Glossary of Common AC Terms

Please let us know how we can assist with your search for improved heating and air conditioning performance. Since people differ in how they define and use certain terms related to air conditioning and heating technology, we’ve provided a glossary of key terms to help make your online experience less confusing and more effective.


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Advanced Reciprocating Compressor – is a type of compressor that uses a more efficient process for compressing refrigerant for better cooling efficiency.

AFUE – (Annual Fuel Utilization Efficiency) tells how much energy is being converted to heat. For example, an AFUE of 90 means that 90% of the fuel is being used to warm your home, while the other 10% escapes as exhaust with the combustion gases.

Air Handler – is a device used to condition and circulate air as part of a heating, ventilating, and air conditioning (HVAC) system. An air handler is usually a large unit that resembles a furnace, runs off electric and is usually used residentially with Heat Pumps.

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BTU – (BritishThermal Unit) is used for both heating and cooling. BTU is a measure of the heat given off when fuel is combusted. For cooling, it’s a measure of heat extracted from your home. One BTU is equal to the heat given off by a wooden kitchen match.

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Carbon Monoxide (CO) – is an odorless, colorless and toxic gas. Because it is impossible to see, taste or smell the toxic fumes, CO can kill you before you are aware it is in your home. At lower levels of exposure, CO causes mild effects that are often mistaken for the flu. These symptoms include headaches, dizziness, disorientation, nausea and fatigue. The effects of CO exposure can vary greatly from person to person depending on age, overall health and the concentration and length of exposure.

Condenser Coil – is the high pressure high temperature refrigerant gas leaves the outdoor compressor and enters the outdoor condensing coil where it is cooled to a liquid state by the condensing unit fan that blows outside air across the condensing coil or by immersion of the condensing coil in cooling water in some designs. The heat produced in these steps is transferred to the outside by a fan which blows outside air across the condensing coil. The liquid refrigerant is then able to return to the indoor components for cooling and dehumidifying the building interior.

Cycling – refers to the process of an HVAC system turning on and off. Some systems require less cycling than others, leading to higher energy efficiency and less wear on the system.

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Damper – is found in ductwork. This movable plate opens and closes to control airflow. Dampers are used effectively in zoning to regulate airflow to certain rooms.

Digital Thermostat – is an electronic thermostat powered by electric, battery or both. It has digital screen which displays temperature and other information. It’s accurate to +/-0.1?C. It ensures reliable and efficient temperature regulation and can be either programmable or non-programmable.

Downflow – is a type of furnace that takes cool air in from the top, heats it and blows warm air out of the bottom–common when a furnace is located in a slab home.

Ductwork – is boxed or pipe-like channels that carry conditioned air throughout your home.

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ECM – (Electronically Controlled Motor) is an ultra-high efficient DC driven motor, which is much more efficient than an AC motor. This allows you to circulate and filter the air in your home continuously for about the same cost as operating a standard light bulb.

Electronic Air Cleaner (EAC) – filters out large particles and contaminants from indoor air. It then electronically pulls out tiny particles that have been magnetized, such as viruses and bacteria, drawing them to a collector plate.

Evaporator Coil – is part of a split-system air conditioner or heat pump located indoors. The evaporator coil cools and dehumidifies the air by converting liquid refrigerant into a gas, which absorbs the heat from the air. The warmed refrigerant is then carried through a tube to the outdoor unit (condenser coil).

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Fan Coil – is an indoor component of a heat pump system, used in place of a furnace. It provides additional heating through electric elements on cold days when the heat pump cannot provide adequate heating.

Freon – is a trademark used for a variety of nonflammable gaseous or liquid fluorinated hydrocarbons employed primarily as working fluids in refrigeration and air conditioning and as aerosol propellants.

Furnace – is an indoor heating unit that works in conjunction with an air conditioner or heat pump.

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Heat Pump – is an outdoor unit designed to warm your home in winter and cool your home in summer.

HEPA Filter – (High Efficiency Particulate Absorbing) is an air filter that removes particles from the air by trapping them as air flows through.

HVAC – is a term used for Heating, Ventilation and Air Conditioning.

HSPF – (Heating Seasonal Performance Factor) is a measure of the heating efficiency of a heat pump. The higher the HSPF number, the more efficiently the heat pump heats your home.

