HVAC Terms: Get Familiar With These Essential Heating and Cooling Concepts

Do you know your capacitors from your compressors? What about your air handlers from your heat exchangers? These are just a few common HVAC terms you might encounter when shopping for a new heating and cooling system. But if you aren’t familiar with them just yet, don’t worry. We’ll explain these concepts (and others) below.

If you read through the terms and are still scratching your head trying to understand them while shopping for a new system or troubleshooting an old one, call a local pro who can advise you on all things HVAC. 

Air Change Rate

An air change rate or air exchange rate measures the speed at which outdoor air replaces indoor air, expressed as air changes per hour (ACH). Air conditioners are often advertised with ACH ratings.

Air Conditioner/Air Conditioning

Representing the “AC” in “HVAC,” an air conditioner can change the temperature, humidity, and quality of your indoor air to cool down your home. There are multiple types of air conditioners on the market, including central AC, window units, and portable air conditioners.

Air Conditioner Capacitor

An air conditioner (AC) capacitor is the part of an air conditioner that provides electricity to begin a cooling cycle. AC capacitors are cylindrical-shaped devices consisting of conductive plates and insulating material.

Air Handler

As part of a central AC unit, an air handler pushes cool air through your home’s ductwork.

Annual Fuel Utilization Efficiency (AFUE)

An AFUE rating measures a furnace’s efficiency (or the amount of fuel it converts into heat versus how much it wastes). Ratings range from 56% to 100%. The higher the percentage, the more efficient the furnace.

Backup Furnace

Some homes have backup furnaces, which can provide heat in case the primary furnace breaks down.

Blower

A blower motor pushes chilled or warmed air from an HVAC system’s cooling or heating device (such as an air conditioner or furnace) into the ductwork and out through vents.

Boiler

A boiler produces hot water or steam and sends it to a radiator or radiant heating system to heat your home.

British Thermal Unit (BTU)

BTUs are units of measurement that explain how much energy your furnace or AC uses per hour. In technical terms, one BTU indicates the amount of energy needed to lower or raise the temperature of water by one degree Fahrenheit.

Chlorofluorocarbons (CFCs)

CFCs were used as refrigerants in HVAC systems until experts discovered that they were harmful to the ozone layer and the overall environment.

Cubic Feet Per Minute (CFM)

CFM in HVAC measures how much air moves through a particular space in one minute.

Combustion Chamber

In a furnace, the combustion chamber (or burner) is an enclosed space where fuel is ignited and converted into heat.

Compressor

Located in the outdoor part of HVAC systems, compressors increase the pressure of refrigerant to cool air and pump it through an HVAC system. 

Condensate

Condensate is a liquid that forms on an air conditioner’s evaporator coil when the AC runs. It collects inside a pan before exiting through the condensate drain line.

Condenser Coil

In outdoor AC units, condenser coils absorb and release heat outside as part of the cooling process.

Controller 

A controller is a device that sends signals to an HVAC system and instructs it on how to operate. It can be automatic or controlled by a user. For example, a controller might tell the system to turn on or off when the home reaches a specific temperature.

Cooling Capacity 

An AC unit’s cooling capacity measures how much heat it removes from a room in one hour.

Cycle

A heating or cooling cycle is the period when an HVAC system is actively running. When the indoor air reaches the desired temperature, the system shuts off, and the cycle ends.

Damper

A damper is a metal plate or valve that sits inside ductwork. Like a dam, it opens and closes to manage airflow within an HVAC system. Dampers can direct air into individual zones, which gives you more control over heating and cooling within specific parts of your home.

Dehumidifier

Dehumidifiers remove or reduce moisture from the air. There are several types available, including single-room, whole-home, and basement dehumidifiers.

Diffuser

A diffuser goes over an air supply duct and directs airflow in a specific direction. It provides more even and effective distribution of heated or cooled air.

Draft 

Draft is the force that draws smoke up your chimney flue and out of your home. A healthy draft keeps the fire burning and prevents smoke from lingering inside your house.

Drain Pan

Also known as a condensate pan, a drain pan is where condensate collects inside an AC unit.

Ducts and Ductwork

Air ducts (or ductwork) are the tubes that distribute warm or cool air throughout your home. 

Ductless System

Ductless systems don’t use ductwork to warm or cool your home. Instead, they use individual HVAC units to cool smaller spaces.

Energy Efficiency Ratio (EER)

EER is a ratio that compares an air conditioner’s cooling capacity (measured in BTUs) to its energy input (in watts). The higher the EER, the more efficient the AC unit.

Energy Star

The Environmental Protection Agency’s Energy Star program highlights appliances and products that meet certain energy efficiency standards.

Evaporator Coil

An evaporator coil uses refrigerant to absorb heat from the warm indoor air, which is a pivotal step in the process of cooling your home. This component is located within an AC’s indoor unit.

Filter

HVAC filters catch dirt, debris, pet hair, and other particles so that they don’t enter (and potentially damage) your HVAC system. They’re found in several parts of an HVAC system, including furnaces, air conditioners, and heat pumps.

Fins 

Fins are small pieces of metal in an AC’s evaporator and condenser coils. Evaporator fins help the evaporator coil absorb heat, while condenser fins help move heat out of your AC unit (and your home).

Flue

A flue is a vent that carries combustion byproducts (including smoke and gases) from your heating appliance to the outside of your home.

Forced-Air System

Forced-air heating systems use a furnace or heat pump to create warm air and circulate it through your home via ductwork.

Freon

Freon is a coolant used in air conditioners. However, due to its environmental risks, the Environmental Protection Agency (EPA) is phasing it out. Since 2010, new AC units have not been allowed to use Freon.

