Absolute HumidityThe humidity of the air measured by the number of grains of water vapor present in one cubic meter of
air.
Absolute PressureThe sum of gauge and atmospheric pressure (psia).
Absolute TemperatureThe temperature measured on the Kelvin scale.
Absolute ZeroThe lowest temperature theoretically attainable on the Kelvin scale (approximately 273.16° C).
AbsorbentA substance with the ability to absorb another substance.
AbsorptionIn physics, the taking up of light, heat, or other energy by molecules. The absorbed energy is converted into heat. Absorption in chemistry is the taking up of one substance by another. For example, a
gas such as oxygen may be absorbed, or dissolved, in water. In the HVAC industry heat energy is absorbed from the medium being cooled and transferred in the refrigerant.
AccumulatorA shell device installed in the suction line of a HVAC system to prevent liquids from entering the compressor.
AccuratorA piston type metering device that feeds the proper amount of liquid refrigerant into the evaporator.
AcroleinAn agent added to methyl chloride to make you aware of refrigerant leaks.
Activated AluminaA type of aluminum oxide that absorbs moisture (used in refrigerant driers).
Activated CarbonA processed carbon used in filter driers and commonly used in air filters to clean the air.
Active CoolingHVAC term for compressor driven air conditioning.
AdiabaticA change in gas condition where no heat is added or removed except in the form of work.
AdsorptionAdsorption, which is often confused with absorption, refers to the adhering of molecules of gases and
liquids to the surfaces of porous solids. Adsorption is a surface phenomenon; absorption is an intermingling or interpenetration of two substances.
AFUEAnnualized 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 DOE
for furnaces is 78%.
Air ChangeThe amount of air required to completely replace the air in a room or building; not to be confused with
re-circulated air.
Air CleaningIn HVAC an IAQ control strategy to remove various airborne particulates and/or gases from the air.
The three types of air cleaning most commonly used are particulate filtration, electrostatic precipitation, and gas sorption.
Air ConditionerA device used to control temperature and humidity of the air.
Air ConditioningIn HVAC the control of the quality, quantity, and temperature-humidity of the air in an interior space.
Air CooledUses a fan to discharge heat from the condenser coil to the outdoors.
Air Cooler, Forced Circulation A factory-made encased assembly of elements by which heat it transferred from air to evaporating refrigerant.
Air DiffuserHVAC term for an air distribution outlet, typically located in the ceiling, which mixes conditioned air
with room air.
AirflowThe distribution or movement of air.
Air Exchange RateIn HVAC the rate at which outside air replaces indoor air in a space. Expressed in one of two ways:
the number of changes of outside air per unit of time - air changes per hour (ACH); or the rate at
which a volume of outside air enters per unit of time - cubic feet per minute (CFM).
Air HandlerHVAC term for a fan-blower, heat transfer coil, and housing parts of a system.
Air Handling Unit (AHU)In HVAC refers to equipment that includes a blower or fan, heating and/or cooling coils, and related
equipment such as controls, condensate drain pans, and air filters. Does not include ductwork, registers or grilles, or boilers and chillers.
Air InfiltrationThe unwanted entrance of air due to leakage, temperature difference, or wind.
Air PassagesOpenings through or within walls, through floors and ceilings, and around chimney flues and plumbing
chases, that permit air to move out of the conditioned spaces of the building.
Air Retarder/BarrierA material or structural element that inhibits air flow into and out of a building's envelope or shell. This
is a continuous sheet composed of polyethylene, polypropylene, or extruded polystyrene. The sheet is
wrapped around the outside of a house during construction to reduce air in-and exfiltration, yet allow
water to easily diffuse through it.
Air StandardHVAC term for air having a temperature of 68 degrees F (20 degrees C) and a relative humidity of
36%% at 14.7 psia.
Air to AirWhere both the condensing and evaporating mediums are air.
Air to Air Energy Recovery Ventilation Equipment(AAERVE) Energy recovery components and packaged energy recovery ventilation which employ Airto-Air Heat Exchangers to recover energy from exhaust air for the purpose of pre-conditioning outdoor
air prior to supplying the conditioned air to the space, either directly or as part of an air-conditioning (to
include air heating, air cooling, air circulating, air cleaning, humidifying and dehumidifying) system.
Air VentHVAC term for a valve, either manual or automatic, that is used to remove unwanted air from the highest point of a piping system.
Alcohol BrineWater and alcohol solution witch remains a liquid below 32 degress F.
AllergenA substance capable of causing an allergic reaction because of an individual's sensitivity to that substance.
Ambient Air –
Generally speaking, the air surrounding an object. In a domestic or commercial refrigerating system
having an air-cooled condenser, the temperature of the air entering the condenser.
Annual Fuel Utilization Efficiency (AFUE)A rating that denotes the efficiency of gas heating equipment. It is the amount of heating your equipment delivers for every dollar spent on fuel. A higher rating indicates more efficient equipment. This
rating is calculated in accordance with the Department of Energy test procedures.
ARIAir-Conditioning and Refrigeration Institute is a non-profit, voluntary organization comprised of heating, air conditioning and refrigeration manufacturers. ARI publishes standards for testing and rating
heat pumps and air conditioners to provide you with a standardized measure of comparison. So, ARI
ensures a level of quality within the industry.
Atmospheric PressureThe weight of a 1 square inch column of the earth's atmosphere. At sea level this pressure is 14.696
pounds per square inch.
AtticThe usually unfinished space above a ceiling and below a roof.
Attic FanA fan mounted on an attic wall used to exhaust warm attic air to the outside.
Attic VentHVAC term for a passive or mechanical device used to ventilate an attic space, primarily to reduce
heat buildup and moisture condensation.
Auto ChangeoverA control package that provides for automatic switching from a primary air conditioning system to a
backup in the event of a failure of the primary system.
Available HeatThe amount of heat energy that may be converted into useful energy from a fuel.
Back Pressure Loose terminology for suction pressure of refrigeration vapor in a system.
Balance PointAn 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.
BioaerosolsMicroscopic living organisms suspended in the air that grow and multiply in warm, humid places.
