0 Items Total: 0.00
Email:   Password:   Register
(866) 476-5101
Support Center

IAQ Terms, Acronyms & Definitions

A-D        E-H        I-L       M-P      Q-T      U-Z


AAAAI a membership organization of more than 6,600 allergist / immunologists and related professionals around the world with advanced training and experience dedicated to allergy, asthma, immune deficiencies and other immunologic diseases.
  • A nationwide survey found that more than half (54.6%) of all U.S. citizens test positive to one or more allergens.
  • In a recent survey, over 50% of homes had at least six detectable allergens present.
  • Allergic diseases affect as many as 40 to 50 million Americans. 


The AAFA is a not-for-profit organization founded in 1953, the leading patient organization for people with asthma and allergies, and the oldest asthma and allergy patient group in the world.
MISSION STATEMENT: AAFA is dedicated to improving the quality of life for people with asthma and allergic diseases through education, advocacy and research.
  • Allergy and asthma control begins at home. Many people with allergies stay indoors when outdoor air is full of pollen and spores. But dust mites, animal dander and even cockroaches can cause problems indoors.
  • Eight out of 10 people in the United States are exposed to house dust mites, and six out of 10 are exposed to cat or dog dander. Cockroaches cause allergic reactions among people who live in the inner cities or southern parts of the United States.
  • Controlling the air quality in your home, office and car can reduce allergy and asthma symptoms.


Reduction or removal of a contaminant.

Absolute Humidity

The amount / grains of water vapor present in a unit volume of air, usually expressed in kilograms per cubic meter.


The uptake of liquid into the fibers of a substance. Vapor Pads are absorbent materials that absorb water to help humidify / add moisture to indoor air.


The ACCA is a non-profit association serving more than 60,000 professionals and 4,000 businesses in the HVACR community, working together to promote professional contracting, energy efficiency, and healthy, comfortable indoor environments. ACCA's roots stretch back to the early part of the 20th century, and the organization was incorporated in its present form over 40 years ago.
The ACCA has one goal: to make the HVACR industry, and every professional contracting business, more successful.

Acceptable Indoor Air Quality

Indoor air that does not contain harmful concentrations of contaminants; air with which at least 80% of building occupants do not express dissatisfaction.


Air Changes Per Hour.  
The number of times indoor air is replaced by outside air, in an hour. Older homes have an ACH from 1 to 2. Newer homes that are more tightly sealed may have an ACH of 5.

Acid Condition in System

Condition in which refrigerant or oil in system is mixed with fluids that are acidic in nature.

Activated Carbon

Specially processed carbon used as a filter drier; commonly used to clean air.
Activated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.


AD is a dynamic community of independent distributors and manufacturers of construction and industrial products. Our 450 independently owned members span seven industries and two countries with collective annual sales in excess of $23 Billion. AD distributors are local market leaders with deep roots in their local communities, great people and strong customer relationships.
The product industries that AD serves include: electrical supplies, industrial supplies, plumbing, PVF, HVAC, drywall and clean energy. The customer markets served include: industrial MRO, industrial OEM, commercial construction, residential construction, utilities, retail and government.


The adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. Solids that are used to adsorb gases or dissolved substances are called adsorbents; the adsorbed molecules are usually referred to collectively as the adsorbate. An example of an excellent adsorbent is the charcoal used in gas masks and air filters to remove poisons or impurities from a stream of air.


Act of combining substance with air. To supply with air or expose to the circulation of air. Aeration (also called aerification) is the process by which air is circulated through, mixed with or dissolved in a liquid or substance.

After Market

The market for parts and accessories used in the upkeep or enhancement of a previous purchase, as of a car, computer or other item; generally not the same brand as the original purchase.  Ideally, any parts purchased for the upkeep / maintenance of a product is purchased through the original dealer to ensure maximum performance and life of your purchase.


Air Filter Testing Laboratory.
This is an independant laboratory capable of testing filters by several standard test methods, and developing special test programs which may be required by client.


Annual Fuel Utilization Efficiency.  
This number represents how efficiently a furnace converts fuel to energy. The ratio of annual output of useful energy or heat to the annual energy input to the furnace. The higher the AFUE, the more efficient the furnace -- higher efficiency translates to more savings on fuel bills. This will range from 80% to 95%. Percentage of fuel used for heating. A measure of heating efficiency, in consistent units, determined by applying the federal test method for furnaces. This value is intended to represent the ratio of heat transferred to the conditioned space by the fuel energy supplied over one year.


OUR MISSION: Building healthier lives, free of cardiovascular diseases and stroke. The American Heart Association is working to improve the cardiovascular health of all Americans by 20% while reducing deaths from cardiovascular disease and stroke by 20% by the year 2020.
What the AHA says about indoor air quality: "Inhaled particles cause adverse cardiovascular effects. As a major, overarching finding, the statement concludes...that the overall evidence is consistent with a causal relationship between PM2.5 (fine particles in the (ambient) air 2.5 micrometres or less in size) exposure and cardiovascular morbidity and mortality."


The Air-Conditioning and Refrigeration Institute is a nonprofit, 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. ARI ensures a level of performance within the industry.  
Ensuring human comfort, productivity, and safety, while practicing environmental stewardship is the mission of the Air-Conditioning, Heating, and Refrigeration Institute (AHRI). Our 300 member companies produce more than 90 percent of the residential and commercial air conditioning, heating, water heating, and commercial refrigeration equipment made in North America. Through our certification program, standards, advocacy, and other activities, we strive to help save energy, improve productivity, and ensure a better environment.

Air Cleaner

Device used for removal of airborne impurities (particles, germs and gases). A device that actively removes impurities from the air, including forced air filtration systems, electronic air cleaners and UV Air Purifiers. Air cleaners may be added to HVAC systems or act as stand-alone, single room units.

Air Cleaning

Generally considered removing contaminants that are in the air; more recently expanded to include sterilizing biological organisms and removing odors.

Air Contaminant

Any material in the atmosphere that adversely affects persons and their environment (pollutant includes materials such as liquids, solids, aerosols, gases and odors). Term is used interchangeably with air pollutant.

Air Diffuser

An air distribution outlet or grille designed to direct airflow into desired patterns. Supply air terminal device, designed to direct airflow into desired patterns, usually placed in the ceiling, generally of circular, square or rectangular shape, and composed of divergent deflecting members; in a manner so as to meet certain specified conditions, such as air change rate, pressure, cleanliness, temperature, humidity, air velocity and noise level.

Air Exchange Rate

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 Handling Unit (AHU)

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. The part of an HVAC system responsible for moving air, which may also clean, heat, or cool the air. This refers to equipment used to provide conditioned air to a space. Does not include ductwork, registers or grilles, boilers and chillers.