Fresh Air Intake

A fresh air intake is a duct that pulls outdoor air into your home. Its primary jobs are to boost your indoor air quality and help your furnace run more efficiently.

Fuel Efficiency

This term describes how much effort is required to convert fuel (like natural gas or coal) into heat.

Furnace

Furnaces are the heating appliance in many HVAC systems. To warm your home, a furnace heats up air and then sends it throughout your home via ductwork. There are a few different types of furnace fuel sources, including natural gas, oil, propane, and electricity.

Grille

Grilles are vent covers that protect your HVAC system by preventing debris from getting into your ducts. They’re often rectangular with narrow openings running across them.

Heat Exchanger

A heat exchanger is the piece of a furnace that transfers heat into the air.

Heat Loss

Heat loss describes how much heat escapes from the inside to the outside of your home. Calculating your heat loss rate is helpful when figuring out how much energy you’ll use to keep your house warm.

Heat Pump

Heat pumps are outdoor HVAC units that provide both heating and cooling. They can be part of a ducted or ductless system.

Heat Transfer

Heat transfer is a process in which heat flows from a warmer space, surface, or material to a cooler one. 

Heating Load

Also called a thermal load, this is a measurement of how much heat is required to maintain a consistent indoor temperature. Your heating load plays an important role in determining the right size HVAC system for your home.

High-Efficiency Particulate Absorbing (HEPA) Filter

HEPA filters are highly effective air filters found in HVAC systems. They block out at least 99.97% of airborne particles, including pollen and dust.

Home Energy Rating System (HERS)

The HERS Index is a standard used to calculate a home’s energy efficiency, with lower scores correlating to more efficient homes. You’ll come across HERS Index scores while browsing real estate listings since sellers often share their home’s score as a way to entice buyers.

Heating Seasonal Performance Factor (HSPF)

This factor measures a heat pump’s efficiency.

Humidifier

A humidifier adds moisture to your home's air. This is particularly important in winter when forced-air heating systems can reduce the amount of moisture in your air. There are multiple types of humidifiers, including portable, wall-mounted, and whole-house humidifiers.

HVAC 

HVAC is an acronym for heating, ventilation, and air conditioning. It refers to all of the appliances and systems that work together to heat or cool buildings.

HVAC Zoning System

HVAC zoning divides your home into different temperature zones. You can control each zone independently, which helps lower energy usage and avoid unnecessary heating or cooling in unused areas.

Indoor Air Quality (IAQ)

This rating describes the quality of your home’s air. The rating is described in terms like “excellent” or “poor” rather than with numbers or percentages. 

Minimum Efficiency Reporting Value (MERV) Rating 

These ratings indicate the size of the holes within air filters. Higher ratings equate to smaller holes (and better filtration).

Packaged Heating and Air System

This type of HVAC system includes heating and cooling all in one unit, which sits outside of your home. When installed on a roof, they’re called rooftop AC units.

Pilot Light 

In a gas furnace, a pilot light is a small flame that burns continuously. When your furnace turns on, the pilot light ignites the gas to create heat for your home.

Plenum

A plenum or plenum box connects your HVAC system’s ductwork to the air handler. It serves a few purposes, including moving air through your ducts and into your living space and providing a pathway for used air to leave your home.

Radiant Barrier

A radiant barrier is a thin foil sheet that helps keep heat out of your house. It goes underneath a home’s roof in the attic.

Refrigerant 

HVAC systems use refrigerant—which can be either liquid or gas—to remove heat from air as part of the cooling process. Refrigerants can be hazardous, so if you think you might have a leak or another refrigerant issue, contact a local HVAC company.

Register

A register is a type of vent that’s similar to a grille, but it also includes a damper that you can open and close to control airflow. 

Resistance Value/R-Value

R-value measures how well insulation keeps hot air in or out of your home. Insulation with a higher R-value is more effective at preventing energy loss and keeping your home’s temperature more comfortable.

Return Vent 

A return air vent sends warmed or cooled air from inside your home back into the HVAC system to begin the heating or cooling process again.

Seasonal Energy Efficiency Ratio (SEER) 

SEER ratings describe an HVAC system’s efficiency. However, SEER2 replaced SEER in 2023. The new rating system takes into account the real-life conditions that can affect an HVAC system’s efficiency.

Split System

These HVAC systems include an indoor unit and an outdoor unit and are available in ducted and ductless configurations. Mini-split systems are an example of a ductless split system.

Supply Vent

A supply vent blows heated or chilled air into a room in your home.

Therm 

A therm is a unit of heat energy that’s equivalent to 100,000 BTUs. You may notice gas companies using this term to describe how much natural gas is required to heat a space.

Thermostat 

A thermostat is a device that controls the operation of an HVAC system. There are a few types of thermostats, including manual, digital, and smart thermostats. Functionality varies by type, but newer models often have smartphone apps and features to run heating and cooling on programmed or automatic schedules.

Ton

In HVAC, a ton isn’t related to weight. Instead, it refers to the cooling capacity of an air conditioning unit. A one-ton AC can remove 12,000 BTUs of heat per hour. You’ll find your AC’s tonnage on the label of your outdoor unit.

Two-Stage Furnace 

A two-stage furnace has two heat settings: low and high. It runs on low when the weather is milder, but when it gets extremely cold outside, it will operate on the high setting. Compared to single-stage furnaces, two-stage models offer more consistent heating and energy efficiency.

Vents

Vents are openings that allow heated or cooled air to flow out of your HVAC system and into your living space (and vice versa).

Ventilation

A ventilation system brings fresh outdoor air inside your home and expels stale air outside. This helps remove moisture, reduces pollutants, and prevents the air in your house from feeling stagnant.