We're pleased to present you with this glossary of popular HVAC terms used througout the industry.


Air Balance

The process of measuring and balancing air flow throughout a home with central heating to ensure even temperatures and comfort throughout the home.

Air Conditioner

An appliance designed to provide cooling and humidity control in an indoor, conditioned space. Air conditioners provide cooling only, whereas heat pumps provide both heating and cooling.

Air Conditioning

The removal of heat and control of humidity in the air within a home or building. Heat pumps, central air conditioners, and ductless heat pumps provide air conditioning.

Air Duct

See duct

Air Handler

An appliance used to circulate and/or heat air in a home or building. A component of an HVAC system, an air handler is usually shaped like a large metal box that houses a blower, air filter, and other components. Air handlers are connected to duct systems and are responsible for moving air throughout an HVAC system. When outfitted with electric strip heat, an air handler can provide heat in a home and can be referred to as an electric furnace. Air handlers can be teamed with electric heat pumps to provide heating and cooling in a home.



A component of a furnace or air handler responsible for circulating air throughout an HVAC system in a home or building.


A vessel in which water is heated. The heated water then exits the boiler and is circulated through a series of pipes or radiators, providing radiant heat and/or domestic hot water for homes.

BTU (British Thermal Unit)

A unit of measurement for heat defined as the amount of energy required to raise the temperature of 1 pound of water 1 degree Fahrenheit. When referring to HVAC heating equipment, BTU's typically refer to BTU's per hour


Carbon Monoxide (CO)

A colorless, odorless, and poisonous gas emitted as a byproduct of the combustion process of carbon compounds such as fossil fuels, including natural gas, gasoline, fuel oil, and coal.

Central Air Conditioning

Air conditioning delivered to a home or building with a single appliance through a system of air ducts. See air conditioning.

Central Heating

A home heating system where heat is delivered with a single appliance, such as a furnace, through a system of air ducts.


A component of air conditioners, heat pumps, and refrigerators used to pressurize refrigerant, allowing it to flow through the refrigerant lines in the system.


A component of air conditioners and heat pumps which allows refrigerant to condense from a gas to a liquid in the decompressing/cooling process.

Condenser Coil

A component of heat pumps or air conditioners. The refrigerant is circulated through the condenser coil while air is blown over the coil with a fan, transferring heat from the refrigerant to the surrounding air. Refrigerant enters the condenser coil as hot vapor and leaves as a liquid.

Condensing Unit

The component of a central air conditioner or heat pump that is designed to transfer heat absorbed by refrigerant from indoors to the outside, or from the outdoors to inside the home (heat pump only).

Conditioned Space

The interior of a home or building that is heated and/or cooled and insulated.

COP (Coefficient of Performance)

A unit of measurement for the heating efficiency of electric heat pumps. COP is the ratio of heat output to energy input by a heat pump. The higher the COP number, the more efficient the heating output of a heat pump. Electric resistance heating, such as baseboards, electric wall heaters, or an electric furnace, operate at a COP of 1. Some models of heat pumps can operate with a COP of 3.5 or higher, or 3.5 times as efficient as electric resistance heat.



Components of an HVAC system that control the amount of air flow to a given part of the system. Return air grilles, supply registers, or air ducts can all have dampers which can be adjusted manually or automatically to restrict or allow air to flow, depending on the heating and cooling demands of an area of the home or building.


A device which can lower the humidity level of indoor air.

Demand Water Heater

See Tankless Water Heater.


A device used to disperse air flow from an air supply grill, allowing the air from the ducts to more effectively mix with the air in a room, and controlling the direction and amount of air flowing into the room.

Duct (Air Duct)

Round or rectangular tubes, constructed of sheet metal or flexible plastic and wire, which distribute air throughout an HVAC system.

Duct Fan

A fan located in an air duct which moves air through the duct.

Ductless Heat Pump

A type of heat pump that does not use air ducts to move conditioned air throughout a home. As opposed to the central air handler component of a standard heat pump system, a ductless heat pump consists of an outdoor condenser unit and one or more smaller wall mounted indoor air handlers. Because ductless heat pumps are not affected by the static pressure of an air duct system, they are one of the most energy-efficient ways available to heat and cool a home.


Electric Furnace

An air handler outfitted with electric resistance strip heat. Electric furnaces are often used as backup heating for heat pumps in case of extremely cold weather.

Electric Strip Heat

See Strip Heat


Filter (Air)

A device that removes contaminants and particulate matter from the air before it enters conditioned space in a home. Filters come with different ratings based on the size of particulate matter they can remove from the air. See HEPA for a description of this rating system.

Fireplace Insert

A device inserted into an existing fireplace, typically constructed of cast iron or metal, which makes the fireplace a more efficient source of heat for the home. Inserts typically have self-cleaning glass doors which allow the flames to be viewed from the outside and may have fans and thermostat controls, allowing them to provide heat more efficiently than an open fireplace.


A duct, pipe, or chimney into which combustion gases flow in order to be conveyed out of a home to the outdoors.

