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Frequently Asked Questions
.: Frequently Asked Questions

Glossary of  HVAC Terms

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13 SEER: This is the new minimum efficiency standard (effective January 2006) for air conditioners & heat pumps. All new units must now meet this standard. Seasonal Energy Efficiency Ratio (SEER) is the measure of cooling efficiency for air conditioners and heat pumps. The higher the SEER value, the more energy efficient the unit (It's similar to comparing miles per gallon in automobiles).


ACCA: Air Conditioning Contractors of America, a private sector trade association representing HVAC Contractors.

A-Coil: Common name for an evaporator coil (i.e. heat exchanger consisting of two diagonal coils that are joined together at the top in a manner that looks like the letter "A".)

AFUE: Annual Fuel Utilization Efficiency. A measure of a gas furnace's efficiency in converting fuel to energy, the higher the rating, the more efficient the unit. For example: A rating of 90 means that approximately 90 percent of the fuel provides warmth to your home, while the remaining 10 percent escapes in exhaust. American Gas Association

AGA: American Gas Association, Inc.

Air Handlers: Equipment which includes a heating element and/or cooling coil and other components (i.e. blower fan) in a cabinet or casing.

ARI: Air-Conditioning and Refrigeration Institute, a non-profit organization composed of heating, air conditioning and refrigeration manufacturers. ARI publishes standards for testing and rating heat pumps and air conditioners.


ASHRAE: American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Founded in 1894, an international organization of over 55,000 professionals with a mission of advancing heating, ventilation, air conditioning and refrigeration to serve humanity and promote a sustainable world through research, standards writing, publishing and education.

Auxiliary Heat: Supplemental back-up heat built into a heat pump system, which switches on when outdoor conditions drop below the unit's set point. These are resistance heat strips housed within the air handler that provide the balance of heat necessary to satisfy winter extremes.


BBB: Better Business Bureau (Washington Metro Area)

BTU (British Thermal Unit): the amount of heat required to raise or lower the temperature of one pound of water one degree Fahrenheit. The heat extracted from your home by an air conditioner is measured in BTUs (or Ton's).

BTU/h (British Thermal Units per Hour): 12,000 BTUh equals one Ton of cooling.

Burner: Device that uses fossil fuel to support combustion.


Capacity: the output or producing ability of cooling or heating systems. Cooling and heating capacities are referred to in British thermal units (BTUs) per hour.

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 of fossil fuel furnaces.

CFC: Chlorofluorocarbons, used as refrigerant in air conditioners and heat pumps, linked to the depletion of the earth's protective ozone layer. As known as HCFC (an acronym for Hydro-chlorofluorocarbons, an example is R22)

CFM: Cubic Feet per Minute. A standard measurement of airflow. A typical system requires approximately 400 CFM per ton of air conditioning.

Charge: Add refrigerant to a system. This is refrigerant contained in a sealed system or in the sensing bulb to a thermostatic expansion valve.

Comfort-R Airflow System: Proprietary features of high efficiency home comfort systems from TRANE/American Standard. This method incrementally ramps up airflow yielding greater humidity control in cooling and warmer air during heating start up.

Compressor: The central part of an air conditioning or heat pump system and located in the outdoor unit. It pumps refrigerant to compress the gas into high-pressure on the condensing side of the cycle and provides a vacuum on the cooling side drawing gas back from the evaporator order to meet the cooling demands of the system.

Condensate: Vapor that liquefies due to the lowering of its temperature to the saturation point. Must be accommodated (conveyed to suitable discharge point) in air conditioning evaporator coils and high efficiency furnaces with secondary heat exchangers.

Condenser Coil (or Outdoor Coil): In an air conditioner, the coil dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid. In a heat pump system, the coil absorbs heat from the outdoors.

Contactor: Switch (Relay) that can be repeatedly cycled, making and breaking an electrical circuit. When sufficient current flows through the coil within the contactor, the resulting magnetic field causes the contacts to be pulled in or closed.

Crankcase heater: An electric resistance heater installed on a compressor crankcases to boil off liquid refrigerant that may have combined with compressor oil. Required on heat pump compressors.


Damper: Device located within ductwork. Movable plate opens and closes to control airflow. Dampers can be used to balance airflow in a duct system (heating verses cooling season). Dampers are also used effectively in zoning to regulate airflow to certain rooms.