Air Quality Control

Since studies have shown indoor air quality can be up to 100 times more polluted than outdoor air, it's important to consider taking steps to improve the quality of the air we breathe.  A recent review of the research on “how particles cause harm” found that the body responds to particles in similar ways to its response to cigarette smoke; helping to illustrate why particle pollution can cause heart attacks and strokes.
Particle pollution also diminishes lung function, causes greater use of asthma medications and increased rates of school absenteeism, emergency room visits and hospital admissions. The impact of even short-term exposure to particle pollution on healthy adults showed up in the Galveston Lifeguard Study, in addition to the harmful effects of ozone pollution. Lifeguards had reduced lung volume at the end of the day when fine particle levels were high. Other adverse effects can be coughing, wheezing, cardiac arrhythmias and heart attacks. According to the findings from some of the latest studies, short-term increases in particle pollution have been linked to:
  • Death from respiratory and cardiovascular causes, including strokes.
  • Increased mortality in infants and young children.
  • Increased numbers of heart attacks, especially among the elderly and in people with heart conditions.
  • Inflammation of lung tissue in young, healthy adults.
  • Increased hospitalization for cardiovascular disease, including strokes and congestive heart failure.
  • Increased emergency room visits for patients suffering from acute respiratory ailments.
  • Increased severity of asthma attacks in children; including hospitalization for some.
The EPA Concludes Fine Particle Pollution Poses Serious Health Threats, such as:
  • Early death (both short-term and long-term exposure)
  • Cardiovascular harm (e.g. heart attacks, strokes, heart disease, congestive heart failure)
  • Respiratory harm (e.g. worsened asthma, worsened COPD, inflammation)
  • Cancer
  • Reproductive and developmental harm
We have the need and fortunately, the ability to change, improve and control the quality of indoor air through the use of air cleaners, humidifiers, dehumidifiers, energy recovery / exchange ventilators and UV Air Purifiers.


A 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.


Is one of three mineral forms of titanium dioxide (TiO2). It is always found as small, isolated and sharply developed crystals. 

Animal Dander

Tiny scales of animal skin.


Agent that kills microbial growth.


The ability of a filter to capture a mass fraction of coarse test dust.


Founded in 1894, ASHRAE is a building technology society with more than 50,000 members worldwide. The Society and its members focus on building systems, energy efficiency, indoor air quality and sustainability within the industry. Through research, standards writing, publishing and continuing education, ASHRAE shapes tomorrow’s built environment today.
Their Mission:  To advance the arts and sciences of heating, ventilating, air conditioning and refrigerating to serve humanity and promote a sustainable world.


A chronic respiratory disease of the airways, often arising from allergies, that is characterized by sudden recurring attacks of labored breathing, chest constriction, wheezing and coughing.
Bronchial tubes that are chronically inflamed may become overly sensitive to allergens (specific triggers) or irritants (nonspecific triggers).
Asthma affects people differently. Each individual is unique in their degree of reactivity to environmental triggers. 80% of asthmatics also have allergies to airborne substances. Triggers can include animal dander, dust mites, grass, tree and weed pollens, mold, foods and food additives and more.

Attic Vent

HVAC term for a passive or mechanical device used to ventilate an attic space, primarily to reduce heat buildup and moisture condensation. Another way to accomplish this is with an Attic Fan, mounted on an attic wall to exhaust warm air to the outside.


Single celled microorganisms ranging from harmless and beneficial to intensely virulent and lethal. 


Is a suspension of airborne particles that contain living organisms or were released from living organisms. Air often contains tiny organisms such as fungi, bacteria, mycotoxins and viruses. Due to evaporation of water, bacterial cells usually die when they become airborne but under high humidity conditions bioaerosol levels are increased. Fungal cells such as spores, molds and yeast can be active at low humidity levels and high or low temperatures.

Biological Contaminants

Sources include bacteria and viruses from people and animals; pollens from outdoor and indoor plants; pets' saliva and dander; dried urine from rats and mice; bacterial and fungal growth in ventilation systems; water left standing in drip pans and humidifiers.   Also included are wet/damp rags left standing and dust mites.


An air handling device for moving air in a distribution system, such as a fan.


BLUE HAWK was formed in May 2005 to maximize the strength of independent HVACR distributors throughout the United States. BLUE HAWK members distribute a wide range of HVACR related products to the residential and commercial construction trades.
We are a purchasing cooperative of HVACR Distributors. Our core product categories include:
  • Air Conditioning
  • Heating
  • Hydronics
  • Refrigeration
  • Tools, Supplies and Accessories
  • Ventilation

Breathing Zone

Area of a room in which occupants breathe as they stand, sit, or lie down. 


Also abbreviated as Btu. British Thermal Unit.
The amount of heat that will raise or lower one pound of water 1 degree F. at 39.2 degrees F. One Btu is the equivalent of the heat given off by a single wooden kitchen match. The British Thermal Unit is a standard of measure for cooling and heating capacities. This is how the capacity of air conditioning is measured. A standard measure of heat energy in the U.S. commonly used to measure the energy content of various fuels and steam. It takes one  Btu to raise the temperature of one pound of water by one degree Fahrenheit at sea level. For example, it takes about 2,000 Btu's to make a pot of coffee. One   Btu is equivalent to 252 calories, 778 foot-pounds, 1055 joules, and 0.293 watt-hours. For your home, it represents the measure of heat givens off when fuel is burned for heating or the measure of heat extracted from your home for cooling. Note: In the abbreviation, only the B is capitalized.

Buffer Zone

An area within the home between the conditioned zones and the outside. Thus it normally is not conditioned (for instance, attics, attached garages, crawlspaces, basements, and enclosed porches).

Bypass Humidifier

Bypass humidifiers are integrated into the residential furnace / HVAC system to humidify your whole house. A valve connects the bypass humidifier to the residential water supply and is installed in the ducting leading out of your furnace. An electric solenoid connected to a humidity sensor regulates the flow of water into the humidifier.
  • A portion of the heated air from the furnace passes through a water-soaked pad where it absorbs additional moisture and then returns for distribution throughout your home.
  • Water is metered into the KineticFlo® Trough and dispersed uniformly across the pad. Unevaporated water drains from the bottom of the unit to eliminate mineral build-up.


An electric circuit element used to store charge temporarily, consisting in general of two metallic plates separated and insulated from each other by a dielectric. Also called condenser.

Carbon Filter

Air purifier using activated carbon as the air cleansing agent for the removal of gaseous contaminants.


Collaborating to create the expertise, information, and tools that people and communities need to protect their health – through health promotion, prevention of disease, injury and disability, and preparedness for new health threats.
What the CDC says about indoor air quality: "Indoor environments are highly complex and building occupants may be exposed to a variety of contaminants (in the form of gases and particles) from office machines, cleaning products, construction activities, carpets and furnishings, perfumes, cigarette smoke, water-damaged building materials, microbial growth (fungal / mold and bacterial), insects, and outdoor pollutants. Other factors such as indoor temperatures, relative humidity, and ventilation levels can also affect how individuals respond to the indoor environment."  