BlowerIn HVAC the device in an air conditioner that distributes the filtered air from the return duct over the
coil/heat exchanger. This circulated air is cooled/heated and then sent through the supply duct, past
dampers, and through supply diffusers to the living/working space.
BoilerA vessel or tank where heat produced from the combustion of fuels such as natural gas, fuel oil, or
coal is used to generate hot water or steam for applications ranging from building space heating to
electric power production or industrial process heat.
Boiler PressureThe pressure of the steam or water in a boiler as measured, usually expressed in pounds per square
inch gauge (psig).
Boiler RatingThe heating capacity of a steam boiler expressed in BTU per hour (BTU/H), horsepower, or pounds of
steam per hour.
Boiling PointThe temperature at which the addition of any heat will begin a change of state from a liquid to a vapor.
Bottled GasA generic term for liquefied and pressurized gas, ordinarily butane, propane, or a mixture of the two,
contained in a cylinder for domestic use.
Breathing ZoneArea of a room in which occupants breathe as they stand, sit, or lie down.
British Thermal Unit (BTU)The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit,
equal to 252 calories.
British Thermal Unit (BTU) Heat required to produce a temperature rise of 1 degree Fahrenheit in 1 lb. of water. The mean BTU
is 1/180 of the energy required to heat water from 32ºF. to 212ºF.
Building EnvelopeElements of the building, including all external building materials, windows, and walls, that enclose the
internal space.
Burner CapacityThe maximum heat output (in BTU per hour) released by a burner with a stable flame and satisfactory
combustion.
CAEThe Combined Annual Efficiency is a measure of the amount of heat produced for every dollar of fuel
consumed for both home and water heating.
CapacityRefrigerating rating system usually measured in BTU ’s per hour (or Btu/h).
Carbon Dioxide (CO2)A colorless, odorless noncombustible gas with the formula CO2 that is present in the atmosphere. It is
formed by the combustion of carbon and carbon compounds (such as fossil fuels and biomass), by
respiration, which is a slow combustion in animals and plants, and by the gradual oxidation of organic
matter in the soil.
Carbon Monoxide (CO)A colorless, odorless but poisonous combustible gas with the formula CO. Carbon monoxide is produced in the incomplete combustion of carbon and carbon compounds such as fossil fuels (i.e. coal,
petroleum) and their products (e.g. liquefied petroleum gas, gasoline), and biomass.
Ceiling PlenumSpace below the flooring and above the suspended ceiling that accommodates the mechanical and
electrical equipment and that is used as part of the air distribution system. The space is kept under
negative pressure.
Central Air ConditioningSee Air Conditioner.
Central Air Handling Unit (Central AHU)This is the same as an Air Handling Unit, but serves more than one area.
Central Heating SystemIn HVAC a system where heat is supplied to areas of a building from a single appliance through a network of ducts or pipes.
Centrifugal CompressorA 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.
Centigrade (Measure of Temperature)A temperature scale with the freezing point of water 00 and the boiling point 1000 at sea level.
CFM Cubic feet per minuteHVAC term for the amount of air, in cubic feet, that flows through a given space in one minute. 1 CFM
equals approximately 2 liters per second (l/s).
Change of Air Introduction of new, cleansed or recirculated air to conditioned space, measured by the number of
complete changes per unit time.
Chill To apply refrigeration moderately, as to meats, without freezing.
Chilled Water SystemA 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.
Chilling Room Room where animal carcasses are cooled after dressing prior to cold storage.
ChimneyA masonry or metal stack that creates a draft to bring air to a fire and to carry the gaseous byproducts
of combustion safely away.
Chimney EffectThe tendency of heated air or gas to rise in a duct or other vertical passage, such as in a chimney,
small enclosure, or building, due to its lower density compared to the surrounding air or gas.
Combination FoundationsBuildings constructed with more than one foundation type; e.g., basement/crawlspace or basement/
slab-on-grade.
CombustionThe process of burning; the oxidation of a material by applying heat, which unites oxygen with a material or fuel.
Combustion AirAir that provides the necessary oxygen for complete, clean combustion and maximum heating value.
Combustion ChamberAny wholly or partially enclosed space in which combustion takes place.
Combustion GasesThe gaseous byproducts of the combustion of a fuel.
Comfort Air Conditioning The simultaneous control of all, or at least the first three, of the following factors affecting the physical
and chemical conditions of the atmosphere within a structure for the purpose of human comfort; temperature, humidity, motion, distribution, dust, bacteria, odors, toxic gasses and ionization, most of
which affect in greater or lesser degree human health or comfort.
Comfort Cooling Refrigeration for comfort as opposed to refrigeration for storage or manufacture.
CompressorA device used to compress air for mechanical or electrical power production, and in air conditioners,
heat pumps, and refrigerators to pressurize the refrigerant and enabling it to flow through the system.
CompressionThe reduction of volume of a vapor or gas by mechanical means.
Compression RatioThe ratio determined by dividing the discharge pressure, in PSI, by the suction pressure in PSI.
CondensationThe process by which a gas is changed into a liquid at constant temperature by heat removal.
Condensation PointThe temperature at which the removal of any heat will begin a change of state from a vapor to a liquid.
CondenserThe device in an air conditioner or heat pump in which the refrigerant condenses from a gas to a liquid
when it is depressurized or cooled.
Condenser CoilThe device in an air conditioner or heat pump through which the refrigerant is circulated and releases
heat to the surroundings when a fan blows outside air over the coils. This will return the hot vapor that
entered the coil into a hot liquid upon exiting the coil.
Condensing UnitThe component of a central air conditioner that is designed to remove heat absorbed by the refrigerant
and transfer it outside the conditioned space.
Conditioned AirAir that has been heated, cooled, humidified, or dehumidified to maintain an interior space within the
"comfort zone." (Sometimes referred to as "tempered" air.)
Conditioned SpaceThe interior space of a building that is heated or cooled.
Constant Air Volume SystemsAir handling system that provides a constant air flow while varying the temperature to meet heating
and cooling needs.