Defrost Cycle: Requirement to periodically eliminate frost buildup on a heat pump outdoor evaporator coil as winter dew point conditions warrant. Accomplished by reversal of refrigeration process. Auxiliary Heat is activated during the defrost cycle. Sometimes mistaken as an equipment malfunction when steam appears rising from the outdoor unit.

Degree-day: Computation that gauges the amount of heating or cooling needed for a building structure. A degree-day is equal to 65 degrees Fahrenheit plus/minus the mean outdoor temperature on that day.

Dehumidifier: A device that removes excess moisture from the air.

Diffuser: Grille over an air supply duct having vanes to distribute the discharging air in a specific pattern or direction.


DOE: Department of Energy, a federal agency that sets industry efficiency standards and monitors the use of various energy sources.

Downflow Furnace: Furnace that intakes (return) air at its top and discharges (supply) air at its bottom.

Drain Pan: Also referred to as a condensate pan. This is a pan used to catch and collect condensate (in residential systems vapor is liquefied on the indoor coil, collected in the drain pan and discharged through a drain line).

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

Ductwork: Conveyance network (Pipes or channels) that carry air throughout your home. In a home comfort system, ductwork is critical to performance in fact, it's as critical as the equipment.

DX: Direct expansion; a system in which heat is transferred by the direct expansion of refrigerant within an evaporator coil. Most common residential system.


Economizer: Ventilators utilized in commercial package units to remove heat when outdoor temperatures are low by increasing the quantity of cool outdoor air in the ventilation air stream.

EER: (Energy Efficiency Ratio) 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.

Emergency Heat: Back-up resistance heat strips built into a heat pump system capable of carrying the entire heating load should the refrigeration equipment suffer catastrophic failure during an extreme winter event. Emergency Heat is Auxiliary Heat plus additional reserve resistance heat strips (locked out during normal operation) combined in aggregate to equal the total design heating load. Emergency Heat is activated manually at the thermostat. (Sometimes mistaken as Supplemental or Auxiliary Heat).

Energy Star

ENERGY STAR®: A government supported branding used to identify energy efficient products. This branding was developed by the US Department of Energy and the US Environmental Protection Agency.

EPA: The United States Environmental Protection Agency. One of the EPA’s many duties is to oversee programs that protect the stratospheric ozone layer.

esco Institute

ESCO: ESCO is approved by the EPA to administer Section 608 certification testing. Section 608 of the Clean Air Act enforces activities that protect the ozone layer as they apply to stationary refrigeration and air conditioning systems. Under this program, certification is required to ensure that HVAC technicians are competent in EPA requirements on service practices and refrigerant reclamation.

Evaporator Coil (or Indoor Coil): The other half of an air conditioning system located inside your home in the indoor (air handler) unit. This is a tubing coil in which a volatile liquid evaporates and absorbs heat. This is where the refrigerant absorbs the heat from the indoor air that passes over the coil.

Expansion Valve: Refrigerant-metering valve with a pressure &/or temperature controlled orifice. Also known as a Thermostatic EXpansion Value (TXV)


Filter: A device used to remove dust and other particles from air for the purposes of reducing the load on the respiratory system and to protect the HVAC equipment. Filters vary greatly in particle arrestance; the higher the MERV rating, the better the filter.

Flue: Any vent or passageway that carries the products of combustion from a furnace.

Freon: A general term used to identify, any of a group of partially or completely halogenated simple hydrocarbons containing fluorine, chlorine or bromine, which are used as refrigerants.

Furnace: That component of a HVAC Comfort system which converts gas, oil, electricity or other fuel into heat for distribution within a structure.

Furnace Heat Exchanger: Located inside the furnace, the heat exchanger allows transfer of heat generated by combustion of fossil fuel to air passing through the exchanger, which is then pumped throughout your home. The structural integrity of this chamber is paramount, should a crack or hole develop this could allow toxic carbon monoxide into your conditioned space. Failure of a Heat Exchanger mandates immediate decommissioning of a furnace.

Fuse: Any device that provides overload protection for an electric circuit. Typically a metallic strip that melts and breaks the circuit when excessive current flows through it.

Fuses are designed to break in order to:

  • Save more expensive electrical components.
  • Protect against fire caused by overheated wire & or other mechanisms.
  • GAMA


    GAMA: Gas Appliance Manufacturing Association is a national trade association serving the interests of manufacturers of gas, oil, and electric appliances and equipment, components and related products used in residential, commercial, and industrial applications.