Ceiling Plenum

The space between the suspended and structural ceiling used as part of the air distribution system that accommodates the mechanical and electrical equipment.

Central Air Handling Unit

This is the same as an Air Handling Unit, but serves more than one area. 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. The part of an HVAC system responsible for moving air, which may also clean, heat, or cool the air. This refers to equipment used to provide conditioned air to a space. Does not include ductwork, registers or grilles, boilers and chillers.


Cubic Feet Per Minute
Refers to the total volume of air, measured in cubic feet that is moved through the system in one minute.  The total SFM that can be delivered through a duct is dependent on the pressure making the air move (the blower), combined with the size of the duct.  Therefore, the larger the duct is, and the more pressure behind the air, the more air that can be moved. Minimum standards set by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE Standard 62-1989) requires that ventilation should provide a minimum of 15 cfm of outdoor air per person / 20 cfm in office spaces.  60 cfm may be required for some spaces (such as smoking lounges). 


Code of Federal Regulations
The Code of Federal Regulations (CFR) annual edition is the codification of the general and permanent rules published in the Federal Register by the departments and agencies of the Federal Government. It is divided into 50 titles that represent broad areas subject to Federal regulation.


Colony-Forming Unit.
A measure of bacterial or fungal numbers.  Unlike direct microscopic counts where all cells, dead and living, are counted, CFU measures viable cells. For convenience the results are given as CFU/mL (colony-forming units per milliliter) for liquids, and CFU/g (colony-forming units per gram) for solids. 

Clean Air Delivery Rate

Cadre.  This is a standard developed by the independent association of Home Appliance Manufacturers (AHAM) to measure the removal of dust, smoke and pollen from the air in cubic feet per minute (CFM). 

Code Valve

Regulates the flow of water.


Equipment that performs heat transfer to air when mounted inside an air handling unit or ductwork. It is heated or cooled by electrical means or by circulating liquid or steam within it.


The heart of an air conditioning or heat pump system. The large part in the condenser (outdoor unit) that pumps refrigerant. The pump of a refrigerating mechanism which draws a low pressure on cooling side of refrigerant cycle and squeezes or compresses the gas into the high pressure or condensing side of the cycle. The compressor maintains adequate pressure to cause refrigerant to flow in sufficient quantities to meet the cooling requirements of the system.


The transformation of the water vapor content of the air into liquid water on cold surfaces.  The beads or drops of water (or frost in extremely cold weather) that accumulate on the inside of the exterior covering of a building when warm. Moisture-laden air from the interior reaches a point where the temperature no longer permits the air to sustain the moisture it holds.


A component in the basic refrigeration cycle that ejects or removes heat from the system. The condenser is the hot side of an air conditioner or heat pump. Condensers are heat exchangers, and can transfer heat to air or to an intermediate fluid (such as water or an aqueous solution of ethylene glycol) to carry heat to a distant sink, such as ground (earth sink), a body of water, or air (as with cooling towers).

Conditioned Air

Air that has been heated, cooled, humidified, or dehumidified to maintain an interior space within the "comfort zone." (Sometimes referred to as "tempered" air.)

Constant Air Volume (CAV)

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


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


The U.S. Consumer Product Safety Commission is charged with protecting the public from unreasonable risks of injury or death from thousands of types of consumer products under the agency's jurisdiction.
What the CPSC says about indoor air quality: "In the last several years, a growing body of scientific evidence has indicated that the air within homes and other buildings can be more seriously polluted than the outdoor air in even the largest and most industrialized cities. Other research indicates that people spend approximately 90 percent of their time indoors. Thus, for many people, the risks to health may be greater due to exposure to air pollution indoors than outdoors."


The Commissioning Specialists Association (CSA) is at the forefront of career development and training for the Building Services, Commissioning and HVAC industries.
Specialising in Distance Learning, the CSA offer modular courses which allow you to study at your own pace. The CSA will provide you with professionally developed learning materials and the support of expert tutors to help guide you through grades 1-6. These qualifications ensure a consistent and uniform standard for the Commissioning industry.


A device used to vary and / or control airflow through an air duct.  A damper may be immovable, manually adjustable or part of an automated system.


Sealing the foundation walls to help prevent outside moisture from entering the basement.


Removing moisture from the air. The reduction of water vapor in air by cooling the air below the dew point; removal of water vapor from air by chemical means, refrigeration, absorption, etc.


A difference in temperature. Often used in the context of the difference between the design indoor temperature and the outdoor temperature.


Materials that readily absorb, remove moisture.

Dew Point

The temperature at which air cannot hold all the moisture in it and dew begins to form. Absolute measure of moisture in the air.


Not able to conduct direct electric current, and therefore used as an insulator.

Diffusers and Grilles

Components 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.


Department of Energy.
The Department of Energy is a federal agency in charge of setting industry efficiency standards and monitoring the consumption of energy sources.

Drain Trap

A dip in the drain pipe of sinks, toilets, floor drains, etc., which is designed to stay filled with water, thereby preventing sewer gases from escaping into the room.

Drum Humidifier

A drum humidifier works on the principle of evaporation. As the rotating wheel turns, the sponge or pad receives constant moisture from the water reservoir. As the furnace sends warmed air into the drum unit, the air moves through the moistened sponge or pad. The humidified air then travels throughout the home's ducts.


A large "pipe" or closed conduit made of sheet metal, fiberglass board, or other suitable material used for conducting air to and from an air handling unit and delivering the air into a home (or other building). Round or rectangle in shape, ducts convey air at relatively low pressures.

Dust Mite

Dust mites feed on organic detritus such as flakes of shed human skin and flourish in the stable environment of dwellings. House dust mites are a common cause of asthma and allergic symptoms worldwide. The mite's gut contains potent digestive enzymes (notably proteases) that persist in their feces and are major inducers of allergic reactions such as wheezing. The average life cycle for a male house dust mite is 10 to 19 days. A mated female house dust mite can last up to 70 days, laying 60 to 100 eggs in the last 5 weeks of her life. In a 10-week life span, a house dust mite will produce approximately 2,000 fecal particles and an even larger number of partially digested enzyme-covered dust particles
Methods of dust mite eradication:
  1. It is best to have a carpet-free house if dust mites or any house pests are dangerous for a person (e.g., because of their allergy), as flat surfaces are easier to clean and vacuum.
  2. Anti-mite mattresses or mattresses which prevent any house pests should be used.
  3. Regular cleaning and washing of areas where dust mites thrive is necessary to keep them and their waste to a minimum. A simple washing will not completely remove all mites or their droppings, but it will remove at least 90%;
  4. Exposure to temperatures over 60 °C (140 °F) for a period of one hour or freezing, exposure to temperatures below 0 °C (32 °F),[11] will typically prove fatal to house dust mites;
  5. Ten minutes in a household clothes dryer at lethal temperatures [near 105 °C (221 °F)] has been shown to be sufficient to kill all the dust mites in bedding.[12]
  6. A relative humidity less than 50% may also help to control the number of mites.[13]
  7. Disodium octaborate tetrahydrate (DOT) powder is often used to eradicate house dust mites.[14] The side-effect of DOT is irritation of eyes. 



