These Are the HVAC Terms You Need to Know

Lawrence Bonk
Written by Lawrence Bonk
Updated January 12, 2022
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, oh gosh, so much more. If you hire a local HVAC contractor and need to talk shop, it’ll help if you have your components and definitions down. Here are common HVAC terms so you won’t be left in the (literal) cold.

Heating 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. 


This 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 this 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.


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 Terms

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

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.

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.

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. 


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.

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