HVAC Terms You Need to Know

Nick P. Cellucci
Written by Nick P. Cellucci
Updated April 20, 2023
A man adjusting the temperature on a digital thermostat
Photo: Caia Image / Photodisk / Getty Images


  • Your HVAC system features a heating section, a cooling section, and various components that tie everything together. 

  • An HVAC’s heating element derives from a furnace, a forced-air system, a boiler, a heat pump, or a hybrid system. 

  • An HVAC’s cooling element typically comes from an outside air conditioner integrated with an air handler, along with filtration elements. 

  • Your home’s ductwork sends both heated and cooled air throughout the house, also helping with ventilation and filtration.

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They say home is where the heart is, but home is also where the air conditioning is. A house’s HVAC system includes heating elements, cooling elements, related ductwork, and so much more. If you hire a local HVAC contractor and need to talk shop, it helps if you have your components and definitions down. Knowing these common HVAC terms will keep you from being left in the (literal) cold.

Heating HVAC Terms 

Your home’s heating system is more complex than you think. Here are some common HVAC terms to get you started.


The Annualized Fuel Utilization Efficiency (AFUE) measures the amount of natural gas or propane a furnace uses to heat your home, as well as the amount of waste created. For instance, a furnace AFUE rating of 95 converts 95% of the gas that you’re paying for to heat your home. The other 5% exits from the flue (exhaust) pipe. 

Backup Furnace

During the winter months, heating is a literal lifesaver. That’s why many homes include a backup, or secondary, furnace. This additional furnace operates as an emergency solution when your primary heater breaks down. 

British Thermal Unit

You’ll see this written out as BTU. British Thermal Units are a measurement indicating the amount of heat needed to raise the temperature of one pound of water by one degree. Why water? The liquid is equally dense across multiple applications, as BTUs are used in many applications beyond heating and cooling. 

Combustion Chamber

A furnace’s combustion chamber is where all the heating action takes place. These enclosed chambers undergo a multitude of chemical reactions that release heat throughout the rest of the system. You’ll also find these chambers in engines. 


Draft is the major force that pulls the smoke up your chimney, allowing a fire to burn in a fireplace. Your fireplace will struggle to get a fire going if your draft is insufficient. 


The flue is a duct or related structure that holds smoke and waste gases created by your heater or any fuel-burning appliance. These waste gases are then jettisoned outside via the flue. 

Forced-Air System

If your HVAC unit includes ductwork, it uses a forced-air system. These systems push and circulate temperature-controlled air through ducts, thus heating your home. 

Fuel Efficiency

Fuel efficiency describes the effort needed to transform a heating source, such as coal or steam, into actual heat. 

Heat Exchanger

Located inside the furnace, a heat exchanger transfers heat into the air, which pumps throughout your home via ducts or related means. 

Heat Loss

This is a measurement describing how much heat from your home “leaks” to the outside, particularly during the winter months. This figure is normally given in BTUs or kilowatts (KW). Heat loss data is handy when figuring out how much energy you’ll need to keep a room or home heated throughout the day. 

Heat Pump

A heat pump is crucial for many HVAC systems. The heat pump, installed outside, pulls in air from outside, heats it, and moves it throughout your home. This same heat pump is also useful during the summer months, as it pulls in cold air when needed.

Heating Load

The heating load is a measurement of the heat flow needed to keep a stable indoor temperature. Heating loads, otherwise known as thermal loads, are typically measured in BTUs. 


The Heating Seasonal Performance Factor (HSPF) measures the efficiency of heat pumps, defined as a ratio of heat output (BTUs) over electricity used. Look for a measurement of BTU/watt-hr to find HSPF. The U.S. Department of Energy raised the minimum energy efficiency standard for heat pumps in 2015 to 8.2 HSPF.


This term refers to a unit of heat equal to 100,000 BTUs. Gas companies use therms to illustrate the volume of gas used to heat a structure. 

Two-Stage Furnace

A two-stage furnace offers two levels of heat output. On mild winter days, a two-stage furnace runs at a low speed for longer periods, and the opposite is true on harsh winter days. Two-stage furnace designs help improve energy efficiency.

Cooling HVAC Terms

The exterior of a villa with an HVAC system
Photo: onurdongel / iStock / Getty Images

Here are some terms that’ll help you keep your cool if your AC breaks down and you have to chat with your HVAC technician. 

Air Exchange Rate

An air exchange rate is the rate at which air from outdoors replaces pre-existing air. This rate measures air changes per hour, or ACH, defined as the number of times outside air replaces inside air within an hour. You’ll often find air conditioners with advertised ACH ratings. 

Air Handler

The air handler, or air handling unit, is the indoor section of a central AC unit, moving cooled air throughout your home’s ductwork. 

CFM/Cubic Feet Per Minute

A measurement of the volume of air that flows through any given space in one minute.

Condenser Coil

The condenser coil is an essential part of an air conditioner. As the fan blows in air from the outside, refrigerant circulates through the condenser coil to create cool air.

Cooling Capacity

Cooling capacity is a measurement of the amount of heat the AC unit removes from a room in a single hour. 


Freon is an organic compound that is a popular air conditioner coolant. The name is a registered trademark of DuPont but is often used interchangeably with coolants of the same type.