A 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. Depending on your local climate, lifestyle and electricity rates, savings will vary. For furnaces. it is the rate at which a furnace maximizes fuel use. This rate is numerically described as a ratio called AFUE (see AFUE). As of January, 1991, no furnaces can be manufactured with efficiencies lower than 78% afue. High efficiency furnaces will be rated 85 to 95% afue.


An electrode is a conductor through which electric current is passed. Found in variable forms, electrodes may be wires, plates, or rods and may be constructed of metal, such as copper, silver, lead, or zinc. Electrodes are used in the GeneralAire® Elite Steam Humidifiers to create steam.  They are housed within a cylinder along with water.  As electric current is passed between the electrodes, the water is boiled; creating steam.
As the humidifier operates, water is evaporated and minerals are left behind on the walls of the cylinder and the electrodes. Electrodes eventually become completely coated with mineral buildup, resulting in poor steam output.  When this happens, you simply need to replace the cylinder within your humidifier. 

Electrostatic Filter

A filter that uses electrostatically enhanced fibers to attract and retain particles.


Substances given off into the air.

Environmental Tobacco Smoke

Mixture of smoke from the burning end of a cigarette, pipe, or cigar and smoke exhaled by the smoker (also secondhand smoke (SHS) or passive smoking). 


The EPA has 7 major priorities for the future: 
  1. Taking Action on Climate Change
  2. Improving Air Quality
  3. Assuring the Safety of Chemicals
  4. Cleaning up our Communities
  5. Protecting America's Waters
  6. Expanding the Conversation on Environmentalism and Working for Environmental Justice
  7. Building Strong State and Tribal Partnerships
What the EPA says about indoor air quality: "In the last several years, a growing body of scientific evidence has indicated that the air within homes and other buildings can be more seriously polluted than the outdoor air in even the largest and most industrialized cities. Other research indicates that people spend approximately 90 percent of their time indoors. Thus, for many people, the risks to health may be greater due to exposure to air pollution indoors than outdoors."


Energy Recovery Ventilator.
Electronic method of ventilation, designed to transfer heat and moisture during the exchange of indoor / outdoor air. Brings fresh air into your home to keep inhabitants healthy, remove odors, reduce moisture, remove indoor pollutants and lower relative indoor humidity.  Every home needs ventilation, and due to weather, safety and noise many people find it impossible to keep doors and windows open long or often enough to properly ventilate their home.


Formerly known as Edison Testing Laboratories.
The ETL Listed Mark is proof of product compliance (electrical, gas and other safety standards) to North American safety standards. Authorities Having Jurisdiction (AHJ’s) in 50 states and Canada and retailers accept the ETL Listed Mark as proof of product safety. Manufacturers are choosing ETL because it gives them a competitive advantage in the marketplace. Today, the ETL mark is featured on millions of products sold by major retailers.

Evaporative Humidifier

The most common humidifier, an "evaporative", "cool mist", or "wick humidifier", consists of just a few basic parts: a reservoir, wick and fan. The wick or vapor pad is a filter that absorbs water from the reservoir and provides a larger surface area for it to evaporate from. The fan is adjacent to the wick / vapor pad and blows air onto the wick / vapor pad to aid in the evaporation of the water. Evaporation from the wick / vapor pad is dependent on relative humidity. A room with low humidity will have a higher evaporation rate compared to a room with high humidity. Therefore, this type of humidifier is self-regulating: As the humidity of the room increases, the water vapor output naturally decreases.


Uncontrolled air leakage out of a building.  Exfiltration air flow outward through a wall, building envelope, etc.


Air removed deliberately from a space, by a fan or other means, usually to remove contaminants from a location near their source. The air flow leaving the treated space. Exhaust may be accomplished by one or more of the following methods:
  1. Extraction: exhaust in such a manner that the air is discharged into the atmosphere.
  2. Relief: exhaust in such a manner that the air is allowed to escape from the treated space if the pressure in that space rises above a specified level.
  3. Recirculation: exhaust in which the air is returned to the air treatment system.
  4. Transfer: exhaust in which air passes from the treated space to another treated space.


A device for removing / straining unwanted matter / impurities from air, oil or liquids. Can be made of paper, cloth, sand, or other materials.  Sample of products that use filters:  air cleaners, automobiles, home heating furnaces, coffee makers, humidifiers, cigarettes, faucets, fountains, landscaping drainage systems, etc.
Filter Performance Rating  (FPR)
A grading system that uses a color/number system on a scale of 4 (green circle) to 10 (blue circle).  Some manufacturers rate their filters using the FPR rather than MERV (the standard determined by the American Society of Heating, Refrigerating and Air Conditioning and Engineers).  In both rating systems the higher the number the more efficient the particulate removal.


A projecting rim, collar, or ring on a shaft, pipe, machine housing, etc., cast or formed to give additional strength, stiffness, or supporting area, or to provide a place for the attachment of other objects.

Forced Air

This describes a type of heating system that uses a blower motor to move air through the furnace and into the ductwork. Heating and or cooling system that connects to the conditioned space with duct-work that uses air as the moving fluid. The heating or cooling can come from any number of sources.


A colorless, pungent-smelling gas widely used by manufacturers of building materials and household products.  It is commonly found in pressed wood products, particleboard, cabinetry, furniture, wood paneling, drawer fronts, furniture tops, older insulation and permanent press clothing.


Any of a group of parasitic lower plants that lack chlorophyll, including molds and mildews.


Chemical substance that is used to get rid of fungi.

Germicidal Lamp

Is a special type of lamp which produces ultraviolet light (UVC). This short-wave ultraviolet light disrupts DNA base pairing, leading to death of bacteria on exposed surfaces. 

Grains of Moisture

A measurement of humidity (water vapor).  There are 7,000 grains of moisture in ONE pound of steam.


An opening of several slits side by side in a wall or metal sheet or other barrier, usually to let air or water enter and/or leave but keep larger objects including people and animals in or out.


