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Absolute Humidity
The humidity of the air measured by the number of grains of water vapor present in one cubic meter of air.
Absolute Pressure
The total sum of gauge and atmospheric pressure (psia).
Absolute Temperature
The temperature measured on the Kelvin and Rankine scale, relative to absolute zero.
Absolute Zero
The lowest temperature theoretically attainable on the Fahrenheit scale, approximately −459.67° and on the Kelvin scale, approximately 273.16° C.
A substance that is able to absorb another substance.
Absorption units
Heat energy is absorbed from the medium being cooled and transferred in the refrigerant. An HVAC system that uses chemicals rather than refrigerant to either heat or cool (normally found in commercial units).
A shell-like device installed in the suction line of a HVAC system to prevent liquids (usually refrigerants) from entering the compressor during the off-cycle.
An agent added to methyl chloride to make you aware of refrigerant leaks.
Activated Alumina
A type of chemical, aluminum oxide that absorbs wetness (often used in refrigerant driers).
Activated Carbon
Known as the universal adsorbent, is a processed carbon used in filter driers and frequently used in air filters to clean the air.
Adiabatic Expansion
The expansion of a gas without the absorption of heat.
AFUE (annual fuel utilization efficiency) is the ratio of yearly heat output of a furnace or boiler compared to the total yearly fossil fuel energy consumed by a furnace or boiler. The better the rating, the more efficient it is.
Air Balance
A process for HVAC system performance measurement, a term for distributing air through a system to precisely match the required amount.
Air Cleaning
HVAC services or tools that remove and control the accumulation of various airborne pollutants and gases from the inside air.
Air Conditioner
A common device in various forms that is used to control temperature and humidity of the inside air.
Air Conditioning
A means of controlling and regulating the quality, quantity and temperature of the air in an inside space.
Air Diffuser
Also known as an air distribution outlet, usually located in the ceiling, which mixes conditioned air with naturally regulated room air.
Air Duct
An air passage connected to an HVAC system, a duct that provides ventilation to a specified area
Air Exchange Rate
The rate at which outdoor air replaces indoor air, indoor pollutant levels can increase, expressed in one of two ways: the amount 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). This rate is controlled through natural and/or mechanical ventilation.
Air Filter
A device for preventing dust, dirt, natural debris etc., from entering the air system of an internal combustion engine or entire HVAC unit.
Air Handler or Air Handling Unit (AHU)
A device used to condition and circulate air as part of an HVAC system, usually 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 Infiltration
The unwanted entry of air due to a crack, leakage, temperature difference or wind.
Air Passages
Openings that permit air to move out of the conditioned spaces of the building through or within walls, through floors and ceilings, and around chimney flues and plumbing chases.
Air Retarder/Barrier
A structural element that inhibits air flow into and out of a building's envelope or shell. Common materials of this sheet are composed of polyethylene, polypropylene, or extruded polystyrene and is wrapped around the outside of a house or dwelling during construction to reduce air in-and exfiltration, yet allow water to easily diffuse through it.
Air Standard
HVAC term for having an air temperature of 68 degrees F/20 degrees C and a relative humidity of 36% at 14.7 psia.
Air to Air
A situation in which both the condensing and evaporating mediums are air.
Air Vent
HVAC term for a valve, either manual or automatic, that is used to remove unwanted air from the highest point of a piping system.
Alcohol Brine
Water and alcohol solution witch remains a liquid below 32 degrees F.
A natural or artificial substance capable of causing an allergic reaction because of an individual's sensitivity to that substance.
Ambient Air
The air external to a building or device.
Attic Fan
A fan set up on an attic wall used to expel warm attic air to the outside.
Attic Vent
A passive or mechanical device used to air out an attic space, mainly to reduce heat buildup and moisture condensation.
Available Heat
The amount of usable heat energy that may be converted into functional energy from a fuel source.
In HVAC terms, the apparatus in an air conditioner that distributes the filtered air from the return duct over the coil/heat exchanger.