Cooling AnticipatorA 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 CapacityThe quantity of heat that a cooling appliance is capable of removing from a room in one hour.
Cooling Degree DayA value used to estimate interior air cooling requirements (load) calculated as the number of degrees
per day (over a specified period) that the daily average temperature is above 65 degrees Fahrenheit
(or some other, specified base temperature). The daily average temperature is the mean of the maximum and minimum temperatures recorded for a specific location for a 24 hour period.
Cooling LoadHeat which flows into a space from outdoors and/or indoors.
COPCoefficient of performance compares the heating capacity of a heat pump to the amount of electricity
required to operate the heat pump in the heating mode. COPs vary with the outside temperature: as
the temperature falls, the COP falls also, since the heat pump is less efficient at lower temperatures.
ARI standards compare equipment at two temperatures, 47 F and 17 F, to give you an idea of the
COP in both mild and colder temperatures.
DampersHVAC term for controls that vary airflow through an air outlet, inlet, or duct. A damper position may be
immovable, manually adjustable or part of an automated control system.
Decibel (db)A decibel describes the relative loudness of a sound. Some common sounds are fairly close to a typical air conditioner or heat pump's sound level: human voice, 7.0 decibels; blender, 8.8 decibels.
Defrost CycleThe process of removing ice or frost buildup from the outdoor coil during the heating season.
Degree DayA unit for measuring the extent that the outdoor daily average temperature (the mean of the maximum
and minimum daily dry-bulb temperatures) falls below (in the case of heating, see Heating Degree
Day), or falls above (in the case of cooling, see Cooling Degree Day) an assumed base temperature,
normally taken as 65 degrees Fahrenheit, unless otherwise stated. One degree day is counted for
each degree below (for heating) or above (in the case of cooling) the base, for each calendar day on
which the temperature goes below or above the base.
Degree HourThe product of 1 hour, and usually the number of degrees Fahrenheit the hourly mean temperature is
above a base point (usually 65 degrees Fahrenheit); used in roughly estimating or measuring the cooling load in cases where processes heat, heat from building occupants, and humidity are relatively unimportant compared to the dry-bulb temperature.
Dehumidification The conservation of water vapor from air by cooling below the dew point or removal of water vapor
from air by chemical or physical methods.
DehumidifierA device for reducing the level of humidity in a room or home.
DehydrationThe removal of water vapor from air by the use of absorbing materials. (2) The removal of water from
stored goods.
Demand (tankless) Water HeaterA type of water heater that has no storage tank thus eliminating storage tank stand-by losses. Cold
water travels through a pipe into the unit, and either a gas burner or an electric element heats the water only when needed.
Dew PointTemperature at which condensation starts if moist air is cooled at constant pressure with no loss or
gain of moisture during the cooling process.
Differential (of a control)The difference between cut-in and cut-out temperature or pressure.
Diffusers and GrillesComponents of the ventilation system that distribute and return air to promote air circulation in the occupied space. As used in this document, supply air enters a space through a diffuser or vent and return air leaves a space through a grille.
Direct Expansion SystemsOne of two types of basic cooling media (the other is chilled water). Direct expansion systems utilize
freon for cooling and dehumidification. The three most common methods of heat rejection are air
cooled, water cooled and glycol cooled.
Direct Water HeaterA type of water heater in which heated water is stored within the tank. Hot water is released from the
top of the tank when a hot water faucet is turned. This water is replaced with cold water that flows into
the tank and down to just above the bottom plate under which are the burners.
DOEThe Department of Energy is a federal agency in charge of setting industry efficiency standards and
monitoring the consumption of energy sources.
DownflowA type of furnace that takes cool air from the top and blows warm air to the bottom.
DraftA column of burning combustion gases that are so hot and strong that the heat is lost up the chimney
before it can be transferred to the house. A draft brings air to the fire to help keep it burning.
Draft DiverterA door-like device located at the mouth of a fireplace chimney flue for controlling the direction and flow
of the draft in the fireplace as well as the amount of oxygen that the fire receives.
Draft HoodA device built into or installed above a combustion appliance to assure the escape of combustion byproducts, to prevent backdrafting of the appliance, or to neutralize the effects of the stack action of the
chimney or vent on the operation of the appliance.
Dry Bulb TemperatureTemperature measured by ordinary thermometer (term used only to distinguish from wet-bulb temperature).
Dual Duct SystemAn air conditioning system that has two ducts, one is heated and the other is cooled, so that air of the
correct temperature is provided by mixing varying amounts of air from each duct.
Dual FuelA comfort system that pairs an electric heat pump with a gas furnace, providing an energy-efficient
alternative to the conventional furnace/air conditioner combination.
Duct A conduit or tube used for conveying air or other gas.
Duct FanHVAC term for an axial flow fan mounted in a section of duct to move conditioned air.
Duct(s) DuctworkThe round or rectangular tube(s), generally constructed of sheet metal, fiberglass board, or a flexible
plastic-and-wire composite, located within a wall, floor, and ceiling that distributes heated or cooled air
in buildings.
EconomizerIn a chiller with a two-stage centrifugal compressor, the discharge from the first stage impeller and the
inlet to the second stage impeller are at a pressure level approximately half way between the cooler
pressure and condenser pressure. With this arrangement, an economizer may be used. This is a shell
within which refrigerant liquid from the condenser drops down to the interstage pressure, flashing off
some of the refrigerant which is drawn directly into the second stage impeller. This reduces the amount
of refrigerant which has to be compressed by the first stage impeller, improving the refrigeration cycle
efficiency.
EffectivenessThe measure of energy recovery effectiveness not adjusted to account for that portion of the psychrometric change in the leaving supply air that is the result of leakage of entering exhaust air rather
than the exchange of heat or moisture between the airstreams. (source: ARI-1060)
EfficiencyA rating on comfort equipment is similar to the miles per gallon rating on your car. The higher the rating
number, the more efficient the system and the lower your fuel consumption will be. You can save a lot of
money with a high efficiency unit, as shown in the chart below. Depending on your local climate, lifestyle
and electricity rates, savings will vary.