Grommets are made of different materials, such as metal, plastic or rubber. Grommets are used to protect a pierced material from further tearing or abrasion. A grommet can be utilized to protect a hose, a pipe, or other objects passing through a panel hole. Aside from that, it works as a spacer as it can easily snap into a pre-drilled panel hole.


HARDI members market, distribute, and support heating, air-conditioning, and refrigeration equipment, parts and supplies. HARDI Distributor members serve installation and service/replacement contractors in residential and commercial markets, as well as commercial/industrial and institutional maintenance staffs. HARDI proudly represents more than 450 distributor members representing nearly 3,400 branch locations and close to 500 suppliers, manufacturer representatives and service vendors.


High Efficiency Particle Arrestor.
Basically HEPA is a type of filter that can trap the smallest particles (0.3 micrometers (µm) in diameter) that other air filtration systems would simply re-circulate back into the air of your home. HEPA filters remove 99.97% of all airborne particles that can cause problems for allergy and asthma sufferers, reducing or eliminating them to make the air cleaner for those residing in your home. Because HEPA Air Filtration Systems combine HEPA filters with carbon-based materials, they stop mold spores, bacteria, viruses, pet dander and dust, and also help to eliminate harmful gasses and odors from your home. Thus, the air that leaves this type of air purifying system is cleansed and almost totally free from contaminants.
In the 1950’s during WWII, HEPA filters were invented by the US Atomic Energy Commission (AEC), who needed something that would remove small radioactive dust particulates from the air. In the last decade HEPA-technology air purifiers have been used to clean indoor spaces from homes to beauty salons to hospitals and operating rooms across the country. The Environmental Protection Agency (EPA) as well as the Occupational Safety and Health Administration have endorsed the HEPA filter as being the absolute best filter in cleansing the air. 


Heat Recovery Ventilator.
Electronic method of ventilation designed to transfer heat during the exchange of indoor / outdoor air.  Brings fresh air into your home to keep inhabitants healthy, remove odors, reduce moisture, remove indoor pollutants and lower relative indoor humidity. Every home needs ventilation, and due to weather, safety and noise many people find it impossible to keep doors and windows open long or often enough to properly ventilate their home.


Process where product or system is used to increase the specific humidity of the air in a given space.


Water vapor present in the air.


Heating, Ventilating and Air Conditioning.


Absorbing moisture; capable of easily absorbing moisture. Examples include:  hardwood floors, carpet, artwork, kitchen cabinetry, molding and more.


Indoor Air Quality.  
Refers to the physical, chemical and biological characteristics of air in a home, building, institution or commercial facility. Indoor air quality can be adversely influenced by many factors, such as:
  • Processes or activities taking place within the building (use of tobacco, use of chemicals, painting, remodeling, etc.)
  • Inadequate supply of outside, fresh air
  • Contamination arising from sources within the building (tobacco smoke, volatile organic compounds from building materials, fabrics, furnishings, carpet, adhesives, fresh paint, animal dander, new paneling, cleaning products and office equipment)
  • Contamination from outside the building brought in by air intakes, open doors and windows (pesticides, automobile emissions, dust, pollen, etc.)
  • Microbial contamination of the ventilation system or building interior (germs, bacteria, etc.)
  • Moisture / humidity (too much or too little)
The cost of poor IAQ?
  • The American Lung Association says that U.S. adults miss approximately 14.5 million work days due to asthma.
  • U.S. Centers for Disease Control & Prevention estimate that sinus infection sufferers miss an average of four work days each year.
  • Some estimates place the cost of poor IAQ to the United States' economy at $168 billion. Part of that cost relates to direct medical care, while some of it can be traced to reduced productivity due to “presenteeism” (when people go to work or school even when they are sick) and absenteeism.


The International Indoor Air Quality Commission was established in 2003 for the certification of indoor air quality professionals and their continued development, and for the endorsement of indoor air quality products. Its mission is to establish, assess, and promote recognized standards of competence and performance in the indoor air quality field.

Indoor Air Pollutant

Particles and dust, fibers, mists, bioaerosols, and gases or vapors.


The Key Wholesaler Group Association is comprised of 22 independent HVAC/R distributors that contain 356 locations nationwide. We focus our purchases through our approved vendor partners that are selected on an annual basis. We are recognized by most manufacturers as a National Player, and have the reputation for fair and successful relationships with our vendors and customers. Our programs and products are designed to help our customers grow and be successful. Our commitment to quality products is the foundation to our success.

Latent Heat

Heat which brings about a change in state with no change in temperature is called latent heat.  Example:  As latent heat (humidity) is removed the temperature remains constant as the moisture in the air is removed and goes down the condensate drain. Humidity is often referred to as the latent heat load.


Light Emitting Diode.
A semiconductor that emits light when a current passes through it. LED's are used as indicator lamps in many devices.

Maximum Production

The setting of a unit (humidifier, air cleaner, etc.) by which it is working at full capacity, or working at its hardest.   A unit that runs continuously at maximum production can result in a shortened life.  

Mechanical Ventilation

Moving air in and out of a building in a controlled quantity that can be conditioned (heated, cooled, cleaned); eliminating many of the problems associated with natural ventilation.


Minimum Efficiency Reporting Value
The MERV rating is the standard method for comparing the efficiency of an air filter. The higher the MERV rating, the better the filter is at removing particles from the air.
MERV ratings range from 1 - 16 and measurements are in microns. Some of the common particles related to MERV ratings are pet dander, insecticide dust, smog, dust, viruses, wood, tobacco smoke, spores, bacteria and pollen.
MERV ratings are determined by adding particles of varying sizes (1-12) into a controlled testing environment. The particles are added upstream of the test filter and a laser particle counter samples the air before it enters the filter and after it leaves the filter. The two particle counts are compared to calculate the Particle Size Efficiency of the tested filter. Once this is determined, a MERV Parameters chart is used to determine the MERV rating.
The most efficient filters have MERV ratings of 13 to 16 and will stop particles as small as .3 microns. These filters are used in hospitals and other super clean environments. 