Forced Air System

An HVAC system where the air is forced through a system of ducts throughout the home by an air handler or furnace.

Fuel Oil

A petroleum product used as fuel in oil furnaces for heating homes. Fuel oil is similar in composition to diesel fuel but without the lower levels of sulfur and/or ash required by the EPA for motor vehicle diesel. By US law, fuel oil is dyed red to distinguish it from motor vehicle diesel fuel for tax and emission purposes.


An appliance that provides central heat to the interior of a home or building. Furnaces, depending on the type, can use a variety of fuels to provide heat, including natural gas, electric, heating oil, and propane.



Heat Loss

Heat that flows from the interior conditioned space of a building to the unconditioned space outdoors. Some heat loss is necessary as a healthy building must breathe. However, unintended heat loss, such as through holes in insulation, poorly constructed windows, or unsealed ducts results in higher than needed heating and cooling costs in a building.

Heat Loss Calculation

A process which calculates the amount of heat lost to the outdoors from a home. This process allows for the proper sizing of heating and cooling equipment for the home. Proper equipment sizing is extremely important for the comfort of the home and to maximize the useful life of the HVAC equipment.

Heat Pump

A device that provides heating or cooling for a home by absorbing heat energy from one place and moving it to another place through the use of a compressor and refrigerant. Most of the heating and cooling energy used by heat pumps is heat absorbed from the outdoor environment rather than the electricity consumed by the heat pump, making heat pumps one of the most energy-efficient ways to both heat and cool a home. Heat pumps are 200% to 350% more efficient than conventional electric resistance heat, such as an electric furnace or electric baseboards.

Heating Load

The amount of heat flow required to maintain the desired temperature in a building, measured in BTU's per hour.

HEPA Filter

High-Efficiency Particulate Arresting filters. In order to qualify for the HEPA standard in the U.S., a filter must remove at least 99.97% of particulates above .3 micrometers from the air. True HEPA filters are MERV 17 to 20 and cause a large amount of static pressure in the system, rendering them impractical and uncommon in most residential HVAC applications.


Heating Seasonal Performance Factor. HSPF is a measurement system used to measure the heating efficiency of air source heat pumps. In the Pacific Northwest, our heating season is much longer than our cooling season, making HSPF the primary rating used to measure heat pump efficiency. In warmer climates, heat pump efficiency is primarily measured using SEER.

Home Energy Rating System (HERS)

A nationally recognized energy rating program designed to calculate the energy efficiency of homes. This rating system allows homes to be rated and compared by home buyers and renters for energy efficiency.


An appliance which raises the level of humidity in the indoor air of a home.


A measurement of the amount of moisture in the air.


An acronym for Heating, Ventilation, and Air Conditioning.



An acronym for Indoor Air Quality.

Ignitor (Hot Surface Ignitor)

A component of modern gas furnaces which lights the gas when heat is needed. Electricity passes through the ignitor, making it glow red hot, allowing it to heat up to temperatures high enough to light the gas. Higher quality ignitors are made of Silicon Nitride and last significantly longer than those made of Silicon Carbide.

Indoor Air Pollutants

Harmful particles, compounds, gases, dust, fibers, viruses, and bacteria carried by indoor air.


Air leakage from unconditioned space or the outdoors into a conditioned space.

Integrated Heating System

An HVAC appliance that handles heating and another function, such as water heating. Some boilers and tankless water heaters can be used to generate heat for an indoor space and to heat water.





MERV Rating (Minimum Efficiency Reporting Value)

A measurement scale used to rate the effectiveness of air filters at removing particles between 0.3 and 10 micrometers in size. MERV ratings fall between 1 and 16. The higher the rating, the higher the percentage of particulate matter the filter removes from the air with each pass through the filter. A MERV 16 filter removes at least 95% of particles between 0.3 and 10 micrometers in size with each pass.


Natural Gas

A naturally occurring hydrocarbon gas mixture used as fuel in a variety of applications, including home heating, water heating, and cooking.

Natural Ventilation

The intentional infiltration of outdoor air into the space through open windows, doors, or other non-powered means.

Negative Pressure

A condition where the air pressure inside a home or space is less than the pressure outside due to more air being exhausted from the space than supplied to it. Under this condition, outdoor surrounding air will infiltrate through any openings into the space.


Organic Compounds

Compounds that contain carbon molecules. VOC's (Volatile Organic Compounds) are compounds that contain carbon which has a low boiling point and high vapor pressure, meaning they can evaporate and exist as vapor in the air at room temperature. Many VOC's are harmful to people and pets and can exist in high quantities in a home.


Pilot Light

A small flame used in gas appliances which are kept continuously alight to ignite the gas burners when needed by the appliance.


An air compartment typically constructed of sheet metal connected to ducts in an HVAC system.

Positive Pressure

A condition where the air pressure inside a home or space is more than the pressure outside due to less air being exhausted from the space than supplied to it. Under this condition, indoor air will exit through any openings into the surrounding air.