A unit, vessel or tank where heat is produced from the combustion of fuels such as natural gas, fuel oil, or coal is used to make hot water or steam for such as building space heating to electric power production or industrial process heat.
Boiler Pressure
The pressure from steam or water in a boiler as measured, generally expressed in pounds per square inch gauge (psig).
Boiler Rating
The heating capacity of a steam boiler expressed in BTU per hour (BTU/H) or pounds of steam per hour.
Bottled Gas
A general term for liquefied and pressurized gas, customarily butane, propane, or a mixture of the two, contained in a cylinder (for domestic use.)
Breathing Zone
An area of a room or living space in which occupants breathe as they stand, sit, or lie down.
British Thermal Unit (BTU)
The amount of heat necessary to increase the temperature of one pound of water one degree Fahrenheit, equal to 252 calories.
Building Envelope
Construction elements of a building, including all external materials, windows, and walls, that enclose the inside space or living area.
Building Related Illness (BRI)
Occurs when the symptoms of a diagnosable illness are identified and can be attributed directly to building contaminants, usually airborne elements.
Burner Capacity
The highest heat output (in BTU per hour) released by a burner with a steady flame and satisfactory combustion.
A device that stores electrical charges in an HVAC system. There are two types of capacitor: run capacitor and start capacitor.
Carbon Dioxide (CO2)
A colorless, odorless gas with the abbreviation CO2 that is present in the atmosphere. It is formed when carbon and carbon compounds combust (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. It’s commonly called ‘the silent killer’ due to its lethal tendencies.
Central Air Conditioning
Air conditioner that goes through a central unit instead of small portable units.
Central Air Handling Unit (Central AHU)
This is the same meaning as an Air Handling Unit (see above), but serves more than one cooling area.
Central Heating System
A HVAC set up that supplies heat to areas of a building from a single piece of equipment through a system of ducts or pipes.
Chimney Effect
The inclination for heated air or gas to ascend in a duct or other up and down passage, such as in a chimney, small enclosed space, or building, due to its lower density compared to the surrounding air or gas.
Combination Foundations
Buildings and dwellings that are constructed with more than one foundation type; e.g., basement/crawlspace or basement/slab-on-grade.
The chemical process of burning; the oxidation of a material by applying heat, which unites oxygen with a material or fuel.
Combustion Chamber
Any complete or partially enclosed space in which combustion (burning of fuel) takes place.
Combustion Gases
The gas byproducts of the combustion of a fuel, result of combustion.
A mechanism used to compress air for mechanical or electrical power production. Found in air conditioners, heat pumps, and refrigerators to pressurize the refrigerant and enabling it to flow through the HVAC system.
The device in an HVAC system in which the refrigerant condenses from a gas to a liquid when it is depressurized or cooled.
Condenser Coil
Is a heat exchanger and can come in different forms depending on the HVAC system.
Conditioned Air
Air that has been treated with an HVAC system: heated, cooled, humidified, or dehumidified to maintain an inside space within the desired comfort zone.
Conditioned Space
The inside space of a building that is heated or cooled by an HVAC system.
Constant Air Volume Systems
An air handling system that provides a steady air flow while varying the temperature to meet heating and cooling needs.
Control Panel
Area on an HVAC system that controls the instruments and preferences of the user.
Cooling Capacity
The measure of the amount of heat that a cooling appliance is able to remove from a room in one hour.
Cooling Degree Day
An assessment used to approximate inside 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).
HVAC term for controls that manipulate the airflow through an air outlet, vent, inlet, or air duct. A damper position may vary: immovable, manually adjustable or part of an automated control system.
A machine for minimizing the level of humidity in a room, home or dwelling.
Demand (tankless) Water Heater
A type of water heater that has no storage tank, therefore eliminating storage tank stand-by losses.
Diffusers and Grilles
Components of the ventilation system that allocate and return air to promote good air circulation in the occupied space.
Direct Water Heater
A kind of water heater in which heated water is housed within the tank. Hot water is released from the top of the tank when a hot water faucet is turned. This water is returned with cold water that flows into the tank and down to just above the bottom plate under which are the burners to warm it for later use.