A micron is a metric measurement used to analyze very small particles. One micron is equal to 1/25,400 of an inch or one millionth of a meter.
Of particular importance when discussing indoor air quality solutions is how effective a product is at capturing/controlling contaminants 2.5 microns or less in size. Any particles less than 2.5 microns are classified as a Respirable Suspended Particle (RSP) and the health hazards with these small particles is great. RSP’s are particles that are small enough to evade mucosal capture by the protective cilia and mucosa of the human upper respiratory system and invade the deeper reaches of the lungs with serious health consequences. In fact, any particles of five microns or less are considered a definite inhalation hazard because of the inability of the human body to filter effectively these particles from the lungs.
Allergenic particles, infectious agents and toxic gases can cause allergic reactions, headaches, sickness and many other illnesses. Since 98% of airborne contaminants are less than one (1) micron in size, knowing the effectiveness of any product at controlling submicron particles is very important when making a decision on an air purification system for your home.
Examples of some common air contaminants and their size in microns:
  • Human Hair (70 - 100 microns)
  • Pet Dander (0.5 - 100 microns)
  • Pollen (5 - 100 microns)
  • Spores from Plants (6 - 100 microns)
  • Mold (2 - 20 microns)
  • Smoke (.01 - 1 micron)
  • Dust Mite Debris (0.5 - 50 microns)
  • Household Dust (.05 - 100 microns)
  • Skin Flakes (0.4 - 10 microns)
  • Bacteria (0.35 - 10 microns) 
- American Lung Association


NAFA is a dynamic, non-profit trade association. NAFA members include air filter and component manufacturers, sales and service companies, and HVAC and indoor air quality professionals across the United States and in several foreign countries. NAFA is comprised of individuals and companies engaged in the sale, service and manufacture of the air filtration products. 
Various programs are undertaken by NAFA in an effort to maintain high levels of professionalism within the industry. These programs include education, certification, publications, meetings and technical seminars.
NAFA’s Long Range Plan is to have all major segments of the user market — industrial, commercial, healthcare, institutional and residential — recognize NAFA certified professionals as benefits of breathing clean air.


The National Association of Manufacturers (NAM) is the preeminent US manufacturers association as well as the nation’s largest industrial trade association, representing small and large manufacturers in every industrial sector and in all 50 states. Manufacturing employs nearly 12 million workers, contributes more than $1.6 trillion to the U.S. economy annually, is the largest driver of economic growth in the nation and accounts for the lion’s share of private sector research and development.
NAM Motto: Leading Innovation. Creating Opportunity. Pursuing Progress.  
NAM Mission Statement:
  • The mission of the NAM is to be the voice for all manufacturing in the United States. 
  • To inform legislators, the Administration, the media, policy influencers and the public about manufacturing’s vital leadership in innovation, job opportunity, technological progress and economic security.
  • We are a respected and focused partner in achieving an economic environment that encourages the expansion of manufacturing in the United States and strengthens our global leadership.


Now known as Oil and Energy OESP Service Professionals.
For more than 50 years, as NAOHSM, we’ve been a force for positive change in the oil heating industry. Now, as the National Association of Oil and Energy Service Professionals, we’ve expanded our frontiers to encompass everyone connected with the critical job of supplying this great country’s energy needs. The organization provides technical skills training, professional development, educational materials, conventions and trade show opportunities, social activities, tech school support and more.  

Natural Ventilation

Air leaks within the structure allows air to enter (infiltration) or exit (exfiltration) the building. It can also simply be opening windows and doors.  This air is uncontrolled/unconditioned, bringing in airborne moisture, dust, pollen, pesticides and anything being stored (chemicals, solvents, pesticides, etc.) in an attached garage.  The exfiltration can force air that has been conditioned and / or cleaned out of the building, reducing efficiency and increasing cost of any air processing units.


The New England Fuel Institute (NEFI) is a nationally recognized trade organization of over 1,200 independent Oilheat, Bioheat and related home energy dealers and associated businesses in the six New England States.
NEFI provides national leadership on legislative and regulatory rules and regulations for the Oilheat industry in Washington, DC, and is a prominent industry voice before the public and media. NEFI offers its members many communications, education and networking opportunities and events.


Original Equipment Manufacturer.
This means that a replacement part you purchase is manufactured by the original company that produced the product. For instance. If you have a GeneralAire® air cleaner, ideally you would purchase all future replacement filters manufactured by GeneralAire®. Other companies make replacement filters, however, many of the generic knock-off's or fake brand name filters may be of inferior quality in order to get the price down. Preserve the life of your product. Always use OEM products.


Also know as "Out-Gassing".
“Off-gassing” refers to the evaporation of synthetic compounds used in manufacturing a host of products, from cars to computers and toys to tennis balls. Two of the most identifiable types of off-gassing are the telltale “new car” and “new carpet” smells. Adhesives, wallpaper, and paints are other common offenders. Their smell makes it obvious that they’re giving off gas. Low levels of formaldehyde, a colorless and sharp-smelling gas, can also accumulate indoors from construction materials and household products like new furniture, cabinetry, and floor coverings. Other common household items that off-gas with little to no odor include bedding, furniture, and cabinetry.
Off-gassing can also be odorless. A study conducted at Stockholm University in Sweden found that certain computer monitors emit a chemical — triphenyl phosphate — that can cause allergic reactions. Triphenyl phosphate is a flame retardant that's added to many plastics (in the case of the Swedish study, it was contained in the computer monitors’ casings). When turned on, the monitors' heat caused the compound, which is not bonded to the plastic, to start evaporating.
Offgassing of products can irritate your respiratory system.  A person can experience red, puffy, watery eyes, a runny nose, congestion or coughing and asthma-type symptoms. Offgassing can also cause skin irritations, like rashes, itchiness and hives.


An instrument for direct measurement in ohms of the resistance of a conductor in ohms. An ohmmeter is useful in troubleshooting electrical or electronic circuits and devices to help pinpoint the cause of component failures.
Never use an ohmmeter on a circuit that has voltage from a source other than the meter itself. Attempting to conduct resistance checks on a circuit that has not been disconnected from its power source will result in a false reading and may damage the meter.

Organic Compounds

Chemicals 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.


OSHA'S mission:  to assure safe and healthful working conditions for working men and women by setting and enforcing standards and by providing training, outreach, education and assistance.
What OSHA says about indoor air quality: Poor indoor air quality (IAQ) has been tied to symptoms like headaches, fatigue, trouble concentrating, and irritation of the eyes, nose, throat and lungs. Also, some specific diseases have been linked to specific air contaminants or indoor environments, like asthma with damp indoor environments. In addition, some exposures, such as asbestos and radon, do not cause immediate symptoms but can lead to cancer after many years.

Ozone  (O3)

Ozone is a colorless odorless gas; the same kind of gas that’s found in the ozone layer. But in the ozone layer – high in the Earth’s stratosphere – ozone protects us from the sun. At ground level, where we live, ozone pollution is unhealthy to breathe. Ground-level ozone forms when pollutants from cars, trucks, power plants, industries, and some consumer products “cook” in the sun. Ozone usually peaks during the afternoon hours, when sunlight is the most intense.


Perceived Air Quality. 
Simply stated, a person's perception about the quality of air inside a space. Does it smell unpleasant?  Does it seem stuffy?  Does it make the person cough or sneeze?  Does it feel heavy?  Stale?