Electronic Air CleanerAn electronic device that filters out large particles and bioaerosols in indoor air.
Emergency Heat (Supplemental or Auxillary Heat)The back-up heat built into a heat pump system.
Energy Efficiency Ratio (EER)Means 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 Recovery Ventilator (ERV)A ventilator combining a full enthalpic air-to-air heat exchanger which transfers both sensible heat and
humidity (latent heat) between air streams with some combination of fans or blowers to provide fresh air
into a building and exhaust air out of a building at minimized cost.
ENERGY STAR®An EPA (Environmental Protection Agency) designation attached to HVAC products that meet or exceed EPA guidelines for high-efficiency performance above the standard government minimums.
Enthalpy ExchangeThe exchange of both sensible and latent heat energies.
EnthalpyA thermodynamic function of a system, equivalent to the internal energy plus the product of the pressure
and the volume.
Environmental AgentsConditions other than indoor air contaminants that cause stress, comfort, and/or health problems (e.g.,
humidity extremes, drafts, lack of air circulation, noise, and over-crowding).
Evaporator CoilThe part of the air conditioner or heat pump that is located inside the air handler or attached to the furnace. Its primary function is to absorb the heat from the air in your house.
Exhaust Air (EA)Stale indoor air leaving a building
External EqualizerIn a thermostatic expansion valve, a tube connection from the chamber containing the evaporation
pressure-actuated element of the valve to the outlet or the evaporator coil. A device to compensate for excessive pressure drop throughout the coil.
Fahrenheit (Represented as degrees "F")The scale of temperature measurement most commonly used in the United States of America.
Filter (air)A device that removes contaminants, by mechanical filtration, from the fresh air stream before the air
enters the living space. Filters can be installed as part of a heating/cooling system through which air
flows for the purpose of removing particulates before or after the air enters the mechanical components.
Fireplace InsertA wood or gas burning heating appliance that fits into the opening or protrudes on to the hearth of a
conventional fireplace.
Flash GasThe gas resulting from the instantaneous evaporation of refrigerant in a pressure-reducing device to
cool the refrigerant to the evaporations temperature obtained at the reduces pressure.
FlueThe structure (in a residential heating appliance, industrial furnace, or power plant) into which combustion gases flow and are contained until they are emitted to the atmosphere.
Flooded SystemSystem in which only part of the refrigerant passing over the heat transfer surface is evaporated, and
the portion not evaporated is separated from the vapor and recirculated. In commercial systems, one
controlled by a float valve.
Flue GasThe gas resulting from the combustion of a fuel that is emitted to the flue.
Forced Air System or FurnaceHVAC term for a type of heating system in which heated air is blown by a fan through air channels or
ducts to rooms.
Free Cooling SystemTypically a water cooled or glycol cooled system with an additional coil that provides chilled water
cooling when the outdoor ambient is cold thereby reducing or eliminating compressor operation. Provides efficient system operation in Nothern climates.
Free DeliveryThere are no ducts and the unit may be installed in the field without ducts if needed. Ground-Source The ground or soil below the frost line is being used as the heat source or heat sink for a heat pump.
FreonA registered trademark for a cholorfluorocarbon (CFC) gas that is highly stable and that has been historically used as a refrigerant.
Fresh Air (FA)Outside ventilation air entering a building.
Frost BackThe flooding of liquid from an evaporator into the suction line accompanied by frost formation in suction line in most cases.
Fuel EfficiencyThe ratio of heat produced by a fuel for doing work to the available heat in the fuel.
Fuel OilAny liquid petroleum product burned for the generation of heat in a furnace or firebox, or for the generation of power in an engine. Domestic (residential) heating fuels are classed as Nos. 1, 2, 3; Industrial fuels as Nos. 4, 5, and 6.
FungiAny of a group of parasitic lower plants that lack chlorophyll, including molds and mildews.
Furnace (Residential)In HVAC a combustion heating appliance in which heat is captured from the burning of a fuel for distribution, comprised mainly of a combustion chamber and heat exchanger. That part of an environmental system which converts gas, oil, electricity or other fuel into heat for distribution within a structure.
Fuel OilAny liquid petroleum product burned for the generation of heat in a furnace or firebox, or for the generation of power in an engine. Domestic (residential) heating fuels are classed as Nos. 1, 2, 3; Industrial fuels as Nos. 4, 5, and 6.
FungiAny of a group of parasitic lower plants that lack chlorophyll, including molds and mildews.
Furnace (Residential)In HVAC a combustion heating appliance in which heat is captured from the burning of a fuel for distribution, comprised mainly of a combustion chamber and heat exchanger. That part of an environmental system which converts gas, oil, electricity or other fuel into heat for distribution within a structure.
Garage HeatersGarage heaters are an excellent product for combating cold garages and workshops during the winter
months. Heating capacities provide warmth for a 1 car garage up to a 4+ car garage application.
Separated combustion models provide heating for hard-to-heat applications. Available in natural or
propane gas, units should be installed by a Lennox dealer/installer.
Gas SorptionHVAC term for devices used to reduce levels of airborne gaseous compounds by passing the air
through materials that extract the gases. The performance of solid sorbents is dependent on the airflow rate, concentration of the pollutants, presence of other gases or vapors, and other factors.
GeneratorIn an absorption cycle, the vessel in which the lithium bromide solution is reconcentrated by boiling off
the previously absorbed water.
Glycol-Cooled SystemA type of air conditioning system that uses freon as a refrigerant and a water/glycol solution as a condensing medium. Typically, the glycol-cooled condenser is located inside the air conditioner with the
rest of the refrigeration components. Water/glycol is piped to the unit from a drycooler or other suitable
source. The glycol keeps the solution from freezing during winter operation.
Grid-tied
A grid-tied system is the most common and least expensive of all residential solar systems. It allows
you to use your own solar-generated electricity to save energy and reduce costs. At times when the
solar system isn’t producing electricity, such as at night, electricity is provided by the utility company’s
grid, or network of power stations.
Ground Water-SourceWater from an underground well is being used as the heat source or heat sink for a heat pump.