A source of energy from burning combustion gases that are so hot and strong that the heat is lost up the chimney before it can heat a dwelling.
Draft Diverter
A door-like mechanism located at the mouth of a fireplace chimney flue for managing the direction and flow of the draft in the fireplace as well as the amount of oxygen that the fire receives.
Draft Hood
A device constructed into or installed above a combustion machine to assure the release of combustion byproducts, to prevent back-drafting of the appliance, or to neutralize the effects of the stack action of the chimney or vent on the operation of the appliance.
Dual Duct System
An 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.
Duct Fan
An HVAC term for an axial flow fan mounted in a section of ductwork to transport conditioned air from one place to another.
Tubes usually constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite, and generally located within a wall, floor, and ceiling that distributes heated or cooled air in buildings.
Environmental Tobacco Smoke (ETS)
The mixture of smoke from a cigarette, pipe, or cigar and smoke exhaled by the smoker (also secondhand smoke (SHS) or passive smoking) that others can inhale.
Exhaust Ventilation
An HVAC term for the mechanical removal of air from a certain portion of a building.
Filter (air)
A device that removes contaminants, by motorized filtration, from the fresh air stream before the air enters the living environment. Filters vary greatly and can be installed as part of an HVAC system through which air flows for the point of removing particulates before or after the air enters the mechanical components.
Fireplace Insert
A wood or gas burning heating accessory that is installed into the opening to the hearth of a conventional fireplace.
The structure (in a residential heating appliance, industrial furnace, or power plant) into which combustion gases flow out/up and are contained until they are emitted to the atmosphere outside the building.
Flue Gas
The gas that occurs from the combustion of a fuel that is emitted to the flue.
Forced Air System or Furnace
An HVAC term for a category of heating system in which heated air is blown by a fan through air channels or ducts into interior living or working spaces.
Fuel Oil
Any liquid petroleum product burned for the creation of heat in a furnace or firebox, or for the generation of power in an engine unit. Domestic (residential) heating fuels are classed as Nos. 1, 2 and 3; industrial fuels as Nos. 4, 5 and 6.
Any of a group of parasitic plants that lack chlorophyll, including common molds and mildews.
Furnace (Residential)
A combustion heating unit in which heat is retained from the burning of a fuel for allotment, comprised mainly of a combustion chamber and heat exchanger.
Geothermal Heat Pump
A ground-source heat pump that uses the natural heat found in the earth and/or the earth’s groundwater to provide heating, cooling and hot water for homes and businesses.
Heat Loss
The heat energy that flows from the inside of a building or dwelling, through the walls and roof of the building (building envelop) to the outside atmosphere.
Heat Rate
A measurement of heat efficiency; the ratio of fuel energy input as heat per unit of net work output, generally expressed as BTU per net kilowatt-hour.
Heat Register
The grilled-like opening into a room or living space by which the amount of heated air from a furnace can be managed, directed or controlled; this may also include a damper.
Heating Capacity
The amount of heat needed to raise the temperature of a specific mass of a substance by one degree.
Heating Degree Day(s) (HDD)
The amount 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 by who is conducting the rating; mainly used to establish indoor space heating needs and heating system sizing. The higher the HDD for a specific location, the colder the daily average temperature(s).
Heating Load
The rate of heat flow necessary to uphold a specific indoor temperature; usually measured in BTU per hour.
High efficiency particulate arrestance or air (filters). Refers to a filter that is manufactured, tested and certified to meet Institute of Environmental Sciences and Technology (IEST) standards, HEPA filters are normally made for applications where microscopic airborne particles or pollutants could cause health issues for people or product quality troubles.
Home Energy Rating Systems (HERS)
A nationally known energy-rating program that provides builders, mortgage lenders, secondary lending markets, homeowners, sellers, and buyers an exact evaluation of energy-losing shortages in homes. They can use this system to estimate the energy value in their home and also have a star rating on their home to compare to other similarly built homes in the area.
Hot Air Furnace
A heating system where heat energy is distributed by means of convection or fans.
An appliance for escalating the humidity in a room or home.
