HVAC Glossary of Terms

A Complete Glossary of HVAC Terms

The HVAC industry includes a large number of specialized terms and acronyms which can be confusing to those outside of the industry. Making informed purchasing decisions and getting the best performance from your home comfort system is easier when you know the lingo. This glossary will help you navigate the most common terms you’ll come across during your buying decision and when communicating with us for maintenance and repairs.

A  B  C  D  E  F  G  H  I  K  L  M  N  O  P  R  S  T  U  V  W  Z

A

AC (Alternating Current):  A type of current where the polarity is perpetually reversing, causing the directional flow in a circuit to reverse at regular intervals.

ACCA: Air Conditioning Contractors of America, the trade association for the AC industry.

AFUE:  Annual Fuel Utilization Efficiency is a measurement of furnace heating efficiency. The higher the AFUE, the more efficient the furnace.

AHRI: The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is a trade association that develops standards for measuring and certifying product performance. AHRI standards and guidelines are used throughout the world. Equipment with the AHRI Certified® mark has AHRI’s assurance that it will perform accurately and consistently.

Air Changes Per Hour: 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. Also called air change rate or air exchange rate.

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

Air Conditioner: A device that changes humidity levels, temperature or quality of air.
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 flow: How much air your duct system moves. Air conditioners are generally designed to move about 400 cubic feet per minute (cfm) for each ton of AC capacity. In dry climates, that number will be higher (maybe up to 500 cfm/ton), and in humid climates it’ll be lower (~350 cfm/ton).

Airflow Volume: Measured in cubic feet per minute (cfm), this is the amount of air circulated in a space.

Air Handler: The indoor component of your air conditioner or heating system that moves air through your home, 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.

Allergen: A substance that causes an allergic reaction. It includes dust, pollen, pet hair and dander, dust mites, mildew, lint, fungus, most tobacco smoke, cooking grease and bacteria.

ASHRAE: American Society of Heating, Refrigerating and Air-Conditioning Engineers is a global society advancing human well-being through sustainable technology for the built environment. The Society and its members focus on building systems, energy efficiency, indoor air quality, refrigeration and sustainability within the industry. Their website can be found at www.ashrae.org.

B

Bioaerosols: Bioaerosols, or biological aerosols, are tiny airborne particles that are biological in nature. High concentrations of bioaerosols in indoor air can lead to respiratory problems and disease. Maintaining proper home humidity levels and installing advanced filtration devices can help reduce bioaerosols and maintain a healthy environment.

Boot: The sheet metal transition piece that connects to the duct on one side and has a grille or register on the other.

BTU: A British Thermal Unit is a measurement of the amount of heat required to raise or lower the temperature of one pound of water one degree Fahrenheit. In the United States the power of HVAC systems (the rate of cooling and dehumidifying or heating) is sometimes expressed in BTU/hour instead of BTU/h: British Thermal Units per hour

Burner: The device that facilitates the combustion of air and gas.

Burner Orifice: The opening in the burner through which the gas or fuel passes prior to combustion.

C

Capacity: HVAC capacity is the output produced by the heating or cooling unit and is measured in BTUs per hour.

Carbon Monoxide CO): Carbon monoxide (sometimes referred to by its chemical abbreviation, CO) is an odorless, colorless, tasteless and poisonous gas that’s produced when fuels such as natural gas burn with insufficient air. When carbon monoxide leaks into your home’s air, it can lead to serious health problems for your family. Regular maintenance of your home’s heating and ventilation system will help ensure it’s not a source of a carbon monoxide leak.

Central Air-Conditioning: Central air conditioning (or central A/C) is a system in which air is cooled at a central location and distributed to and from rooms by one or more fans and ductwork.

Centrifugal Fan: A centrifugal fan is a mechanical device for moving air or other gases.

CFM (Cubic Feet per Minute): A measurement of airflow volume. A typical air conditioning system produces 400 CFM per ton of air conditioning.

Charging a System: Adding coolant, or refrigerant, to an HVAC system.

Chiller: 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.

Coil: There are typically two HVAC coils in a system—the condenser coil in the outside air conditioning condenser unit and the evaporator coil in the indoor unit. The coils are what increase or decrease the temperature via heat transfer. Installing a correctly sized and rated evaporator coil is essential for getting the highest performance and comfort from your central air conditioning or heat pump system.

Combined Annual Efficiency (CAE): Combined Annual Efficiency (CAE) is a measure of the amount of heat produced for every dollar of fuel used to heat your home’s air and water. It is used to compare efficiencies of integrated water and space heaters.

