Learn HVAC Terms and Definitions

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Below is a glossary of HVAC terminology to provide understanding of these frequently used terms in the industry.

Absolute Pressure

In the larger context of pressure measurement, absolute pressure is part of the “zero reference” trinity. This includes Absolute Pressure (AP), Gauge Pressure and Differential Pressure. AP is zero referenced against a perfect vacuum.

Absolute Temperature

Absolute temperature is the temperature of an object on a scale where 0 is considered as absolute zero. Absolute temperature scales are Kelvin and Rankine. Absolute zero is the lowest temperature during which the system is in a state of lowest possible (minimum) energy. No electronic device can function at this temperature.

Absolute Zero Pressure

Absolute zero, temperature at which a thermodynamic system has the lowest energy. It corresponds to −273.15 °C on the Celsius temperature scale and to −459.67 °F on the Fahrenheit temperature scale.


Ability to take in and retain liquid: able to absorb liquid.

Absorption Refrigerator

An absorption refrigerator is a refrigerator that comes with a heat source (e.g., solar energy, a fossil-fueled flame, waste heat from factories or district heating systems) that provides the energy necessary to power the cooling process. The principle can even air condition buildings using the waste heat from a gas turbine or water heater. This multipurpose appliance is very efficient, since the gas turbine then creates electricity, hot water and air-conditioning (called cogeneration/trigeneration).


The ACCA (The Indoor Environment & Energy Efficiency Association) is a nonprofit association whose membership exceeds more than 60,000 professionals and 4,000 businesses in the indoor environment and energy services community. ACCA works together to promote professional contracting, energy efficiency, and healthy, comfortable indoor environments.


An accumulator is a storage tank that takes liquid refrigerant from the evaporator and prevents it from affecting suction lines before vaporizing


Annual Fuel Utilization Efficiency (AFUE) is the amount of fuel converted to space heat in proportion to the amount of fuel entering the boiler or furnace. It won’t include the electricity consumed by the fan and controls.


The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is a trade association representing manufacturers of HVACR and water heating equipment all over the globe.


The Better Business Bureau (BBB) helps people learn about and recommend businesses, brands and charities they can trust.


British Thermal Unit (BTU) is a simple measure of thermal (heat) energy. One BTU is the amount of energy needed to heat one pound of water one degree Fahrenheit, measured at its heaviest point.


Consortium for Energy Efficiency (CEE) is a U.S. and Canadian association of gas and electric efficiency program administrators. CEE strives to accelerate the manufacturing and availability of energy-efficient products and services for maximum public benefit.


Coefficient of Performance (COP) is a measure of efficiency for heat pumps in the heating mode that makes up the ratio of total heating capacity to electrical energy input. For example, if a heat pump has a COP of 3, it should generate three units of energy for every one unit of electricity consumed.


Energy Efficiency Ratio (EER) of any cooling device is the ratio of output cooling energy (in BTU) to input electrical energy (in Wh) at a defined operating point. EER is generally calculated using a 95 °F outside temp and an inside (actually return air) temp of 80 °F and 50% relative humidity.


Heating Seasonal Performance Factor (HSPF) is a measure of a heat pump’s energy efficiency over one heating “season.” It is made up of the total heating output of a heat pump (including supplementary electric heat) across the normal heating season (in BTUs) as compared to the total electricity consumed (in watt hours) during the same season.


HVAC is a common acronym that stands for Heating Ventilation and Air Conditioning. This is the most frequently used term when referring to combination systems that do both heating and cooling.


Mini-split systems are ductless room air conditioners or heaters. When only a few rooms need heating and cooling, mini-split heat pumps and air conditioners provide a smart, all-around solution. Exceptional efficiency in a small package.


NATE, North American Technician Excellence Inc., is an independent, third-party, nonprofit certification body for HVACR technicians. NATE certification tests demand real-world working knowledge of HVACR systems and applicants can earn installation and/or service certification in one or more particular areas. NATE certification tests are developed by the NATE technical committee. The NATE technical committee is made up of industry experts who developed and manage NATE’s Knowledge Areas of Technician Expertise (KATEs). Each exam is laid out based on these knowledge areas of expertise. The technical committee offers a wide range of perspectives including contractors, educators, manufacturers and utilities. Members also are diversely spread throughout the country.


Seasonal Energy Efficiency Ratio (SEER) is the cooling output across a usual cooling season divided by the total electric energy input over the same period. The higher the unit’s SEER rating the more energy efficient it is.


Solar Heat Gain Coefficient (SHGC) is a measure of how efficiently a window reflects heat caused by sunlight.


U-factor is a measure of how well a window design is capable of stopping heat flow. Look for windows with a lower U-factor (the lower the U-factor, the better).

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