A “Particulate” is a tiny solid or liquid particle or piece of matter. It usually refers to particles in the air (airborne particulates).
There are many sources for particulates in the air. Among them are soil, plants, fires, road dust and dust mites. Other sources include fumes from combustion processes and products, like tobacco smoke, car exhaust, power plants, wood stoves, oil burners or other heating systems. Even burning candles or oil in lanterns can be sources of particulates. Also included: dust from mechanical processes like grinding or sweeping, and common household dust that may include mold, pollen, and small insect parts. Fibrous building material such as fiberglass may also be a source of particulates.   Mist from spray painting also creates airborne particulates. 


An infection agent.  Germ, virus, bacteria, prion or fungus that causes disease in its host.


A measure, in percent, of the material passing through a filter. Mathematically penetration is 100 - Efficency (percent). If a filter is 98% efficient, its penetration is 2% (100 - 98). Penetration is used to measure the performance of very high efficiency filters.

Photocatalytic Purification (PCO)

In the presence of organic pollutants such as solvents, alcohols, carbon monoxide, dyes and fuel oils, the activated photocatalyst attacks the pollutants’ chemical bonds, converting the toxic compounds to benign constituents such as water and carbon dioxide. The GeneralAire® PCO2450 VectorFlo® UV Air Purifier incorporates this technology; making it the most effective UV Light air treatment available.  Read more from Penn State's Study on Photocatalytic Oxidation (PCO).
From an article by Jeanie Wong:  Because PCO technology is an extremely powerful purification method, PCO units have the ability to eliminate particles as small as 0.001 microns, including the tiny penetrating particles that can absorb into your lungs and cause damage (HEPA filters are only able to filter out particles as small as 0.3 microns). Studies have also shown that in addition to VOCs, toxic gases, and chemicals, photocatalytic units can also render gases like carbon monoxide and nitrous oxide harmless - pollutants which can often escape standard units utilizing HEPA or ionic technology alone. 

Positive Pressure

Condition 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.


Parts Per Billion.  
PPB (or ppb) is used to measure the concentration of a contaminant in soils and sediments. In that case 1 PPB equals 1 µg of substance per kg of solid (µg/kg).
PPB (or ppb) is also sometimes used to describe small concentrations in water, in which case 1 ppb is equivalent to 1 µg/l because a liter of water weighs approximately a 1000 000 µg. This use of  PPB tends to be phased out in favor of µg/l.
PPB (or ppb) is often used to describe concentrations of contaminants in air (as a volume fraction). In this case the conversion of PPB to µg/m3 depends on the molecular weight of the contaminant.
For example, 1 ppb chlorine represents one part of chlorine in one thousand million parts of air by weight, which is 1.45 µg/m3.
The  PPB unit is mainly used in North America.


When employees come to work even when they're sick.  

Pressure Drop

The pressure drop of a filter is a measure of its resistance to air flow through it. Resistance is measured in inches w.g. in the Inch-Pound system of measurement (1" w.g. = 0.036 psi). It is measured in Pascals in the SI system.


Also known as Psychrometry.  Psychrometrics is the measurement of water vapor and heat in an air sample. It is used in the refrigeration industry, the design of clean rooms, certain manufacturing processes, and for theoretical applications. Psychrometry is a branch of thermodynamics and deals with terms like dry-bulb temperature, wet-bulb temperature, specific heat, and relative humidity. An understanding of psychrometry is a prerequisite for precise control of the humidity of contained environments. The key concept in psychrometrics is the relationship between wet-bulb temperature, dry-bulb temperature, and relative humidity.

Radiant Heating

Heating by means of heaters such as radiators, baseboard heaters, and electric coils, not by forced hot air. Radiant heating systems involve supplying heat directly to the floor or to panels in the wall or ceiling of a house. The systems depend largely on radiant heat transfer: the delivery of heat directly from the hot surface to the people and objects in the room via the radiation of heat, which is also called infrared radiation. Radiant heating is the effect you feel when you can feel the warmth of a hot stovetop element from across the room. When radiant heating is located in the floor, it is often called radiant floor heating or simply floor heating.

Relative Humidity

(RH) percentage of water vapor in air: the ratio of the amount of water vapor in the air at a given temperature to the maximum amount air can hold at the same temperature, expressed as a percentage.

Return Air

Air drawn into a heating unit after having been circulated from the heater's output supply to a room. The air removed from an occupied space and returned to the air handler to be exhausted or recirculated.


Sick Building Syndrome.
A condition in which a building is very tightly insulated against infiltration, its ventilation is reduced for energy conservation, and airborne contaminants are sufficiently elevated to cause adverse health effects in occupants. Occupants may experience fatigue, headache, dry eyes, and respiratory complaints affecting workers in certain buildings with limited ventilation. The symptoms seem to be caused by a combination of chemical agents in low concentrations rather than a specific irritant. Also known as tight building syndrome (TBS).


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. 

Sensible Heat

When a change in termperature of a substance occurs. This change can be felt or measured when increased or decreased.


A reduction of climate control energy demand in HVAC controls when a building is unoccupied.


The temperature to which a thermostat is set to result in a desired heated space temperature. 

Source Control

A preventive strategy for reducing airborne contaminant levels through removal of the material or activity generating the pollutants. An example would be to remove carpet. Carpet contains many allergy and asthma triggers; from the carpet fibers themselves, to dust, animal dander, dust mites and other particulates that carpet holds.


Sources of indoor air pollutants. Indoor air pollutants can originate within the building or be drawn in from outdoors. Common sources include people, pets, room furnishings such as carpeting, photocopiers, art supplies, etc.


Molds reproduce by means of spores. Spores are microscopic; they vary in shape and size (2-100 micrometers). Spores may travel in several ways--they may be passively moved (by a breeze or water drop), mechanically disturbed (by a person or animal passing by), or actively discharged by the mold (usually under moist conditions or high humidity).


Formed by leading contractors, the SRT is a membership program that offers an incredible array of HVAC, Electrical, Solar, and Plumbing best practice tools to help you generate leads, close more sales, market your company, recruit technicians and plumbers, price for profit, manage your service agreements, and create an exit strategy.
The SRT uses the Internet to drive costs down, passing the savings onto you. The SRT provides access to over $3 million of business building sales letters, post cards, web tools, ads, brochures, consumer newsletters, recruiting pieces, pricing calculators, training videos and more.

Steam Humidifier

Essentially, a steam humidifier boils water, releasing steam and moisture into the air. GeneralAire® steam humidification is produced by passing electric current through water inside a plastic steam generator cylinder between highly-conductive metal electrodes. Steam output is directly proportional to the conductivity of the water between clean electrodes. The Steam Cylinder, therefore, is the engine of the humidifier. The GeneralAire® steam humidifier can either be incorporated into your home's HVAC system, or affixed to a wall to provide humidification to your whole house.  

Supply Side

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.