Compressor: The heart of an air conditioning or heat pump system. It is part of the outdoor unit that pumps refrigerant. 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.

Condensate: As warm air is pulled or pushed across the cool evaporator coil the coil perspires, creating liquid, or condensate which is mechanically drained away from the equipment.

Condenser: A condenser (or AC condenser) is the outdoor portion of an air conditioner or heat pump that either releases or collects heat, depending on the time of the year.

Condenser Coil: Generally the outdoor coil, it removes heat from the refrigerant in the summer months, allowing the refrigerant to be converted from vapor to liquid and complete the refrigeration process.

Condenser Fan: A fan that accelerates the movement of air over the condenser coil, facilitating the removal of heat from the refrigerant.

Constant Air Volume: A system designed to provide a constant air flow. This term is applied to HVAC systems that have variable supply-air temperature but constant air flow rates. Most residential forced-air systems are small CAV systems with on/off control.

Controller: A device that controls the operation of part or all of a system. It may simply turn a device on and off, or it may more subtly modulate the set point of components. Most controllers are automatic but have user input such as temperature set points, e.g. a thermostat. Controls may be analog or digital.

Cubic Feet Per Minute (CFM): Cubic Feet Per Minute (CFM) is a measurement of airflow volume, determined by how many cubic feet of air pass by a stationary point in one minute. Having an accurate estimate of your home’s CFM is key to choosing an HVAC system with the right capacity for your space and needs.

Cycling: 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.

D

DC: Direct Current. A type of electrical current that only flows in one direction.

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.

Degree-Day: Calculated by subtracting the average outdoor temperature for an area from 65º Fahrenheit. This measurement is used to estimate the amount of heating or cooling a home or building will need.

Dehumidifier: A device that removes humidity, or moisture, from the air. It works by cooling air to the point where water turns to liquid from vapor form and then the liquid is removed.

Delta T (ΔT): Temperature difference. If all is working well in your AC, you should get about a 20° F ΔT when the air passes through the cooling stage of the refrigeration cycle.

Diffuser: A diffuser is placed over ductwork, and it separates air with vanes going in differing directions. It evenly distributes air flow in the desired directions.

DOE: Department of Energy. Their website can be found at www.energy.gov.

Downflow Furnace: A furnace with an intake on the top and an air discharge at the bottom.

Drain Pan: Also a condensate pan. As the refrigerant vapor is liquefied, the drain pan collects the condensate and funnels it to the drain line.

Dry Bulb Temperature: Dry bulb temperature is the temperature of air measured by a thermometer which is freely exposed to the air while it is shielded from radiation and moisture. It is usually thought of as air temperature, and it is the true thermodynamic temperature. It is a measurement of heat intensity independently of humidity and a dry bulb thermometer is used to measure it.

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 comfort system that pairs an electric heat pump with a gas furnace and alternates between the two fuel sources to maximize comfort and efficiency.

Duct work: Ductwork refers to the system of ducts (metal or synthetic tubes) used to transport air from heating, ventilation and air-conditioning (HVAC) equipment throughout your home. Properly installed and well-maintained air ducts are a key component of indoor air quality and home comfort.

Ductless mini-split: What the rest of the world uses for air conditioning. It’s a split system heat pump that’s smaller and (usually) has no ducts. The blower and evaporator coil are in the head, which is mounted on a wall or ceiling in the room you’re trying to cool.

E

Electronic Air Cleaner: Electronic air cleaners, sometimes referred to as ionizers or electronic air purifiers, use electrically charged filters to reduce the number of airborne contaminants in your home. As air passes through your heating and cooling system, the cleaner uses electrically charged filters to attract and trap harmful particles to prevent them from recirculating through your home.

EER: The Energy Efficiency Ratio of a particular cooling device is the ratio of output cooling energy (in BTU) to input electrical energy at a given operating point.

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.

EPA: The United States Environmental Protection Agency is an agency of the US federal government which was created for the purpose of protecting human health and the environment by writing and enforcing regulations based on laws passed by Congress. Their website can be found at www.epa.gov.

Evaporator Coil: 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.

Expansion Valve: A valve that meters the levels of refrigerant through a temperature or pressure control.

F

Fahrenheit: A temperature scale in which water freezes at 32 degrees and boils at 212 degrees at normal atmospheric pressure.

Fan: A device consisting of a motor and a blower wheel that creates air flow.

