HVAC Glossary

Making informed purchasing decisions and getting the best performance from your home comfort system is easier when you know the common terms.

Air Conditioning, Heating and Refrigeration Institute (AHRI)

AHRI is an abbreviation for the Air Conditioning, Heating and Refrigeration Institute, a trade association that develops standards for measuring and certifying product performance. AHRI standards and guidelines are used throughout the world. Equipment with the AHRI Certified® mark has AHRI’s assurance that it will perform accurately and consistently.

Annualized Fuel Utilization Efficiency (AFUE)

A measure of furnace heating efficiency. The higher the AFUE, the more efficient the furnace.

British Thermal Unit (BTU)

A measurement of heat energy. One Btu is the amount of heat required to raise one pound of water by one degree Fahrenheit. The higher the Btu rating, the greater the heating capacity of the system.

Carbon Monoxide (CO)

Carbon monoxide (sometimes referred to by its chemical abbreviation, CO) is an odorless, colorless, tasteless and poisonous gas that’s produced when fuels such as natural gas burn with insufficient air. When carbon monoxide leaks into your home’s air, it can lead to serious health problems for your family. Regular maintenance of your home’s heating and ventilation system will help ensure it’s not a source of a carbon monoxide leak.

Cubic Feet Per Minute (CFM)

Cubic Feet Per Minute (CFM) is a measurement of airflow volume, determined by how many cubic feet of air pass by a stationary point in one minute. Having an accurate estimate of your home’s CFM is key to choosing an HVAC system with the right capacity for your space and needs.

Decibel (dB)

A decibel (dB) is a unit used to measure the relative intensity of sound. For example, sound levels in a library are normally about 40 dB, normal conversations range from about 50 to 60 dB, and an operating motorcycle or garbage truck can be as high as 100 dB.

Energy Efficiency Ratio (EER)

The ratio of cooling capacity to the power input (in watts). The higher the EER rating, the more efficient the air conditioner.

Heating Seasonal Performance Factor (HSPF)

The heating efficiency rating for heat pumps. The higher the rating, the more efficient the heat pump.

HVAC Zoning System

An HVAC zoning system (also referred to as “zoned HVAC”) is a heating and cooling system that uses dampers in the ductwork to regulate and redirect air to specific areas of the home. This allows for the creation of customized temperature zones throughout the home for increased comfort and efficiency.

MERV Rating

The Minimum Efficiency Reporting Value (MERV) rating is used to rank the efficiency of air filters. Every air filter has holes that allow air and particles to pass through. The smaller the holes, the fewer particles that can pass through. Higher MERV ratings indicate smaller holes, and therefore a more efficient filter.

Programmable Thermostat

A programmable thermostat is a thermostat with the ability to adjust home heating and cooling equipment according to a pre-set schedule. For instance, a programmable thermostat could be set to maintain a home temperature of 72 degrees throughout the day, and 65 degrees from 11:00 p.m. to 7:00 a.m. every night.

R-22 Refrigerant

The old standard for residential air conditioners, has been phased out by the U.S. EPA.

R-410A Refrigerant

A chlorine-free refrigerant that meets the U.S. EPA’s newest, most stringent environmental guidelines.

Refrigerant Lines – Lineset

Two copper lines that connect the outdoor air conditioner or heat pump to the indoor evaporator coil.

Seasonal Energy Efficiency Ratio (SEER)

Seasonal Energy Efficiency Ratio (SEER) is an energy efficiency rating for air conditioners. The higher the SEER, the more efficient the unit.

Time Delay Relay (TDR)

Protects the system against short cycling of the compressor from power interruption, and thermostat problems (due to vibrations, door slamming, or quick “putzing” adjustments by the user). The time delay relay is usually set for a 5 minute delay after the last time the compressor was energized.


Unit of measurement for determining cooling capacity. One ton equals 12,000 Btuh.

Two-Stage Operation

Provides two levels of heating or cooling output for enhanced temperature control, energy efficiency and indoor air quality.

Variable Speed Motor

A motor that operates at a wide range of speeds to precisely control the distribution of heated and cooled air throughout the home.