A measure of the moisture content of air. It may be expressed in the following ways: absolute, mixing ratio, saturation deficit, relative, or specific.
HVAC is an acronym for heating, ventilation, and air-conditioning.
Indoor air pollution.
Indoor air quality.
Ignition Point
The minimum temperature (Fahrenheit, Celsius) at which combustion of a solid or fluid can take place.
Indoor Air
The air the people breathe inside a constructed environment.
Indoor Air Pollutant
Anything that interferes with clear air: particles of dust, fibers, mists, bioaerosols, and gases or vapors.
Air leakage inward through any kind of crack and through ceilings, floors, and walls of a space or building.
Any kind of particle that is small enough to be inhaled, but big enough so that they are not quickly exhaled.
Integrated Heating Systems
An HVAC term for a kind of heating machine that performs more than one function.
A kind of heating fuel derived by refining crude oil that has a boiling range at atmospheric pressure from 400 degrees to 550 degrees Fahrenheit.
Manufacturers’ Minimum Standards
The standards set by manufacturers, in this case HVAC manufacturers, which specify for dealers, maintenance representatives and buyers the minimum levels of certain variables for a particular system. Usually to ensure that it is operating and maintained well after purchase.
Mechanically Ventilated Crawlspace System
An HVAC system constructed to increase ventilation within a crawlspace, attain higher air pressure in the crawlspace relative to air pressure in the soil beneath the crawlspace, or get lower air pressure in the crawlspace relative to air pressure in the living spaces, by use of a fan system.
MERV Rating
Minimum Efficiency Rating Value. An industry-standard rating system for air filters that makes it easier to compare different filters from different manufactures for various HVAC needs. A MERV rating goes from 1 to 16, 16 being the highest-rated efficiency.
Natural Ventilation
Known as the movement of outside air into a space via intentionally designed and constructed openings, such as windows and doors, or through non-powered ventilators.
Negative Pressure
A condition that occurs when less air is supplied to a space than is exhausted from the same space, so the air pressure within that space is less than that in the surrounding areas. Under this condition, if an opening exists, air will flow from surrounding areas into the negatively pressurized space to equalize pressure. Opposite of Positive Pressure.
Outdoor Air
External air, not previously circulated through an indoor HVAC system.
Positive Pressure
A condition that occurs 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 is made, air will flow from the positively pressurized space will move into the surrounding areas to equalize pressure. Opposite of Negative Pressure.
Preventative Maintenance
Habitual, regular and systematic cleaning, inspection and replacement of malfunctioning and old parts, materials, and operational pieces.
Programmable Thermostat
A 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.
A hydrocarbon gas occurring in crude oil, natural gas, and refinery cracking gas used as a fuel, a solvent, and a refrigerant.
A gauge of the capacity of a material to resist heat transfer; the larger the R-Value of a material, the greater its insulating properties.
Radiant Barrier
A thin, reflective foil sheet that exhibits low radiant energy transmission and under some kinds of conditions can restrict radiant heat transfer; usually installed in attics to reduce heat flow through a roof assembly into the living space.
Radiant Ceiling Panels
Ceiling panels that have electric resistance heating elements implanted within them to supply radiant heat to a room.
Radiant Energy
Energy that transmits away from its source in all directions within a dwelling space.
Radiant Floor
A type of radiant heating system where the floor contains channels, vents or tubes through which heated substances are circulated. The whole floor is evenly heated so the room warms from the bottom up.
Radiant Heat Transfer
Radiant heat transfer happens when there is a substantial difference between the temperatures of two surfaces that are exposed to each other, but are not actually touching.
Radiant Heating System
An HVAC term for a heating system where heat is radiated into a room by means of heated surfaces, such as electric resistance elements, hot water radiators.
A room heat delivery (or exchanger) part of a hydronic (hot water or steam) heating system; hot water or steam is channeled to the radiator it by natural convection or by a pump from a boiler.
Radiator Vent
A device that releases pressure from a radiator when the pressure inside exceeds the operating limits of the vent.