Fan Coil Unit: A fan coil unit (FCU) is a device that uses a coil and a fan to heat or cool a room without connecting to ductwork. Indoor air moves over the coil, which heats or cools the air before pushing it back out into the room. FCUs can be less expensive to install than ducted systems, and are available in ceiling, floor-mounted and freestanding configurations.

Filter: A central heating and cooling system may use multiple filters. The air filter is integral to the system intake ducting, prevents contaminants from entering the equipment and must be maintained or replaced at regular intervals. There is also a filter in the refrigeration system, also referred to as a drier, which acts like a strainer to remove dirt and undesired particles from the system.

Flue: A vent that removes the byproducts of combustion from a furnace.

Fresh Air Intake: 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.

Furnace: A component of an HVAC system that adds heat to air or an intermediate fluid by burning fuel (natural gas, oil, propane, butane, or other flammable substances) in a heat exchanger.

Fuse: A delicate metal strip connecting two parts of an electrical circuit. This strip works as a safety, or circuit protector, and breaks, or melts, in the event of excess electrical charge, breaking the electrical circuit.

G

GAMA: Gas Appliance Manufacturers Association

Gas Furnace: A gas furnace is the part of an HVAC system that converts natural gas or propane into high-temperate heat for the home.

Gas Furnace Heat Exchanger: A gas furnace heat exchanger is responsible for the transfer of heat from inside the furnace into the air outside the furnace. The duct system then transfers this exchanged air to different rooms in the building or space.

Grille: 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.

H

Heat Exchanger: A heat exchanger is a heating component that is located in the furnace and transfers heat to the surrounding air, which is then pumped throughout the home.

Heat Gain / Heat Loss: Terms for the amount of cooling (heat gain) or heating (heat loss) needed to maintain desired temperature and humidity in controlled air. Regardless of how well-insulated and sealed a building is, buildings gain heat from sunlight, conduction through the walls, and internal heat sources such as people and electrical equipment. Buildings lose heat through conduction during cold weather. Engineers use heat load calculations to determine the HVAC needs of the space being cooled or heated.

Heating Coil: A heating coil is the part of the system that conducts heat. It allows electricity to act as fire.

Heat Pump: A heat pump is an HVAC unit that heats or cools by moving heat. During the winter, a heat pump draws heat from outdoor air and circulates it through a home’s air ducts. In the summer, it reverses the process and removes heat from the house and releases it outdoors.

Heat Transfer: The flow of heat from one area to another by conduction, convection, and/or radiation. Heat flows naturally from a warmer to a cooler material or space. It is an important and vital step in the process of cooling a space.

HEPA Filter: A High Efficiency Particulate Absorbing filter. A HEPA filter removes particles from the air by trapping them as air flows through.

Home Automation: Anything that gives you remote or automatic control of things around your home, including but not limited to your HVAC, lighting or security system.

Horizontal Flow: Describes an air handler or furnace that is positioned on its side and circulates air in one end and out the other. Ideal for attic or crawl space installations.

HSPF: Heating Seasonal Performance Factor (HSPF) refers to the efficiency of the heating mode of heat pumps over an entire heating season: The higher the number, the more efficient the unit.

Humidifier: A device that adds humidity, or moisture, to the air.

Humidistat: The device that works with a home’s heating and cooling system to automatically adjust the amount of moisture in the air to maintain a specific humidity level throughout the home and turns the humidifier on and off.

Humidity: The amount of moisture in the air. Variable speed systems can reduce this moisture for added comfort.

HVAC: The classic acronym for Heating, Ventilation and Air Conditioning. Today, an HVAC system also includes air cleaning and moisture control.

I

Ignition: Elevating the temperature of a substance to the point of causing a combustive reaction.

Indoor Coil: The other, less visible half of your outdoor unit. It’s 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.

Interconnection Agreement: A connection or link between power systems that enables them to draw on each other’s reserve capacity in time of need.

K

Kilowatt (kW): 1,000 watts.

L

Latent Heat: The heat you have to remove from the air to remove the moisture.

Line Set: The two refrigerant lines that connect the condensing unit to the evaporator coil. The smaller, hotter, uninsulated copper tube is the liquid line. The larger, colder, insulated tube is the suction line.

Load Calculation: A detailed analysis of your home’s energy needs that determines how much heat a house gains or loses through the building envelope, from duct losses, and by internal gains (people, appliances…). It’s one part of the HVAC design process.

Louver: 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.

M

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.

Media: The fine material of a filter that traps dirt, dust, mildew or bacteria.