Head PressureOperating pressure measured in the discharge line at the outlet from the compressor.
HeatA form of thermal energy resulting from combustion, chemical reaction, friction, or movement of electricity. As a thermodynamic condition, heat, at a constant pressure, is equal to internal or intrinsic energy
plus pressure times volume.
Heat ExchangerLocated in the furnace, the heat exchanger transfers heat to the surrounding air, which is then pumped
throughout the home.
Heat GainThe 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, Latent Heat characterized by change of state of the substance concerned, for a given pressure and always at a
constant temperature for a pure substance, i.e., heat of vaporization or fusion.
Heat LossThe heat that flows from the building interior, through the building envelope to the outside environment.
Heat PumpA 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 your home's air ducts. In the summer, it reverses the
process and removes heat from your house and releases it outdoors.
Heat Recovery Ventilator (HRV)A ventilator combining an air-to-air heat exchanger which transfers sensible heat between air streams
with some combination of fans or blowers to provide fresh air into a building and exhaust air out of a
building.
Heat RateThe ratio of fuel energy input as heat per unit of net work output; a measure of a power plant thermal
efficiency, generally expressed as Btu per net kilowatt-hour.
Heat RegisterThe grilled opening into a room by which the amount of warm air from a furnace can be directed or controlled; may include a damper.
Heat SourceA body of air or liquid from which heat is collected. With any heat pumps, the air outside the home is
used as the heat source during the heating cycle.
Heating Capacity (Also Specific Heat)The quantity of heat necessary to raise the temperature of a specific mass of a substance by one degree.
Heating Degree Day(s) (HDD)The number of degrees per day that the daily average temperature (the mean of the maximum and minimum recorded temperatures) is below a base temperature, usually 65 degrees Fahrenheit, unless otherwise specified; used to determine indoor space heating requirements and heating system sizing. Total
HDD is the cumulative total for the year/heating season. The higher the HDD for a location, the colder
the daily average temperature(s).
Heating Degree Day(s) (HDD)The number of degrees per day that the daily average temperature (the mean of the maximum and minimum recorded temperatures) is below a base temperature, usually 65 degrees Fahrenheit, unless otherwise specified; used to determine indoor space heating requirements and heating system sizing. Total
HDD is the cumulative total for the year/heating season. The higher the HDD for a location, the colder
the daily average temperature(s).
Heating LoadThe rate of heat flow required to maintain a specific indoor temperature; usually measured in Btu per
hour.
HEPAHigh efficiency particulate arrestance (filters).
High SideParts of refrigerating system under condenser pressure.
Home Energy Rating Ssystems (HERS)A nationally recognized energy rating program that gives builders, mortgage lenders, secondary lending
markets, homeowners, sellers, and buyers a precise evaluation of energy losing deficiencies in homes.
Builders can use this system to gauge the energy quality in their home and also to have a star rating on
their home to compare to other similarly built homes.
Horizontal FlowWhen an air handler or furnace is positioned on its side and circulates air in one end and out the other.
Ideal for attic or crawl space installations.
Hot Air FurnaceA heating unit where heat is distributed by means of convection or fans.
HSPFHeating seasonal performance factor is similar to SEER, but it measures the efficiency of the heating
portion of your heat pump. Like SEER, industry minimums have been raised recently, and the minimum
is now 6.80 HSPF.
HumidifierA device for increasing the humidity in a room or home.
HumidityA measure of the moisture content of air; may be expressed as absolute, mixing ratio, saturation deficit,
relative, or specific.
Humidity RatioThe weight of water divided by the weight of air containing the water vapor. This corresponds directly to
the dewpoint temperature and does not change with air temperature.
HumidistatAn automatic device used to maintain humidity at a fixed or adjustable set point.
HVACHVAC is an acronym for heating, ventilation, and air-conditioning system.
IAPIndoor air polution.
IAQIndoor air quality.
IgniteTo heat a gaseous mixture to the temperature at which combustion takes place.
Ignition PointThe minimum temperature at which combustion of a solid or fluid can occur.
Indoor AirThe air the people breathe inside a built environment.
Indoor Air PollutantParticles and dust, fibers, mists, bioaerosols, and gases or vapors.
Indoor CoilsA homes comfort system consist of two components: the outdoor unit (air conditioner or heat pump) and
the indoor unit (coil or blower coil). Combinations of various units will result in vastly different efficiency
ratings. Unreasonably high efficiency ratings can be created by using unrealistic indoor and out door
equipment combinations. The term "most popular coil" indicates the actual tested combinations; other
ratings may be simulated and unrealistic. Be sure that the efficiency ratings you are comparing are for
"most popular coil." You'll know the ratings are attainable and close to reality.
Indoor UnitThis is usually located inside the house and contains the indoor coil, fan, motor, and filtering device,
sometimes called the air handler.
InfiltrationAir leakage inward through cracks and interstices and through cielings, floors, and walls of a space or
building.
InhalableParticles small enough to be inhaled, but large enough so that they are not quickly exhaled.
InsulationAny material that slows down the transfer of heat.
Integrally Controlled Motor (ICM)A variable-speed motor that operates at low RPM when possible for efficiency and quiet operation. ICM
motors are more than 90% efficient versus 60% efficiency for conventional motors.
Integrated Heating SystemsHVAC term for a type of heating appliance that performs more than one function, for example space and
water heating.
InverterSome outdoor units use inverter compressor technology (Variable Frequency Drive) to provide exceptional indoor, high-speed cooling and heating. By responding to indoor temperature changes, these systems reduce power consumption by varying the compressor speed for extra energy savings. The system operates only at the levels needed to maintain a constant and comfortable indoor environment.
IonAn electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged.
IonizerA device that removes airborne particles from breathable air. Negative ions are produced and give up
their negative charge to the particles. These new negative particles are then attracted to the positive
particles surrounding them. This accumulation process continues until the particles become heavy
enough to fall to the ground.
ISO 9000A family of international standards for quality management and assurance.
Isolation ValvesValves used for the transfer and isolation of refrigerant charge in the cooler or condenser, allowing refrigerant to be stored inside a chiller during servicing.