Tight Building Syndrome (TBS)

A condition in which a building is very tightly insulated against infiltration, its ventilation is reduced for energy conservation, and airborne contaminants are sufficiently elevated to cause adverse health effects in occupants. Occupants may experience fatigue, headache, dry eyes, and respiratory complaints affecting workers in certain buildings with limited ventilation. The symptoms seem to be caused by a combination of chemical agents in low concentrations rather than a specific irritant. TBS is often used synonymously with sick building
syndrome (SBS).

Titanium Dioxide

Titanium dioxide (TiO2), particularly in the anatase (crystal) form, is the best in UV light photocatalysts.
GeneralAire® coats a 24 Sq. Ft. surface area of the honey combed aluminum cells within the PCO2450 VectorFlo® UV Light Purifier with titanium dioxide for the most advance germicidal kill rate technology available. The photocatalytic oxidation process breaks down harmful pollutants (VOC's) into harmless carbon dioxide and water molecules to give you clean, healthy, pure air.
The PCO2450 VectorFlo® incorporates titanium dioxide photocatalytic technology (PCO) and uses broad-spectrum UV light to eliminate harmful VOC particles as small as 0.001 microns; microns even smaller than what a HEPA filtration system can capture (0.3).


A device that transfers electrical energy from one circuit to another, either increasing (stepping up) or reducing (stepping down) the voltage through inductively coupled conductors—the transformer's coils.

Tungsten Wire

Tungsten is the most widely used of the refractory metals. In wire form, it is used in electrical applications. Tungsten is essential for the production of lighting products such as wire filament, and other goods where its high temperature properties are of use. Among its properties are a melting point of 3410° C, a low coefficient of thermal expansion and low vapor pressure at elevated temperatures, along with good electrical and thermal conductivity.


UL is a global independent safety science company offering expertise across five key strategic businesses: Product Safety, Environment, Life & Health, Knowledge Services and Verification Services. Our breadth, established objectivity and proven history mean we are a symbol of trust and enable us to help provide peace of mind to all.


Metric unit commonly used to identify particle sizes. Also known as micron and micrometer. These ratings range from those with very large particles to those as small as 0.1-µm diameter. For comparison, 5 µm is the approximate diameter of the cross section of a grain of talcum powder.

Vapor Pad®

Also known as Water Pad, Water Panel, Humidifier Pad, Humidifier Filter, or Evaporator Pad, the Vapor Pad® is usually made with an aluminum frame coated with a hydrophilic material that holds water.  As warm air from your furnace passes through the water-soaked pad, humidity is delivered throughout your home.

It is important to replace the Vapor Pad® at the start of the heating season each year instead of trying to wash it.  If you attempt to wash the pad, you also wash away the coating that holds the water, and your humidifier is not able to produce the humidity you need.

Return to the Top

Variable Air Volume (VAV)

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


The process of supplying or removing air from the outdoors by natural (opening windows and doors) or mechanical (powered air components such as an ERV or HRV) means to or from any space. Such air may or may not have been conditioned or treated.

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").

Venturi Effect

Applies to both air and water.  As air (or water) flows through a narrowing or constricted area of a given space, the pressure decreases while the velocity increases. If holes are incorporated in the narrow or constricted part of the space, a vacuum is created and air (or water) is drawn in through the holes from the outside.
This effect is incorporated in the GeneralAire® PCO2450 VectorFlo® (VectorFlo®=Venturi Effect).  Installed in your HVAC system, the PCO2450 VectorFlo® uses the Venturi Effect (assisted also by your HVAC system moving air through your duct) to draw air into the unit for maximum exposure, making it the most effective UV Light treatment available for reducing levels of air borne bacteria, mold, fungus and other infestations.


A microscopic particle composed of DNA or RNA surrounded by a protein coat. Viruses replicate by entering a cell, releasing their own DNA or RNA and controlling the DNA of the cell to manufacture more of the virus DNA or RNA and the surrounding protein coating. In the process the cell is usually destroyed, its walls disrupted, and the virus released into the surrounding enviroment.


Volatile Organic Compounds.
One of a class of chemical compounds; indoor sources include tobacco smoke, building products, furnishings, cleaning materials, pesticides, correction fluids, building materials, cleaning supplies, paint strippers, solvents, and office supplies. In sufficient quantities, VOCs can cause eye, nose, and throat irritations; dizziness; and headaches. Some VOCs are suspected carcinogens.
Data for health effects resulting, from exposure to the characteristically low levels of VOCs in the indoor environment are scarce. Compounds that vaporize (become a gas) at room temperature. Common sources which may emit VOCs into indoor air include housekeeping and maintenance products, and building and furnishing materials.
In sufficient quantities, VOCs can cause eye, nose, and throat irritations, headaches, dizziness, visual disorders, memory impairment; some are known to cause cancer in animals; some are suspected of causing, or are known to cause, cancer in humans.


Unit of electric potential difference: the unit of electromotive force and electric potential difference equal to the difference between two points in a circuit carrying one ampere of current and dissipating one watt of power.

Water Conductivity

Conductivity is a measure of the ability of water to pass an electrical current. Conductivity in water is affected by the presence of inorganic dissolved solids such as chloride, nitrate, sulfate, and phosphate anions (ions that carry a negative charge) or sodium, magnesium, calcium, iron, and aluminum cations (ions that carry a positive charge). Organic compounds like oil, phenol, alcohol, and sugar do not conduct electrical current very well and therefore have a low conductivity when in water. Conductivity is also affected by temperature: the warmer the water, the higher the conductivity. For this reason, conductivity is reported as conductivity at 25 degrees Celsius (25 C).
Pure water is not a good conductor of electricity. Because the electrical current is transported by the ions in solution, the conductivity increases as the concentration of ions increases. Certain ions, such as nitrates, chloride and phosphates, are better carriers than sugar, alcohol and oil. Softened water is generally not recommended for a steam humidifier.


Unit of electrical power: the international SI unit of power equal to the power produced by a current of one ampere acting across a potential difference of one volt.

Whole House System

Air treatment system, generally installed in a residential home's HVAC system, that treats the whole house Vs just one room.
Portable (stand alone single room) air cleaners, humidifiers and dehumidifiers generally treat only one room in the home. Portable units can take up ample space in a room and sometimes be quite noisy and unattractive.  Maintenance can also be daunting.  Airborne impurities exist throughout a home, so while one room is treated it can also be quickly re-infected as people travel from other contaminated rooms into the treated room (bringing contaminants with them). While portable units may be the only option for those who live in apartment homes or other rental property (where one does not have the authorization or room to make such additions to your home's HVAC system), whole house systems are preferred because:
  • They treat / protect the whole home.
  • They protect you 24/7, 365 days per year.
  • Maintenance is usually minimal.
  • The operation is generally more quiet.
  • They can be easily installed / incorporated within the home’s HVAC system and out of site.


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.