one metre (1 newton metre or N·m), or in passing an electric current of one ampere through a resistance of one ohm for one second. One Btu equals 1,055 joules.

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Kilowatt Hour (symbol kWh, kW·h, or kW h)
The kilowatt hour is a unit of energy equal to 1,000 watt-hours, or 3.6 megajoules. If the energy is being transmitted or used at a constant rate (power) over a period of time, the total energy in kilowatt-hours is the product of the power in kilowatts and the time in hours. The kilowatt-hour is commonly used as a billing unit for energy delivered to consumers by electric utilities.

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1. Components made of multiple smaller blades, sometimes adjustable, placed in ducts or duct entries to control the volume of air flow. When used inside of ducts, their function is similar to that of a damper, but they can be manufactured to fit larger openings than a single-piece damper.
2. Blades in a rectangular frame placed in doors or walls to permit the movement of air.

Latent Heat
Heat that produces a change of state without a change in temperature; i.e., ice to water at 32 oF or water to steam at 212 oF.

  • Latent Heat Of Condensation  The amount of heat energy in BTU’s that must be removed to change the state of one pound of a vapor to one pound of liquid at the same temperature.
  • Latent Heat Of Fusion – The amount of heat energy, in BTU’s required to change the state of one pound of a liquid to one pound of solid at the same temperature.
  • Latent Heat Of Melting – The amount of heat energy, in BTU’S, that must be removed to change the state of one pound of solid to one pound of liquid at the same temperature.
  • Latent Heat Of Vaporization – The amount of heat energy in BTU’s required to change the state of one pound of a liquid to one pound of vapor at the same temperature.

Liquid Line
A tube used to convey the liquid refrigerant from the condenser outlet to the refrigerant control device of the evaporator.

Load Calculation
A detailed analysis of your home’s energy needs conducted by your dealer to help determine which comfort system is best for your home.

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Makeup Air Unit (MAU)
An air handler that conditions 100% outside air. Typically used in industrial or commercial settings, or in “once-through” (blower sections that only blow air one-way into the building), “low flow” (air handling systems that blow air at a low flow rate), or “primary-secondary” (air handling systems that have an air handler or rooftop unit connected to an add-on makeup unit or hood) commercial HVAC systems.

A tube filled with a liquid used to measure pressures.

Matched System
A heating and cooling system wherein all components are matched in capacity and efficiency. This enables your system to perform at its best, and most efficient, for longer.

MBH is an expression for the rate of energy consumption or production: M is for thousands(M is the Roman numeral prefix for 1000), B is for Btu’s (British Thermal Units) and H is “per hour”One MBH is equivalent to 1,000 BTU’s per hour.

MERV Rating (Minimum Efficiency Reporting Value)
A measurement scale designed in 1987 by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to rate the effectiveness of air filters. The MERV rating of a filter describes the size of the holes in the filter that allow air to pass through. The higher the MERV rating, the smaller the holes in the filter, the higher the efficiency.

An electrical component consisting of integrated circuits, which may accept, store, control, and output information.

A unit of measure equal to one millionth of a meter, or one thousandth of a millimeter.

Minimum Outside Air
The lowest amount of fresh air flow that can be allowed into a recirculating system. This limit is sent to ensure that the interior air remains safe and comfortable to breathe.

Modulating Heating
Fully modulating heating provides greater fuel efficiency and ideal comfort control by constantly adjusting to changing temperatures in your home.

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North American Technician Excellence (NATE)
This is the nationwide certification program for home heating and cooling technicians. It’s the only certification that is recognized by the entire industry.

Nexia™ Home Intelligence
Nexia™ Home Intelligence is a home automation system that, when paired with an American Standard AccuLink™ Platinum XV Control or an AccuLink™ Remote Control, makes it possible to manage your home’s HVAC system, locks, lights, security and more by computer or most web-enabled cell phones.


Outdoor Coil
Located in the outdoor unit, the coil dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid. In American Standard units, the coil is called Spine Fin™.

Outdoor Unit
A component of an air-conditioning system which contains compressor, propeller fan, circuit board, and heat exchange coil. It pumps refrigerant to/from indoor unit. It’s the outdoor portion of a split system, such as an air conditioner or heat pump. It may also be a packaged air conditioning and/or heating system in which all components are located in one cabinet.

Outside Air Damper
An automatic louver or damper that controls the fresh air flow into an air handler and modulates to the most energy efficient setting.

Outside Air Temperature (OAT)
A measure of the air temperature outside a building. The temperature and humidity of air inside and outside the building are used in enthalpy calculations to determine when outside air can be used for free heating or cooling.

Oil Separator 
A device for separating out oil entrained in the discharge gas from the compressor and returning it to the crankcase.

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Packaged Terminal Air Conditioner (PTAC)
An air conditioner and heater combined into a single, electrically powered unit, typically installed through a wall and often found in hotels.