MERV: The Minimum Efficiency Reporting Value rating is used to rank the efficiency of air filters. Every air filter has holes that allow air and particles to pass through. The smaller the holes, the fewer particles that can pass through. Higher MERV ratings indicate smaller holes, and therefore a more efficient filter. Not all filters have a MERV rating.

Manufacturer Approved System: If replacing a condensing unit, furnace or air handler, the system must be manufacturer approved and Air Conditioning, Heating and Refrigeration Institute (AHRI) matched. NOTE: Installation of unmatched systems is strongly discouraged.

Micron: A unit of measure equal to one millionth of a meter, or 1/25,000 of an inch. Airborne particles – such as dust, dander, mold and viruses – are measured in microns. These tiny particles can circulate in your home’s air and have harmful effects on your family’s health and comfort.

Minimum Outside Air: The lowest amount of fresh air flow that can be allowed into a recirculating system. This limit is set 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.

N

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

NEC: National Energy Council / National Electric Code

O

Outdoor Coil: Located in the outdoor unit, the coil dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid.

Outdoor Unit: The outdoor portion of a split system, such as an air conditioner or heat pump. 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: 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.

P

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.

Particles: Particles are tiny substances measuring less than 100 microns in diameter. Indoor air can be filled with particles, some of which can be seen with the naked eye, such as dust and dirt. Others can only be seen with a microscope, such as bacteria and viruses, which typically measure only one micron or smaller in diameter. All of these airborne particles can have significant impacts on your family’s health and comfort.

Particulates: The fine liquid or solid particles contained in combustion gases. The quantity and size of particulates emitted by cars, power and industrial plants, wood stoves, etc are regulated by the U.S. Environmental Protection Agency.

Plenum: The box attached to either side of the air handler to which the other ducts are attached. On the return side, there may be a filter between the plenum and the air handler. On the supply side, the evaporator coil may be in a separate housing from the blower. Look for the box that the ducts are attached to. It’s usually made of insulated sheet metal or duct board. One function of the plenum is to manage and equalize pressure for more even distribution.

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.

Programmable Thermostat: A type of thermostat that allows the user to program into the devices’ memory a pre-set schedule of times and temperatures enabling or disabling the HVAC equipment.

PSI: A pound per square inch is a unit of pressure resulting from the force of one pound-force applied to an area of one square inch.

PSIA: Pounds per square inch, absolute is used to clarify that the pressure is relative to a vacuum rather than the ambient atmospheric pressure. Since atmospheric pressure at sea level is around 14.7 psi, this will be added to any pressure reading made in air at sea level.

PSIG: Pounds per square inch gauge designates that the pressure is relative to atmospheric pressure.

Psychrometric: The study of the behavior of air-water vapor mixtures. Water vapor plays an important role in energy transfer and human comfort in HVAC design.

R

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

R-410A Refrigerant: A chlorine-free refrigerant that meets the U.S. 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.

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

Reciprocating Compressor: A type of compressor used in cooling systems to compress refrigerant by using a piston action.

Refrigerant: A chemical used in air conditioners, heat pumps, and refrigerators that produces a cooling effect while expanding or vaporizing. Most residential air-conditioning units contain the standard R-22 refrigerant, or freon.

Refrigerant Charge: The amount of refrigerant in a system.

Refrigerant Lines: Two copper lines that connect the outdoor air conditioner or heat pump to the indoor evaporator coil.

Return: The side of the duct system that pulls air from the house back to the air handler to be conditioned again.

Reversing Valve: The part responsible for the difference between a heat pump and an air conditioner.

S

Scroll Compressor: A specially designed compressor that works in a circular motion, as opposed to up-and-down piston action. Used in both lower and higher efficiency air conditioners, scroll compressors feature fewer moving parts than reciprocating compressors. This translates to more efficient operation, higher tolerance to liquid refrigerant, less mechanical failure and smoother, quieter operation.

SEER: The efficiency of air conditioners is often rated by the Seasonal Energy Efficiency Ratio which is defined by the Air Conditioning, Heating, and Refrigeration Institute as the cooling output during a typical cooling-season divided by the total electric energy input during the same period.

Self-contained System: A package unit.

Sensible Heat: Heat added or subtracted that causes a change in temperature.

Sensible Heat Ratio (SHR): The ratio of the sensible cooling load or capacity to the total (sensible + latent) cooling load or capacity.

Sensor: A device that reacts to a change in conditions.

Single-Speed: A single-speed motor runs at top speed until it satisfies your temperature setting and then shuts off. They’re generally louder at start-up, consume more energy than alternative motor types and can cause more stress on mechanical parts.