An air circulation situation that happens when air being exhausted from a building is immediately brought back into the system through the air intake and other openings in the building envelope, usually unintentional.
Recirculated Air
Air that is removed from the conditioned, interior space and used for ventilation, heating, cooling, humidification, or dehumidification.
A compound fluid used in air conditioners, heat pumps, and refrigerators to shift heat into or out of an interior space.
Refrigeration Capacity
A measure of the useful cooling capacity of a refrigerator, expressed in BTU per hour or in tons, where one ton of capacity is that of the heat required to melt 2,000 pounds of ice in 24 hours or 12,000 BTU per hour.
Relative Humidity
A measure of the percent of moisture truly in the air compared with what would be in the air if it were fully saturated at that temperature. When the air is fully saturated, its relative humidity is 100 percent.
Return Air
Air that is returned to a heating or cooling device from a heated or cooled space.
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. Read more about it here.
Setback Thermostat
A thermostat that can be set to automatically lower temperatures in an unused or uninhabited house or dwelling and go up again before the occupants returns, minimizing energy waste.
Sick Building Syndrome (SBS)
Describe situations in which building occupants, such as workers, tenants or anyone who spends a significant amount of time in one place, experience health problems that appear to be linked to a building, but no specific illness or cause can be identified.
Space Heater
A portable or sometimes fixed heater used to warm individual rooms or spaces.
Split System Air Conditioner
An HVAC term for an AC system that is made up of two to five parts: one piece contains the compressor, condenser, and a fan; the other pieces 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 dwelling. Each evaporator is individually controlled, allowing different rooms or zones to be cooled or heated to varying degrees.
A smokestack or flue for expelling the products of combustion from a combustion unit.
Stand-by Heat Losses
A term used to explain heat energy lost from a water heater tank.
Static Pressure
A condition that happens when an equal amount of air is supplied to and exhausted from a specific space. Equilibrium has been reached at static pressure.
Steam Boiler
A kind of furnace in which a fuel source is burned and the heat is used to produce steam.
Storage Water Heater
A water heater that releases hot water from the top of a tank when a hot water tap is opened. To restore that hot water, cold water will enters the bottom of the tank to create a full tank.
Sump Pump
A pump used to remove water that has accumulated in a water collecting sump basin, such as basement and other low-level, indoor spaces.
Supply Duct
An HVAC term for the duct(s) of a forced air heating/cooling system through which heated or cooled air is supplied to individual rooms by a fan of the central heating or cooling unit.
Tankless Water Heater
A water heater that heats water before it is directly dispersed for end use as needed; on-demand water heater.
Temperature Zones
When individual rooms or zones in a building or dwelling has temperatures controlled separately from other rooms or zones.
A unit of heat containing 100,000 British thermal units (BTU).
A sensor for measuring temperature, consisting of two dissimilar metals, joined together at one end. When the junction of the two metals is heated or cooled, a voltage is produced that can be correlated back to the temperature.
Variable Air Volume System (VAV)
An air handling system that conditions the air to a constant temperature and varies the outside airflow to ensure thermal comfort.
A component of a heating or ventilation system used to conduct fresh air into, or waste air or combustion gases out of, an appliance or interior space. Sizes and shapes vary.
Vent Damper
HVAC term for a part mounted in the vent connector that opens and closes the vent when the heating unit is not firing or a person wishes to open a vent.
Vent Pipe
A tube in which combustion gases from a combustion unit are vented out of the appliance to the outdoors.
Vented Heater
A type of combustion heating device in which the combustion gases are directed outside, either with a fan (forced) or by natural convection.
The method of moving air (changing) into and out of an inside space either by natural or mechanically induced (forced) means.
Ventilation Air
Known as the total 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 Rate
The 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").
In HVAC caulking and weatherstripping to reduce air infiltration and exfiltration into/out of a building.
A material used to close gaps and air leaks around windows and exterior doors.
In 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.
The combination of rooms in a building or dwelling according to similar heating and cooling patterns to maximize energy. Zoning uses more than one thermostat to control heating, cooling, and ventilation equipment.

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