Horizontal Flow – is a type of furnace, installed on its “side,” that draws in air from one side, heats it and sends the warm air out the other side. Most often used for installations in attics or crawl spaces.

Humidifier – is a piece of equipment that adds water vapor to heated air as it moves out of the furnace. This adds necessary moisture to protect your furnishings and reduce static electricity. Humidity levels should range from 35-50%.

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Indoor/Outdoor System – refers to a comfort system consisting of components in two locations. Common examples include an outside unit, such as an air conditioner, and an indoor unit, such as a furnace or air handler with a coil.

Indoor Coil – is the other, less visible half of your air conditioning unit. It’s attached to your furnace or air handler. As indoor air flows across it, heat and moisture are drawn out, leaving air that is cool, comfortable and conditioned.

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Load Estimate – is a series of studies performed to determine the heating or cooling requirements of your home. An energy load analysis uses information such as the square footage of your home, window or door areas, insulation quality and local climate to determine the heating and cooling capacity needed by your furnace, heat pump or air conditioner. When referring to heating, this is often known as a Heat Loss Analysis, since a home’s heating requirements are determined by the amount of heat lost through the roof, entry ways and walls. When referring to cooling it’s considered a Heat Gain.

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Matched System – is a heating and cooling system comprised of products that have been certified to perform at promised comfort and efficiency levels when used together, and used according to design and engineering specifications.

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NATE – (North American Technician Excellence) is the nationwide certification program for home heating and cooling technicians. It’s the only certification that is recognized by the entire industry.

Natural Gas – is an energy fuel that is a mixture of naturally occurring combustible gases found underground and is widely used for heating and cooking, as well as for a variety of industrial applications.

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Operating Cost – is the day-to-day cost of running your home comfort equipment, based on energy use.

Outdoor Coil – is located in the outdoor unit and dissipates heat from the refrigerant by changing the refrigerant from vapor to liquid.

Outdoor Unit – is the outdoor portion of a split system, such as an air conditioner or heat pump. May also be a packaged air conditioning and/or heating system in which all components are located in one cabinet.

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Packaged System – is an air conditioning and/or heating system in which all components are located in one cabinet. Used in certain localities and for certain building types, the all-in-one system is installed either beside or on top of your home.

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

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R410A Refrigerant – is an environmentally sound refrigerant designed to protect the earth’s ozone layer. Federal law requires all manufacturers phase out ozone depleting refrigerants within a certain date. It is approved by the US Environmental Protection Agency as a replacement for Freon 22.

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SEER – (Seasonal Energy Efficiency Ratio) is a measure of the cooling efficiency of your air conditioner or heat pump. The higher the SEER number, the more efficient the system is at converting electricity into cooling power.

Setback Thermostat or Programmable Thermostat – is a thermostat with built-in memory, that can be programmed for different temperature settings at different times of the day.

Scroll Compressor – is a special designed compressor that works in a circular motion, as opposed to up-and-down piston action. It has fewer moving parts and provides smooth, efficient and reliable operation.

Split System – refers to an air conditioner or heat pump that has components in two locations. Usually, one part of the system is located inside (evaporator coil) and the other is located outside your home (condenser coil).

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Thermostat – is a temperature-control device, typically found on a wall inside the home. It consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system.

Ton – is a unit of measurement used for determining cooling capacity. One ton is the equivalent of 12,000 BTUs per hour.

Two-Stage Furnace – is a furnace that can operate on both low and high heat settings. Low heat is typically used around 80% of the time. Low stage heat has a lower BTU output and runs quieter.

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Upflow – is a type of furnace that draws air in from the bottom, heats it and then blows the warmed air out the top into the duct work. This type of furnace is usually installed in a basement or an out-of-the-way closet.

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Variable Speed Motor – is a fan motor inside higher efficiency indoor and outdoor units that works with the thermostat and is designed to change its speed based on your home’s heating and air conditioning requirements. The variable-speed motor also increases dehumidification and is quiet because it runs at a lower speed most of the time. Plus, the consistent air circulation eliminates noisy startups and shutdowns.

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Zoning – is a way to increase your home comfort and energy efficiency by controlling when and where heating and cooling occurs in a home. Thermostats are used to control operating times of the equipment. Dampers are used to direct airflow to certain parts or “zones” of the home.

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