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.

Split System: An outdoor unit combined with an indoor unit (as opposed to a package unit), generally providing more efficiency and configuration options. This is the most common type of system.

Static Pressure: Something that almost never gets checked but has a big effect on the efficiency and effectiveness of your air conditioner. It’s the pressure inside your duct system and is often too high to get the proper air flow.

Supply: The side of the duct system that pushes conditioned air back into the house. Supply vents have cool air coming out of them when the AC is on.

System: General term used to refer to the set or a subset of components that perform a specific HVAC function within a building.

S

Scroll Compressor: A specially designed compressor that works in a circular motion, as opposed to up-and-down piston action. Used in both lower and higher efficiency air conditioners, scroll compressors feature fewer moving parts than reciprocating compressors. This translates to more efficient operation, higher tolerance to liquid refrigerant, less mechanical failure and smoother, quieter operation.

SEER: The efficiency of air conditioners is often rated by the Seasonal Energy Efficiency Ratio which is defined by the Air Conditioning, Heating, and Refrigeration Institute as the cooling output during a typical cooling-season divided by the total electric energy input during the same period.

Self-contained System: A package unit.

Sensible Heat: Heat added or subtracted that causes a change in temperature.

Sensible Heat Ratio (SHR): The ratio of the sensible cooling load or capacity to the total (sensible + latent) cooling load or capacity.

Sensor: A device that reacts to a change in conditions.

Single-Speed: A single-speed motor runs at top speed until it satisfies your temperature setting and then shuts off. They’re generally louder at start-up, consume more energy than alternative motor types and can cause more stress on mechanical parts.

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.

Split System: An outdoor unit combined with an indoor unit (as opposed to a package unit), generally providing more efficiency and configuration options. This is the most common type of system.

Static Pressure: Something that almost never gets checked but has a big effect on the efficiency and effectiveness of your air conditioner. It’s the pressure inside your duct system and is often too high to get the proper air flow.

Supply: The side of the duct system that pushes conditioned air back into the house. Supply vents have cool air coming out of them when the AC is on.

System: General term used to refer to the set or a subset of components that perform a specific HVAC function within a building.

T

Thermostat: A wall mounted device that monitors and controls the output of an HVAC system.

Thermal Zone: An individual space or group of neighboring indoor spaces that the HVAC designer expects will have similar thermal loads. Building codes may require zoning to save energy in commercial buildings. Zones are defined in the building to reduce the number of HVAC subsystems, and thus initial cost. For example, for perimeter offices, rather than one zone for each office, all offices facing west can be combined into one zone. Small residences typically have only one conditioned thermal zone, plus unconditioned spaces such as garages, attics, and crawlspaces, and basements.

Thermostatic Expansion Valve: A device that creates a constant evaporator temperature by regulation of refrigerant flow through the system.

Ton: A unit of measurement used for determining cooling capacity. One ton is 12,000 BTUs per hour.

Trunk: A big duct attached directly to the plenum from which the branches go out to the vents.

Two-Speed: The base required for a high-efficiency air conditioner, two-speed motors cycle on in low gear and attempts to satisfy the cooling load for the home, shifting to high gear if necessary. Once it reaches the desired temperature, it cycles back down to low before shutting off. With just two speeds, it reduces start-up noise, operates with greater energy efficiency and causes less stress on mechanical parts compared to single-speed motors.

U

Unitary Controller: A unitary controller is a device that controls only one zone in a building.

Upflow Furnace: A furnace that pulls in air from the bottom and releases it through the top.

V

Vacuum: A space where the pressure is significantly below that of standard atmospheric pressure.

Variable Air Volume: 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. Most new commercial buildings have VAV systems. 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: Ideal for high-efficiency air conditioners, a variable-speed motor functions much like a two speed, only with several speeds of operation. When compared with single- or two-speed motors, it facilitates smoother cycling and more precise performance control, as well as the quietest operation, highest energy efficiency and least stress on mechanical parts.

Ventilation: The process of moving air (changing) into and out of an interior space either by mechanically induced (forced) means.

Volt: Is the derived unit for electrical potential and electromotive force.

Voltage: The force pushing electrical current along wires and cables.

W

Watt: Is defined as joule per second and can be used to express the rate of energy transformation with respect to time.

Wet Bulb Thermometer: A thermometer that measures the relative humidity in the air.

Z

Zoning: A system that divides a home, office or space into different zones which are controlled independently of each other in order to better control the temperature and effectiveness of a heating and cooling system.