    Gastite: Flexible gas piping which supplies natural gas or liquefied petroleum (LP) gas to appliances in commercial and residential applications. Approved by building codes throughout North America, Gastite is made flexible 304 stainless steel with a protective, UV-resistant polyethylene jacket.

    Greenhouse Gases: Components of the atmosphere that contribute to the greenhouse effect and thus linked to global warming. The concentrations of several significant greenhouse gases have increased over time, most notably for HVAC chlorofluorocarbons (CFCs) used in refrigeration systems. CFCs are controlled by the Montreal Protocol, motivated by their effect on stratospheric ozone rather than by their effect on greenhouse warming. Note that ozone depletion has only a minor role in greenhouse warming though the two processes often are confused.


    HCFC: Hydrochlorofluorocarbons, used as a refrigerant in air conditioners and heat pumps. HCFCs are believed to contribute to the depletion of the earth's protective ozone layer. An example is the still widely used R22 (Chlorodifluoromethane). Please review EPA's Phaseout Schedule

    Heat Exchanger: See Furnace Heat Exchanger above.

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

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

    HFC: Hydroflorocarbon, used as a refrigerant in air conditioners and heat pumps. An example is R410a an azeotropic mixture of difluoromethane and pentafluoroethane. HFC has little or no effect on the earth's protective ozone layer, however it is considered to be a greenhouse gas thus must be properly recovered like other refrigerants. R134a is another widely used HFC in automotive air conditioning.

    Heat Pump: A single refrigeration system designed to provide both heating and cooling. Compare to a furnace and an air conditioner, which are separate units that only heat or cool.

    HRV: Heat Recovery Ventilator, a device that brings fresh air into a home through a process that preheats the air so it has less impact on your utility bill. See "Economizer"

    HSPF: Heating Seasonal Performance Factor. This rating is used in measuring the heating efficiency of a Heat Pump. The higher the number, the more efficient the unit. Analogous to MPG in automobiles. The current industry minimum is 6.80 HSPF.

    Humidifier: A device that adds moisture to warm air for your home.

    Humidistat: A humidity-sensing control device that cycles a humidifier on and off based upon a relative humidity setpoint.

    HVAC: Abbreviation for Heating, Ventilation and Air Conditioning, sometimes noted as HVACR for Heating, Ventilation, Air Conditioning and Refrigeration.


    IAQ: Indoor Air Quality, the status of indoor air as measured by numerous factors: temperature, humidity, airflow, pollutants, etc.

    Ignition: The lighting of a fossil fuel to cause combustion. Furnace ignition system technology has evolved rapidly over the past several decades to allow introduction of ever increasing efficiencies. Failure of a furnace's ignition system is a frequent cause of "no-heat" service calls.

    Infiltration: Passage of outside air into your home through holes, gaps, and cracks, (e.g., plumbing or electrical holes, the heating and air conditioning system, doors, and windows etc.). Important factor in determining heating loads.

    Insulation: Any material that slows down the transfer of heat.


    Jumper: Simple single pole switches on a furnace or air handler control board which allows a blower motor to run at various speeds to accommodate differing airflow rates required by different size evaporator coils. Allows vendors to use a single blower for a wide range of unit capacities.

    Junction Box: A box or container housing a group of electrical terminals.


    (K) Factor: The insulating value of any material. Also known as conductivity.

    Kilowatt: Unit of electrical power, equal to 1000Watts


    Latent Heat: The energy absorbed in water when changing state to moisture vapor in the air.

    Light Commercial: Term used to refer to low-rise non-residential structures, typically professional office buildings that lack three phase electrical service. This allows application of single phase HVAC equipment which can accomodate heating/cooling loads of such structures. Single phase equipment has a lower initial cost, albeit higher operating expense.

    Limit Control: Control device used to open or close electrical circuits as temperature &/or pressure limits are reached. Failure of a furnace limit control switch is a common cause of "no-heat" service calls.

    Load Calculation: Analytical determination of required cooling and heating (BTUs) an HVAC system must deliver for occupant comfort. Based on a variety of factors including: volume of conditioned space, building orientation, number of occupants, size and placement of rooms, number and size of windows and doors, amount of insulation, number of floors, and climate. Required by jurisdictional code on new and significantly altered structures.

    LP Gas: Liquefied Petroleum used as a fuel gas.