Packaged Unit
An air-handling unit, defined as either “recirculating” or “once-through” design, made specifically for outdoor installation. They most often include, internally, their own heating and cooling devices. Very common in some regions, particularly in single-story commercial buildings. Also called a rooftop unit (RTU)

Plenum Space
An enclosed space inside a building or other structure, used for airflow. Often refers to the space between a dropped ceiling and the structural ceiling, or a raised floor and the hard floor. Distinct from ductwork as a plenum is part of the structure itself. Cable and piping within a plenum must be properly rated for its fire and smoke indices.

Precharged Line
Refrigerant line’s which are filled with refrigerant and are sealed at both ends. The seals are broken when the lines are installed and the line charge becomes part of the total system charge.

Pressure Drop
The decrease in pressure due to friction of a fluid or vapor as it passes through a tube or duct or/and lift.

Pressure – Temperature Relationship
The change effected in temperature when pressure is changed or vice versa. Only used at saturated conditions. An increase in pressure results in a temperature increase. A decrease in temperature results in a pressure decrease.

Programmable Thermostat
A thermostat which is designed to adjust the temperature according to a series of programmed settings that take effect at different times of the day. Programmable thermostats may also be called setback thermostats or clock thermostats. They come with settings that allow you set the temperature of your house based on your family’s schedule.

Process of pumping refrigerant out of the evaporator and suction line at the end of the on- cycle by closing a solenoid valve in the liquid line and letting the compressor shut-off by the low pressure control.

A devices having both a dry and wet bulb thermometer. It is used to determine the relative humidity in a conditioned space. Most have an indexed scale to allow direct conversion from the temperature readings to the percentage of relative humidity.

Psychrometric chart
A chart on which can be found the properties of air under varying conditions of temperature, water vapor content, volume, etc.


Quick Connect
Name given to the end connections on precharged lines which screw on to mated fittings of the outdoor and indoor sections. Tightening the quick connections ruptures the seals on the fittings and the line charge becomes part of the total system charge.

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R-22 Refrigerant
The old standard for residential air conditioners, R-22 refrigerant is now being phased out by the U.S. EPA

R-410A Refrigerant
A chlorine-free refrigerant that meets the EPA’s newest, most stringent environmental guidelines.

Radiant Ceiling Panels
Usually metal panels suspended under the ceiling, insulated from the building structure. The primary cooling/heating agent temperature is close to the room’s temperature.

Radiant Floor
A type of radiant heating system where the building floor contains channels or tubes through which hot fluids such as air or water are circulated. The whole floor is evenly heated. Thus, the room is heated from the bottom up. Radiant floor heating eliminates the draft and dust problems associated with forced air heating systems.

The transfer of heat directly from one surface to another (without heating the intermediate air acting as a transfer mechanism.

A chemical that produces a cooling effect while expanding or vaporizing.

Refrigerant Control
A device used to meter the amount of refrigerant to an evaporator. It also serves as a dividing point between the high and low pressure sides of the system.

Refrigerant Distributor
A device which meters equal quantities of refrigerant to independent circuits in the evaporator coil.

Refrigerant Lines
Insulated copper tubing that connect the outdoor air conditioner or heat pump to the indoor evaporator coil. Refrigerant flows to and from indoor and outdoor units through these tubes.

Refrigerant Migration
The movement of refrigerant through the system to the compressor crankcase during the off-cycle, caused by its attraction to oil.

Refrigerant Operating Charge
The total amount of refrigerant required by a a system for correct operation.

Refrigerant Velocity
The rate at which refrigerant is moving at a given point in a system, usually given in feet per minute (FPM).

Refrigerant Effect
The amount of heat a given quantity of refrigerant will absorb in changing from a liquid to a vapor at a given evaporating pressure.

Rooftop Unit (RTU) Same as Air handler.

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Scroll Compressor
A specially designed compressor that works in a circular motion, as opposed to up-and-down piston action.

Seasonal Energy Efficiency Ratio (SEER)
This is a rating that measures the cooling efficiency of a heat pump or air conditioner. The higher the number, the more efficient the unit.

Seasonal Extreme Environmental Test (SEET)
This is the American Standard testing facility for heating and air conditioning systems, where the equivalent of five years of operation is condensed into 16 weeks of testing under harsh conditions.

Single Package
A heating and cooling system contained in one outdoor unit.

Smart Home
A smart home features an advanced system that offers remote or automatic control of the systems around your home, including but not limited to, your HVAC system, lighting or security system.

Smoke Damper
A damper or adjustable louver designed to augment the ventilation of a space during a fire.

Spine Fin™
A revolutionary technology that makes stronger, more efficient cooling coils. Spine Fin™ consists of thousands of tiny fins bonded to continuous aluminum refrigerant tubing. The tiny fins create a greater surface area, helping it to more efficiently transfer more heat from your home.

Split System
Refers to a comfort system consisting of components in two locations. Common examples include an outside unit, such as an air conditioner, and an indoor unit, such as a furnace with a coil.

Split-Ducted System
A system comprised of a remote outdoor condensing unit connected by refrigerant pipes to a matching, ducted indoor air handler with minimal field installed ductwork and a wired remote controller.

Split-Ductless System
A system comprised of a remote outdoor condensing unit connected by refrigerant pipes to a matching, non-ducted indoor air handler and a remote controller. Special cases for introducing ventilated air may call for limited ducting to air handler from outside.