An acronym for high-efficiency particulate arrestance, an extremely popular type of air filter found in air conditioners. 

MERV Rating

Minimum efficiency reporting value ratings describe the size of the holes in air filters. Filters with higher MERV ratings have smaller holes, allowing only the smallest particles to pass through.

Radiant Barrier

A radiant barrier prevents heat from entering a building. Typically located under the roof in the attic, these barriers are typically thin foil sheets that help maintain a cool internal temperature. 


Also known as coolant, refrigerant is a liquid found in HVAC systems that helps remove heat from the air. 


An air conditioner is just one part of your cooling system, as insulation is equally important. Materials with high R-Values offer better insulation against temperature changes, while materials with low R-Values offer poorer insulation. 


In the world of air conditioning, a ton has nothing to do with weight. A ton is the amount of refrigeration needed to melt ice and, thus, cool indoor air. One ton of air equals 12,000 BTUs, so you’ll find air conditioners rated in tons instead of BTUs. In other words, a 4-ton AC equals 48,000 BTUs of cooling power.

Miscellaneous HVAC Terms

It’s always good to learn important HVAC terms and understand how your system works. But ultimately, the best way to determine what kind of system you need and avoid making a big HVAC mistake is to talk to a professional.

There are many HVAC terms that defy simple categorization, as they help with both heating and cooling.


A controller is any device that controls the operation of part or all of an HVAC system. Many are automatic devices with a simple function such as turning a system on or off. Others have user input that allows you to control some component of your HVAC system, like a thermostat, which is a controller that allows you to set your system to a specific temperature.


A dehumidifier is a device that reduces or removes humidity from indoor air by cooling it to the point where any water vapor turns into a liquid. That liquid is then contained for manual removal or drained from the system to the outdoors.

Ducts and Ductwork

Ductwork refers to the system of ducts, metal or synthetic tubes, that run throughout your home. These ducts transport hot air and cold air and help assist with air filtration and overall ventilation. 

Energy Star Rating

Created by the Environmental Protection Agency, the Energy Star Program rates energy-efficient products used in homes, including heating and cooling systems. The Energy Star rating determines how energy efficient a product or appliance is. When shopping for Energy Star products, look for the tell-tale blue or yellow label.

Fresh Air Intake 

Fresh air intake refers to an open duct that draws air from outside into your furnace. When your home’s ventilation system exhausts indoor air to the outdoors, outside air is then drawn in to replace it through this opening.


A grille refers to the facing across an open duct—essentially a vent cover. It often looks like a rectangular panel with parallel slotted openings running across. These slots, or vanes, are often angled in a particular direction to prevent debris from passing through as air comes out of the ducts into ventilated rooms.

HERS/Home Energy Rating System

The HERS index is a nationally recognized energy rating program used to calculate a home’s overall energy efficiency and performance. You’ll run into a HERS index score when buying or selling a home, as a low score allows sellers to raise that asking price. 

HVAC Home Zoning

An HVAC home zoning system creates different temperature-regulated zones throughout the house. This helps reduce your energy usage and eliminate hot and cold spots, as unused rooms are left colder or hotter than frequently used areas. Many homes benefit from HVAC zoning techniques.

IAQ/Indoor Air Quality

As the name suggests, the IAQ describes a given indoor space's overall air quality and pollution level. An IAQ rating is not a number, but typically just “acceptable” or “not acceptable.” Knowing your IAQ allows you to take steps to improve indoor air quality.

Packaged Unit or Rooftop Unit

A packaged unit is an all-in-one heating and cooling system that is installed outside your home. The internal heating and cooling devices are all pre-assembled before installation, and the combined unit takes up much less space than a split system. Packaged units are also sometimes called rooftop units (RTUs) when installed on the roof of a building.


The plenum box connects to your ducts throughout the home, distributing heated or cooled air. Most HVAC systems include two plenum boxes, one for distribution and one for sending used air outside.

Return Duct

A return duct carries air to the furnace or air handler before being temperature-regulated and circulated through the supply vents. Any indoor air reused to heat or cool a home travels through this duct. 


Standing for Seasonal Energy Efficiency Ratio, SEER rates the efficiency of an HVAC system. Illustrated via numeral, this ratio is the primary measurement used to determine an appliance’s Energy Star rating. The higher the SEER number, the more efficient the system. 

Supply Duct

Supply ducts operate with forced air systems. Forced air units push either heated or cooled air through these supply ducts.


A thermostat monitors your home's temperature and controls its heating and cooling systems. You can use it to set your desired indoor temperature—whether that’s closer to an igloo or a sauna is up to you. Some thermostats are programmable, allowing you to schedule different temperatures at different times of day, like turning down the heat during hours you’ll be asleep.

Variable Air Volume

A variable air volume (VAV) system is a type of HVAC system that supplies a stable airflow at one constant temperature. VAV systems adjust the temperature by varying fan speeds and conserve energy with lower speeds when there isn't as much demand for temperature control.

In contrast, constant air volume (CAV) systems use more energy to supply air at a constant rate, adjusting the air temperature according to thermostat settings.

Lawrence Bonk contributed to this piece.

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