Preventative Maintenance

The regular cleaning and inspection of HVAC equipment, along with the replacement of parts and materials that are worn or past their useful life. Regular preventative maintenance gives HVAC equipment a much better chance of having an extended useful life, saving money and preventing premature catastrophic equipment failure.

Programmable Thermostat

A type of thermostat with the ability to be programmed with user behaviors, allowing the system to run only when needed. Programmable thermostats can help homeowners save money over the long run by increasing the efficiency of their HVAC system.




A measure of a material's thermal resistance, or resistance to the transfer of heat. R-Value is used to describe the heat resistant capabilities of insulation material. The higher the R-value of a material, the greater it's heat resistant and insulating capacity.

Radiant Floor

A type of radiant heating system that is installed in a floor, emitting heat from the floor up into a space. Usually, radiant floor heating is achieved by installing tubes with circulating hot water in the floor.

Radiant Heating System

A type of heating system that emits heat from a surface, such as hot water radiators, in-floor hot water tubes, or electric baseboards, into a living space. Because no moving air is required to heat a space with radiant heat, some of the draft and dust problems of lower quality central heating systems are eliminated.


A component of a radiant heating system that radiates heat from its surface into a room. The heat is usually supplied to the radiator through hot water or steam circulation.


Occurs when the air or gases being exhausted from a building are immediately pulled back into the building through an air intake or other opening.


A fluid used in air conditioners, heat pumps, refrigerators, and other cooling equipment to transfer heat from one place to another. Because of its low boiling point, refrigerant easily evaporates, making it highly effective at transferring heat energy. There are many different refrigerant compounds, including R-22 (Ozone depleting) and R-410A (Ozone friendly). Under current federal law, it is illegal to manufacture and ship new equipment loaded with R-22 refrigerant due to its Ozone-depleting properties. However, units are still allowed to be manufactured and "dry-shipped" without R-22, then subsequently charged with R-22 after installation.

Relative Humidity

The capacity of air to hold moisture changes depending on its temperature. As air temperature rises, it's capacity to hold moisture increases. Relative humidity is a measure of the amount of moisture in the air as a percentage of its capacity to hold moisture at its current temperature.

Return Air

Air returning from a conditioned space into a heating or cooling appliance in order to be reheated or cooled.

Return Duct

Ducts which house return air, returning it to the heating or cooling appliance. Ducts are connected to return air grilles. Due to the lower pressure in return air ducts, air moves from the conditioned space of a room, through the return grilles, and into return air ducts to be sent back through the heating or cooling appliance.



Seasonal Energy Efficiency Ratio. This is a measure of the cooling efficiency of an HVAC appliance, such as an air conditioner or heat pump. SEER takes into account the fluctuation in temperatures that occur during different seasons. In Western Washington, because our cooling season is so short each year, SEER is much less important as an efficiency measurement for heat pumps than HSPF, which measures heat pump heating efficiency.

Setback Thermostat

A thermostat that can be programmed to "set back" at certain times of the day which do not require as much temperature control by an HVAC system. "Setbacks" are commonly used during the day when home occupants are at work, or at night when occupants are sleeping, in order to save energy.

Space Heater

A self-contained appliance used for heating a room or enclosed space.

Static Pressure

The resistance in a duct system that moving air has to overcome. Static pressure is measured as inches of water column (IWC). The higher the number, the more static pressure that exists in a duct system.

Storage Water Heater

A water heater with a tank that stores heated water to be dispensed when needed. Hot water is dispensed from the top of the tank and is refilled by cold water in the bottom of the tank.

Strip Heat

An electric device used to heat the surrounding air. These are often installed in air handlers to provide heating for a home. An air handler equipped with strip heat is also referred to as an electric furnace.

Supply Air

Conditioned or heated air distributed by the fan in an HVAC appliance into the conditioned space of a room or home.

Supply Duct

The duct through which supply air is distributed into the conditioned space of a room or home.


Tankless Water Heater

A water heater that heats water on demand as needed as opposed to a storage water heater, which heats water and stores it for later use. Tankless water heaters are also called "demand water heaters."

Temperature Zones

Areas of a building, such as rooms or other sections, where the temperature can be controlled independently.


A unit of measurement for heat which contains 100,000 BTU's (British Thermal Units).


A device containing a temperature sensor used to control the output of an HVAC system in order to maintain the desired temperature in a building.


A unit of measurement for cooling capacity. 1 ton of cooling represents 12,000 BTU per hour or the approximate cooling power of 1 ton of ice melting over a 24 hour period.



Vent Pipe

A tube that is used to vent combustion gases out of a combustion appliance and into the outside air.

VOC (Volatile Organic Compound)

Compounds that contain carbon which have a low boiling point and high vapor pressure, meaning they can evaporate and exist as vapor in the air at room temperature. Many VOC's are harmful to people and pets and can exist in high quantities in a home.



The process of modifying a home or building to protect it from the outside elements, decreasing the amount of energy required to heat and cool the building.



An HVAC system equipped with temperature zones where the temperature can be controlled independently from other zones in the system.

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