    Matched System: An air conditioner or heat pump system composed of equipment that has been certified by ARI to work together to deliver the specified heating and cooling capacity at the stated efficiency rating

    Media: The material in a filter that traps and holds impurities.

    Montreal Protocol: A 1987 international agreement to reduce and eventually eliminate production of CFC's due to the adverse impact CFC's have upon the earth's protective ozone layer.


    NATE® : North American Technician Excellence, a nonprofit organization that conducts a vendor neutral certification program for HVAC technicians. It is currently the only technical credential supported by the entire industry.

    Natural-Draft Furnace: A furnace in which the natural flow of air from around the furnace provides the air to support combustion. It also depends on the pressure created by the heat in the flue gases to force them out through the venting system.

    Natural Gas: A mixture of primarily methane and other hydrocarbon used a fuel.

    National Comfort Institute

    NCI: National Comfort Institute (NCI) a private sector trade association representing HVAC Contractors serves the HVAC industry with advanced training and certification rooted in testing, mesurement, and proven performance.


    NEC: The National Electrical Code (NEC), or NFPA 70, is a standard for the safe installation of electrical wiring and equipment. It is part of the National Fire Codes series published by the National Fire Protection Association (NFPA). While the NEC is not itself a U.S. law, NEC use is commonly mandated by state or local law. The NEC codifies the requirements for safe electrical installations into a single, standardized source. The "Authority Having Jurisdiction" inspects for compliance with these minimum standards.

    NEMA: National Electrical Manufacturing Association.


    NFPA: National Fire Protection Association.


    Ozone: O3 is produced photo-chemically (smog) ; by electrical discharge (lightning) and also in the upper atmosphere by ultraviolet light. The ozone layer is the outermost layer of the earth's atmosphere that absorbs ultraviolet light from the sun thereby shielding the earth's surface from harmful radiation. An ozone concentration of 0.1ppm is generally considered the maximum permissible eight hour exposure. For continuous occupancy ozone should not exceed 0.01ppm (parts per million).


    Package Unit: A heating and cooling system contained in one outdoor unit. A package unit can be situated in a mechanical room, at grade adjacent to the structure, or on the rooftop.

    Payback Analysis: Overall measure of the efficiency and value of your home comfort system. By combining your purchase price and ongoing operating costs, a payback analysis determines the number of years required before monthly energy savings offset the purchase price.

    Plumbing-Heating-Cooling Contractors Association

    PHCC: Plumbinng-Heating-Cooling Contractors Assocation.

    PSI: Pounds per Square Inch.

    PVC: Polyvinyl chloride; a type of plastic commonly used in pipe construction. Condensate drain lines are 1 inch PVC. Also used to vent flue gas (typically 3 to 4 inch PVC) in condensing furnaces owing to their low stack temperature (below 212 degrees Fahrenheit).


    R-22: Chlorodifluoromethane, the most common refrigerant used in air conditioning and heat pump systems today. The EPA has mandated that equipment using R-22 cannot be manufactured after 2010 and all production of R-22 will cease by 2020 because it has been linked to the depletion of the ozone layer and global warming. Most commonly referred to by its trademarked name, Freon.

    R-410A: Azeotropic mixture of difluoromethane and pentafluoroethane, Unlike R-22 it does not contain chlorine nor does it contribute to ozone depletion, and is therefore becoming more widely used as ozone-depleting refrigerants are phased out. Commonly referred to by its trademarked names of Puron, AZ-20, and Suva 9100.

    Refrigerant: A chemical that produces a refrigerating effect while expanding and vaporizing. Most residential air conditioning systems contain R-22 refrigerant. R-22 is regulated by international controls under the Montreal Protocol and in the United States by the Environmental Protection Agency. It is scheduled to be in production until the year 2020. It's still used in approximately 90 percent of air conditioning equipment manufactured in the U.S. today.

    Refrigerant Charge: The required amount of refrigerant for a system to operate at its design efficiency. Residential Split Systems come with a "Factory Charge" which normally accommodates 15 feet of refrigeration tubing between outdoor and indoor units. If the distance is greater an installing technician with add the necessary refrigerant to attain the full charge required. During commissioning of a new system high side and low side pressures are carefully monitored to ensure that the manufacturers specifications are meet.

    Register: Combination grille and damper assembly covering an air opening of an air duct.