Split-zoning System
A system comprised of a remote outdoor condensing unit connected by refrigerant pipes to a matching, indoor air handler that conditions single or multiple room space that is conditioned to a set temperature and is independent from other rooms within the same structure.

Suction Line
A tube used to convey the refrigerant vapor from the evaporator outlet to the suction inlet of compressor.

Suction Line Accumulator
A device located in the suction line that intercepts quantities of a liquid refrigerant and thereby prevents damage to the compressor.

A The condition where liquid refrigerant is colder than the minimum temperature required to keep it from boiling which would change it from a liquid to a gas phase. Subcooling is the difference between its saturation temperature and the actual liquid refrigerant temperature.

A condition where a substance changes from a solid to a gas without becoming a liquid.

Suction Line
A tube used to convey the refrigerant vapor from the evaporator outlet to the suction inlet of compressor.

Suction Line Accumulator
A device located in the suction line that intercepts quantities of a liquid refrigerant and thereby prevents damage to the compressor.

The number of degrees a vapor is above its boiling point at a specific pressure.

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Terminal unit (TU)
A small component that contains a heating coil, cooling coil, automatic damper, or some combination of the three. Used to control the temperature of a single room.

Thermal Zone
An individual space or group of neighboring indoor spaces that the HVAC designer expects will have similar thermal loads. Zones are defined in the building to reduce the number of HVAC subsystems.

A thermostat is a system that monitors and regulates a heating or cooling system. It can be used to set the desired temperature at which it keeps the environment either heated or cooled. Thermostats allows you to adjust your indoor comfort at the touch of a switch.

Thermostatic Expansion Valve (txv)
A thermostatic expansion valve is a piece of equipment that meters the flow of liquid refrigerant into the evaporator while measuring the vapor refrigerant leaving the evaporator. It thereby controls the superheating at the outlet of the evaporator.

Unit of measurement that is used to determine cooling capacity. One ton equals 12,000 Btuh (British Thermal Units per Hour).

Ton of Refrigeration
The amount of heat of fusion absorbed by melting 1 short ton (0.893 long ton or 0.907 t) of pure ice at 0 °C (32 °F) in 24 hours. Equivalent to the consumption of one ton of ice per day during the transition from stored natural ice to mechanical refrigeration.

A depression or dip in refrigerant piping in which oil will collect. A trap may be placed at the base of a suction or hot gas riser to improve oil return up the riser.

Two-stage (cooling and heating) or Two-stage operation
A two-stage air conditioner is designed to operate on high and low settings during different weather conditions and seasons. The high setting is used during extreme weather, and the low setting is used during moderate weather. This type of air conditioner produces a balanced temperature and is in use for a longer period of time.

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Underfloor Air Distribution (UFAD).
A method for providing ventilation and space conditioning by using the air plenum below a raised floor to distribute conditioned air through diffusers directly to the occupied zone.

Air enters at the bottom or side of the unit and is discharged vertically out the top. This happens when an air handler or furnace is installed in an upright position. It circulates air through the side or bottom and out through the top. Typically used in basement, closet and attic installations.

U.S. Green Building Council – Non-profit organization dedicated to sustainable building design and construction. Developers of the LEED building rating system.

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Any pressure below atmospheric pressure.

Vapor Barrier
An impervious layer of material superimposed upon a layer of insulation. Vapor barriers are always applied on the warm side of the insulation layer.

Vapor Pressure
The pressure exerted by vapor.

Variable Air Volume (VAV)
An HVAC system that has a stable supply-air temperature, and varies the air flow rate to meet the temperature requirements. Compared to constant air volume systems, these systems conserve energy through lower fan speeds during times of lower temperature control demand. VAVs may be bypass type or pressure dependent. Pressure dependent type VAVs save energy while both types help in maintaining temperature of the zone that it feeds.

Variable Speed Motor
A motor that automatically adjusts the flow of warm or cool air for ultimate comfort. The fan motor inside higher efficiency indoor and outdoor units is designed to change its speed based on the home’s heating and air conditioning requirements. Working in conjunction with your thermostat, it keeps the appropriate-temperature air (e.g., warm air on cold days) circulating throughout your home, reducing temperature variances. The variable-speed motor also increases dehumidification and is quiet because it runs at a lower speed most of the time. Plus, the consistent air circulation eliminates noisy startups and shutdowns.

Velocity Pressure
The pressure capable of causing an equivalent velocity as applied to move the same fluid through an orifice such that all pressure energy expanded is converted into kinetic energy.

Mechanically moves and exchanges stale, recirculated indoor air with filtered outside air.

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Water Manometer
Used to measure pressure in inches of water.

Wet Bulb Temperature
Temperature read with a thermometer whose bulb is encased in a wetted wick.

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zoning system
A zoning system sections a building or a space into regions or sectors which are controlled independently of each other. This is beneficial when different areas or rooms of a building have different temperatures as well as when the desired temperatures in different rooms are different. Temperature is controlled by different thermostats.

Programmable thermostats are used to control operating times of the equipment. Dampers are used to direct air flow to certain zones.

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hvac glossary

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