(K) FactorThe insulating value of any material. Also known as conductivity.
KeroseneA type of heating fuel derived by refining crude oil that has a boiling range at atmospheric pressure from
400 degrees to 550 degrees F.
Kilowatt (kW)A kilowatt equals 1,000 Watts. A kilowatt hour (kWh) is the amount of kilowatts of electricity used in one
hour of operation of any equipment.
Latent EnergyPotential heat energy as water vapor / Humidity.
Latent Cooling CapacityAn A/C system's capability to remove moisture from the air.
Latent HeatThe heat energy needed to change the state of a substance (i.e.: from a liquid to a gas) but not it's temperature.
LCDAn abbreviation for a screen type - Liquid Crystal Display.
Liquid Line The tube or pipe carrying the refrigerant liquid from the condenser or receiver of a refrigerating system
to a pressure-reducing device.
Load CalculationA mathematical design tool used to determine the heat gain and heat loss in a building so that properly
sized air conditioning and heating equipment may be installed.
Low SideParts of a refrigerating system under evaporator pressure.
Matched SystemA heating and cooling system comprised of products that have been certified to perform at promised
comfort and efficiency when used together, and used according to design and engineering specifications.
Mechanically Ventilated Crawlspace SystemIn HVAC a system designed to increase ventilation within a crawlspace, achieve higher air pressure in
the crawlspace relative to air pressure in the soil beneath the crawlspace, or achieve lower air pressure
in the crawlspace relative to air pressure in the living spaces, by use of a fan.
MERV RatingThe MERV (Minimum Efficiency Reporting Value) 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.
MicroprocessorElectrical component consisting of integrated circuits which may accept, store, control, and output information.
MitigationMeasure taken to reduce adverse effects on the environment.
Natural-Draft FurnaceA 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
vent system.
NFPANational Fire Protection Association.
Natural VentilationIn HVAC the movement of outdoor air into a space through intentionally provided openings, such as
windows and doors, or through nonpowered ventilatiors or by infiltration.
Negative PressureCondition that exists when less air is supplied to a space than is exhausted from the space, so the air
pressure within that space is less than that in surrounding areas. Under this condition, if an opening exists, air will flow from surrounding areas into the negatively pressurized space.
Operating CostThe day-to-day cost of running your home comfort equipment, based on daily energy use.
Organic CompoundsChemicals that contain carbon. Volatile organic compounds vaporize at room temperature and pressure.
They are found in many indoor sources, including many common household products and building materials.
Outdoor AirAir taken from the external atmosphere and, therefore, not previously circulated through the system.
Outdoor Air SupplyHVAC term for air brought into a building from the outdoors (often through the ventilation system) that
has not been previously circulated through the system. Also known as "Make-Up Air".
Packaged SystemA piece of air conditioning and heating equipment in which all components are located in one cabinet.
Used occasionally in residential applications, the packaged unit is installed either beside or on top of the
home.
Particulate MatterA state of matter in which solid or liquid substances exist in the form of aggregated molecules or particles. Airborne particulate matter is typically in the size range of 0.01 to 100 micrometers.
Packaged UnitA self-contained heating and/or air conditioning system.
ParticulatesFine liquid or solid particles such as dust, smoke, mist, fumes, and fog found in air and emissions.
PlenumHVAC term for an air compartment connected to a duct or ducts.
Positive PressureCondition that exists when more air is supplied to a space than is exhausted, so the air pressure within
that space is greater than that in surrounding areas. Under this condition, if an opening exists, air will
flow from the positively pressurized space into surrounding areas.
Pressure DropPressure drop through the heat exchanger shall be expressed as the difference in static pressure between the Entering Supply Airflow and the Leaving Supply Airflow.
Pressure, StaticIn flowing air, the total pressure minus velocity pressure. The portion of the pressure that pushes equally
in all directions.
PRESSURE, TOTALIn flowing air, the sum of the static pressure and the velocity pressure.
Pressure, VelocityIn flowing air, the pressure due to the velocity and density of the air.
Preventative MaintenanceRegular and systematic inspection, cleaning, and replacement of worn parts, materials, and systems.
Preventive maintenance helps to prevent parts, material, and systems failure by ensuring that parts, materials and systems are in good working order.
Programmable ThermostatA type of thermostat that allows the user to program into the devices' memory a pre-set schedule of
times (when certain temperatures occur) to turn on HVAC equipment.
PropaneA hydrocarbon gas, C3H8, occurring in crude oil, natural gas, and refinery cracking gas. It is used as a
fuel, a solvent, and a refrigerant. Propane liquefies under pressure and is the major component of liquefied petroleum gas (LPG).
R-410A RefrigerantA chlorine-free refrigerant that meets the EPA's newest, most stringent environmental guidelines.
R-ValueA measure of the capacity of a material to resist heat transfer. The R-Value is the reciprocal of the conductivity of a material (U-Value). The larger the R-Value of a material, the greater its insulating properties.
Radiant BarrierIn HVAC a thin, reflective foil sheet that exhibits low radiant energy transmission and under certain conditions can block radiant heat transfer; installed in attics to reduce heat flow through a roof assembly
into the living space.
Radiant Ceiling PanelsCeiling panels that contain electric resistance heating elements embedded within them to provide radiant heat to a room.
Radiant EnergyEnergy that transmits away from its source in all directions.
Radiant FloorA 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 heats from
the bottom up. Radiant floor heating eliminates the draft and dust problems associated with forced air
heating systems.
Radiant Heat TransferRadiant heat transfer occurs when there is a large difference between the temperatures of two surfaces
that are exposed to each other, but are not touching.
Radiant Heating SystemHVAC term for a heating system where heat is supplied (radiated) into a room by means of heated surfaces, such as electric resistance elements, hot water (hydronic) radiators, etc.
RadiatorA room heat delivery (or exchanger) component of a hydronic (hot water or steam) heating system; hot
water or steam is delivered to it by natural convection or by a pump from a boiler.