    Relative Humidity: The percent of moisture actually in the air compared to the maximum amount of moisture the air can hold at a given temperature.

    Relay: Switch (Contactor) that can be repeatedly cycled, making and breaking an electrical circuit. When sufficient current flows through the coil within the relay, the resulting magnetic field causes the contacts to be pulled in or closed.

    Return: The path the air takes to get to an air-handling unit or furnace so it can be cooled or heated. It is the "return" path. The return side should be "balanced" with the supply side to ensure proper air flow occupant comfort.

    Reversing Valve: A device in a heat pump that reverses the flow of refrigerant as the system is switched from cooling to heating.


    RSES: The RSES Educational Foundation is approved by the EPA to administer Section 608 certification testing. Section 608 of the Clean Air Act enforces activities that protect the ozone layer as they apply to stationary refrigeration and air conditioning systems. Under this program, certification is required to ensure that HVAC technicians are competent in EPA requirements on service practices and refrigerant reclamation.


    SEER: Seasonal Energy Efficiency Ratio. A measure of cooling efficiency for air conditioners and heat pumps. The higher the seer, the more energy efficient the unit. The government's minimum SEER rating is 13. Analogous to MPG in automobiles.

    Sensible Heat: The actual temperature of the air. This type of heat is measured with a thermometer.

    Setpoint: A temperature &/or pressure at which a controller is set with the expectation that this will be a nominal value depending on the range of the controller.

    Split System: A two-component heating and cooling (heat pump) or cooling only (air conditioner) system. The condensing unit is installed outside, the air handling unit is installed inside (preferably in conditioned space). Refrigerant lines and wiring connect them together.

    Supplemental Heat: Auxiliary back-up heat built into a heat pump system, which switches on when outdoor conditions drop below the unit's set point. These are resistance heat strips housed within the air handler that provide the balance of heat necessary to satisfy winter extremes.

    Supply: The part of an HVAC system that takes (supplies) the conditioned air from the air-handling unit or furnace to your home. The supply side should be "balanced" with the return side to ensure proper air flow and comfort.


    Therm: Quantity of heat equal to 100,000BTU.

    Thermostat: A device consisting of a series of sensors and relays that monitor ambient temperature and control the functions of a heating & cooling system.

    TXV: Thermostatic Expansion Valve, a refrigerant metering device that maintains a constant evaporator temperature by monitoring suction vapor superheat; also called a thermal expansion valve.

    Ton: Legacy terminology from early era ice based refrigeration and is cooling yielded by one ton of ice melting (changing state) over 24hours. Typically expressed as its equivalent of 12,000BTU's per hour.


    TracPipe® : A flexible stainless steel piping material used for natural gas and propane installations.


    Unitary System: A factory assembled heating/cooling system in one package and usually designed for conditioning one space or room. Underwriters Laboratories

    UL: Underwriters Laboratories. UL has developed more than 800 Standards for Safety.

    Upflow Furnace: A furnace in which air is drawn in through the sides or bottom and discharged out the top.

    UV: Ultraviolet, invisible radiation waves with frequencies shorter than visible light and longer than X rays. Used in devices designed eradicate microbial growth on evaporator coils.


    Vacuum: A pressure below atmospheric pressure. A perfect vacuum is 30 inches Mercury (periodic symbol "Hg"). A residential split system (refrigeration) that as been opened to the atmosphere must be evacuated in order to boil off water vapor (always present) which enters any system that has lost its refrigeration charge. This process is known as "pulling a vacuum" and must be accomplish by means of a standalone dedicated vacuum pump.

    Vapor Barrier: A moisture-impervious layer applied to the surfaces enclosing a humid space to prevent moisture travel to a point where it may condense due to lower temperature.

    Ventilation: The process of supplying or removing air, by natural or mechanical means, to or from any space. Such air may or may not have been conditioned.

    Ventilator: A ventilator captures heating or cooling energy from stale indoor air and transfers it to fresh incoming air.


    Watt: A unit of electrical power:
    1 watt equals 1 volt X 1 ampere
    1 watt equals 3.41 BTU
    746 watts equals 1 horsepower.


    Zone System: A method of dividing a home into zones and enabling you to control the amount of comfort provided to each.

    Zoning: The practice of providing independent heating and/or cooling to different areas in a structure. Zoning typically utilizes a system controller, zoning dampers controlled by a thermostat in each zone, and a bypass damper to regulate static pressure in the supply duct.

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