HVAC Glossary

This glossary covers key terms related to HVAC systems and ventilation, providing a basic understanding of the terminology used in this website.

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.

An air handler, or air handling unit, is the indoor part of an air-conditioning or heat pump system. Air handlers work much like furnaces in that they efficiently move air throughout the home. The main difference is that the air handlers only circulate heat when paired with a heat pump.

A component of HVAC systems that traps dust, pollen, and other particles to improve indoor air quality.

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.

A British Thermal Unit (Btu) is a measurement of heat energy. One Btu is the amount of heat required to raise one pound of water by one degree Fahrenheit. Btus per hour (Btuh) is the benchmark used to estimate the capacity of heating systems.

Combined Annual Efficiency (CAE) is a measure of the amount of heat produced for every dollar of fuel consumed for both home and water heating.

Central air conditioning (or central A/C) is a system in which air is cooled at a central location and distributed to and from rooms by one or more fans and ductwork. The work of the air conditioner compressor is what makes the whole process of air conditioning possible. The compression of the refrigerant gas enables it to discharge heat out of the house, which is how the cool air is created.

An AC or heat pump compressor is the part of an outdoor air conditioner or heat pump that compresses and pumps refrigerant to meet household cooling requirements.

A condenser (or AC condenser) is the outdoor portion of an air conditioner or heat pump that either releases or collects heat, depending on the time of the year.


Both split air conditioner and heat pump condensers are made of the same basic parts. The condenser cabinet contains the condenser coil, a compressor, a fan, and various controls. The condenser coil can be made of copper tubing with aluminum fins or all-aluminum tubing so heat can be rapidly transferred.


The condenser fan is a vital component and circulates the air across the coil to facilitate heat transfer. The compressor is the heart of the system since it compresses the refrigerant and pumps it to a coil in the form of a hot gas.


In air conditioners, the refrigerant is cooled at the condenser into a warm liquid, and passes through a pipe into the evaporator coil where it expands and cools. In heat pumps, the hot gas is pumped directly to the evaporator coil to provide heat.


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. Larger homes circulate more air (since there’s more space for it to move around), so they will have a higher CFM. 

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.


Older furnaces, air conditioners and heat pumps often have relatively high operating sounds, along with the noisy stops and starts associated with single-stage operation. Direct Air offers a full line of home heating, cooling and ventilation products that are designed to be as quiet as they are efficient.

A comfort system that pairs an electric heat pump with a gas furnace and alternates between the two fuel sources to maximize comfort and efficiency.

Ductwork refers to the system of ducts (metal or synthetic tubes) used to transport air from heating, ventilation and air-conditioning (HVAC) equipment throughout your home. Properly installed and well-maintained air ducts are a key component of indoor air quality and home comfort.

An HVAC system that doesn’t require ductwork, consisting of an outdoor compressor unit and one or more indoor air-handling units.

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

ENERGY STAR® is a program of the U.S. Environmental Protection Agency (EPA) designed to help consumers and businesses save money by choosing energy-efficient products and technologies. Heating, ventilation and air-conditioning (HVAC) equipment with the ENERGY STAR® label meets or exceeds federal guidelines for energy-efficient performance.

The part of the air conditioner or heat pump located inside the air handler or attached to the furnace. Its primary function is to absorb the heat from the air in your house.

A fan coil unit (FCU) is a device that uses a coil and a fan to heat or cool a room without connecting to ductwork. Indoor air moves over the coil, which heats or cools the air before pushing it back out into the room. FCUs can be less expensive to install than ducted systems, and are available in ceiling, floor-mounted and freestanding configurations.

A heat exchanger is a heating component that is located in the furnace and transfers heat to the surrounding air, which is then pumped throughout the home.

A heat pump is an HVAC unit that heats and cools by extracting and transferring heat. Unlike traditional air-conditioners, heat pump systems have the ability to both cool a home during the summer months and heat it during winter. They do this through a reversible refrigerant cycle within the system. During cold weather, a heat pump extracts from outdoor air and transfers it inside to warm the home. This same process is reversed during warm weather for cooling, with the heat pump extracting heat from inside and transferring it outside.

A ventilation system that recovers heat from outgoing air and uses it to preheat incoming air, improving energy efficiency.

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

A humidistat (sometimes called a humidistat control) is a device that works with a home’s heating and cooling system to automatically adjust the amount of moisture in the air to maintain a specific humidity level throughout the home.

HVAC stands for Heating, Ventilation, and Air Conditioning and is used to describe a complete home comfort system used to heat and cool a home.

Indoor air quality (IAQ) is an HVAC industry term that refers to the air quality inside a home or building.

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.

Everything you need to know about Mini-Split ductless heat pumps and air conditioners.

A ductless multi-zone system is comprised of two or more indoor units connected to a single outdoor unit. The indoor units can be all the same or a mixture of different styles of indoor unit to fit the application.

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

A chemical that produces a cooling effect while expanding or vaporizing. Most residential air-conditioning units contain the standard R-22 refrigerant, or freon.

A packaged unit that controls both heating and air conditioning. An air handler typically refers to a unit that provides a fan to support either heating or cooling.

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

Split system HVAC, also called a home comfort system or central heating and air-conditioning system, is essential to keeping a home warm in the winter or cool in the summer. It also can provide protection from allergens and pollutants year round. A split system typically has five main parts:


    1. Indoor unit, such as a furnace or an air handler
    2. Outdoor unit, either an air conditioner or a heat pump
    3. Thermostat, programmable or nonprogrammable
    4. Indoor air quality component, like a filtration system or humidity control product
    5. Ductwork that allows conditioned air to move from equipment to rooms inside the home

A thermostat is a device that monitors the indoor temperature and automatically adjusts your heating or cooling system to maintain the desired level.

A ton is a unit of measurement for determining cooling capacity. One ton (or tonne) equals 12,000 Btuh.

A ventilator is a device that replaces the stale, recirculated air inside your home with fresh, filtered outdoor air. An energy recovery ventilator (ERV) is ideal for warm and humid climates, since it cools and dehumidifies the incoming outdoor air. A heat recovery ventilator (HRV) is more appropriate for colder climates, as it transfers heat from recirculated air to the fresh incoming air.

A motor which varies its speed based on demand.

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