Radiator VentA device that releases pressure within a radiator when the pressure inside exceeds the operating limits
of the vent.
Re-entrySituation that occurs when the air being exhausted from a building is immediately brought back into the
system through the air intake and other openings in the building envelope.
Recirculated AirAir removed from the conditioned space and used for ventilation, heating, cooling, humidification, or dehumidification.
RefrigerantThe compound (working fluid) used in air conditioners, heat pumps, and refrigerators to transfer heat
into or out of an interior space. This fluid boils at a very low temperature enabling it to evaporate and
absorb heat.
RefrigerationThe process of the absorption of heat from one location and its transfer to another for rejection or recuperation.
Refrigeration CapacityA measure of the effective cooling capacity of a refrigerator, expressed in Btu per hour or in tons, where
one (1) ton of capacity is equal to the heat required to melt 2,000 pounds of ice in 24 hours or 12,000
Btu per hour.
Refrigerant LinesTwo copper lines that connect the outdoor air conditioner or heat pump to the indoor evaporator coil.
RegisterCombination grille and damper assembly covering an air opening or end of an air duct.
Relative HumidityA measure of the percent of moisture actually in the air compared with what would be in it if it were fully
saturated at that temperature. When the air is fully saturated, its relative humidity is 100 percent.
RespirationProduction of CO2 and the heat by ripening of perishables in storage.
Return AirAir that is returned to a heating or cooling appliance from a heated or cooled space.
Return DuctThe central heating or cooling system contains a fan that gets its air supply through these ducts, which
ideally should be installed in every room of the house. The air from a room will move towards the lower
pressure of the return duct.
Reversing valveA device in a heat pump that reverses the flow of refrigerant as the system is switched from cooling to
heating. Also called a four-way valve.
Scroll CompressorA specially designed compressor that works in a circular motion, as opposed to up-and-down piston action.
Seasonal Energy Efficiency Ratio (SEER)A measure of seasonal or annual efficiency of a central air conditioner or air conditioning heat pump. It
takes into account the variations in temperature that can occur within a season and is the average number of Btu of cooling delivered for every watt-hour of electricity used by the heat pump over a cooling
season.
SEERSeasonal energy efficiency ratio is the amount of cooling your equipment delivers per every dollar spent
on electricity. SEER applies to air conditioners and heat pumps. In the past, a unit with a SEER of 8.00
was considered standard efficiency, and a unit with a 10.00 SEER was considered high efficiency. After
January 1, 1992, the minimum SEER required by the DOE is 10.00 and 15.00+ SEER is considered
high efficiency.
Sensible Cooling CapacityAn A/C system's capability to remove heat from the air. (lower the temperature).
Sensible HeatTemperature. Heat which is associated with a change in temperature; specific heat x change of temperature; in contrast to a heat interchange in which a change of state (latent heat) occurs.
Setback ThermostatA thermostat that can be set to automatically lower temperatures in an unoccupied house and raise
them again before the occupant returns.
SetpointThe temperature to which a thermostat is set for desired comfort level.
Single PackageA year-round heating and air conditioning system that has all of the components completely encased in
one unit outside the home.
SoffitA panel which covers the underside of an roof overhang, cantilever, or mansard.
Sound RatingsAlthough sound does not affect the efficiency of a unit, it will certainly affect your comfort. If your unit
has a low sound level, you will hardly notice it is operating. But if it has a higher sound level, it may
mean your good night's sleep is disturbed every time it runs!
Sound Rating Number (SRN)Sound is measured in bels (a bel equals 10 decibels). The SRN of a unit is based on ARI test, performed at ARI standard rating conditions. Average sound rating range from 7.0 to 8.0 decibels. The
lower the SRN rating, the quieter the unit.
Space HeaterA movable or fixed heater used to heat individual rooms.
Specific HeatEnergy per unit of mass required to produce one degree rise in temperature, usually BTU per lb. degree
F. numerically equal to cal. per gram degree C.
Specific HumidityThe weight of water vapor, per unit weight of dry air.
Split-Ductless SystemComprised of a remote outdoor condensing unit connected by refrigerant pipes to a matching, nonducted indoor air handler. Special cases for introducing fresh air may call for limited ducting to air handler from outside.
Split System Air ConditionerHVAC term for an air conditioning system that comes in two to five pieces: one piece contains the compressor, condenser, and a fan; the others have an evaporator and a fan. The condenser, installed outside the house, connects to several evaporators, one in each room to be cooled, mounted inside the
house. Each evaporator is individually controlled, allowing different rooms or zones to be cooled to varying degrees.
StackA smokestack or flue for exhausting the products of combustion from a combustion appliance. Stack
(Heat) Loss Sensible and latent heat contained in combustion gases and vapor emitted to the atmosphere.
Stack EffectThe overall upward movement of air inside a building that results from heated air rising and escaping
through openings in the building super structure, thus causing an indoor pressure level lower than that
in the soil gas beneath or surrounding the building foundation.
Standard AirAir weighing 0.075 lb. per cu. ft. which is closely air at 68ºF. dry bulb and 50% relative humidity at barometric pressure of 29.92 in. of mercury of approximately dry air at 70ºF. at the same pressure.
Stand-by Heat LossesA term used to describe heat energy lost from a water heater tank.
Static PressureCondition that exists when an equal amount of air is supplied to and exhausted from a space. At static
pressure, equilibrium has been reached.
SteamWater in vapor form; used as the working fluid in steam turbines and heating systems.
Steam BoilerA type of furnace in which fuel is burned and the heat is used to produce steam.
Storage Water HeaterA water heater that releases hot water from the top of the tank when a hot water tap is opened. To replace that hot water, cold water enters the bottom of the tank to ensure a full tank.
Supercooled LiquidLiquid refrigerant cooled below its saturation point.
SubcoolerThis is a section of some condensers in which the temperature of the condensed refrigerant liquid is reduced. This improves the energy efficiency of the chiller.
SubcoolingCreating a drop in temperature by removing sensible heat from a refrigerant liquid.
Suction lineThe tube or pipe which carries the refrigerant vapor from the evaporator to the compressor inlet.
SuperheatTemperature of vapor above its saturation temperature at that pressure.
Superheated VaporRefrigerant vapor heated beyond its saturation point.
SuperheatingCreating a rise in temperature by adding heat energy to a refrigeration vapor.
Supplementary HeatThe auxiliary or emergency heat, usually electrical resistance heat, provided at temperatures below a
heat pump's balance point.
Supply DuctHVAC term for the duct(s) of a forced air heating/cooling system through which heated or cooled air is
supplied to rooms by the action of the fan of the central heating or cooling unit.
Switchover ValveA device in a heat pump that reverses the flow of refrigerant as the system is switched from cooling to
heating. Also called a reversing valve or four-way valve.
Tankless Water HeaterA water heater that heats water before it is directly distributed for end use as required; a demand water
heater.
Temperature, Wet-BulbEquilibrium temperature of water evaporating into air when the latent heat of vaporization is supplied by
the sensible heat of air.
Temperature ZonesIn HVAC individual rooms or zones in a building where temperature is controlled separately from other
rooms or zones.
ThermostatUsually found on an inside wall, this device operates as a control to regulate your heating and cooling
equipment, allowing you to adjust your home comfort at the touch of a switch.
ThermA unit of heat containing 100,000 British thermal units(BTU).
Thermal ValveA valve controlled by a thermally responsive element, for example, a thermostatic expansion valve
which is usually responsive to suction or evaporator temperature.
ThermistorA resistor having semiconductor properties where resistance varies rapidly and predictably with
changes in temperature.
ThrowThe distance air will carry, measured along the axis of an air stream from the supply opening to the position, is the stream at which air motion reduces to 50 fpm.
Ton (Air Conditioning)A unit of air cooling capacity; 12,000 Btu per hour. It is important to note that actual capacity is not constant and will change based on outdoor or indoor temperatures. The published capacity rating of air conditioners and heat pumps is based on performance at the ARI standard temperature levels of 95 F outside, 80 F inside.
Ton of Refrigeration A rate of heat interchange of 12,000 BTU per hour; 200 BTU per min.
Two-Stage OperationProvides two levels of heating or cooling output for greater temperature control, energy efficiency and
improved indoor air quality.
Unit CoolerAdapted from unit heater to cover any cooling element of condensed physical proportions and large surface generally equipped with fan.
Unit VentilatorHVAC term for a fan-coil unit package device for applications in which the use of outdoor- and return-air
mixing is intended to satisfy tempering requirements and ventilation needs.
UpflowWhen an air handler or furnace is installed in an upright position and circulates air through the side or
bottom and out through the top. Typically used in basement, closet and attic installations
Vacuum PumpA pump used to remove air and moisture from a refrigeration system at a pressure below atmospheric
pressure.
Vapor BarrierA 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.
Vapor RetarderA material that retards the movement of water vapor through a building element (walls, ceilings) and
prevents insulation and structural wood from becoming damp and metals from corroding. Often applied
to insulation batts or separately in the form of treated papers, plastic sheets, and metallic foils.
Variable Air Volume System (VAV)Air handling system that conditions the air to constant temperature and varies the outside airflow to ensure thermal comfort.
Vapor SealA vapor seal is an essential part of preventing moisture infiltration into or migration out of a critical
space, such as a data processing center or other room that contains sensitive electronic instrumentation. Essentially, a vapor seal is a barrier that prevents air, moisture, and contaminants from migrating
through tiny cracks or pores in the walls, floor, and ceiling into the critical space. Vapor barriers may be
created using plastic film, vapor-retardant paint, vinyl wall coverings and vinyl floor systems, in combination with careful sealing of all openings (doors and windows) into the room.
Variable Speed MotorA motor that automatically adjusts the flow of warm or cool air for ultimate comfort.
VentilatorA system that exchanges stale, recirculated indoor air with fresh, filtered outside air.
VentA component of a heating or ventilation appliance used to conduct fresh air into, or waste air or combustion gases out of, an appliance or interior space.
Vent DamperHVAC term for a device mounted in the vent connector that closes the vent when the heating unit is not
firing. This traps heat inside the heating system and house rather than letting it draft up and out the vent
system.
Vent PipeA tube in which combustion gases from a combustion appliance are vented out of the appliance to the
outdoors.
Vented HeaterA type of combustion heating appliance in which the combustion gases are vented to the outside, either
with a fan (forced) or by natural convection.
VentilationThe process of moving air (changing) into and out of an interior space either by natural or mechanically
induced (forced) means.
Ventilation AirDefined as the total air, which is a combination of the air brought inside from outdoors and the air that is
being re-circulated within the building. Sometimes, however, used in reference only to the air brought
into the system from the outdoors; this document defines this air as "outdoor air ventilation."
Ventilation RateThe rate at which indoor air enters and leaves a building. Expressed in one of two ways: the number of
changes of outdoor air per unit of time (air changes per hour, or "ach") or the rate at which a volume of
outdoor air enters per unit of time (cubic feet per minute, or "cfm").
Water Cooled SystemA type of air conditioning system that uses freon as a refrigerant and water as a condensing medium.
Typically, the water-cooled condenser is located inside the air conditioner with the rest of the refrigeration components. Water is piped to the unit from a cooling tower or other suitable source.
Water SourceWater is being used as the heat source or heat sink for a heat pump. Sources of underground water are
wells and sources of surface water are lakes, large ponds, and rivers.
WattA unit of power that equals one joule per second. Named after James Watt.
WeatherizationIn HVAC caulking and weatherstripping to reduce air infiltration and exfiltration into/out of a building.
WeatherstrippingA material used to seal gaps around windows and exterior doors.
Year-RoundAir Conditioner which uses gas, oil or electricity for heating.
ZoneIn HVAC an area within the interior space of a building, such as an individual room(s), to be cooled,
heated, or ventilated. A zone has its own thermostat to control the flow of conditioned air into the space.
ZoningThe combining of rooms in a structure according to similar heating and cooling patterns. Zoning requires
using more than one thermostat to control heating, cooling, and ventilation equipment.