HVAC Glossary
129 heating, cooling, and indoor-air terms defined in plain English. Bookmark this page or jump to a letter below.
A
- A-Coil
- An evaporator coil shaped like the letter 'A', commonly installed in upflow and downflow furnace or air-handler systems to absorb heat from indoor air.AC
- AC (Air Conditioning)
- Equipment that removes heat and humidity from indoor air to cool a space, transferring that heat outdoors via the refrigeration cycle.AC
- AC or DC
- Alternating Current (AC) periodically reverses direction and powers most home HVAC equipment; Direct Current (DC) flows one way and is used by many modern variable-speed motors.
- ACCA
- Air Conditioning Contractors of America — the trade association that publishes the Manual J, S, and D standards used to correctly size and design HVAC systems.ACHeating
- Acoustical
- Relating to sound; in HVAC it describes materials or design choices that reduce equipment and airflow noise.
- AFUE (Annual Fuel Utilization Efficiency)
- A percentage rating of how efficiently a furnace converts fuel to heat over a year; a 95% AFUE furnace turns 95% of its fuel into usable heat.Heating
- AGA
- American Gas Association — an organization that tests and certifies the safety and performance of gas-fired appliances such as furnaces.Heating
- AHRI
- Air-Conditioning, Heating, and Refrigeration Institute — certifies the rated performance (SEER2, AFUE, HSPF2) of matched HVAC systems.ACHeating
- Air Cleaner (Air Filtration System)
- A device that removes particles, allergens, or contaminants from circulating air to improve indoor air quality.Indoor Air QualityAir Purification
- Air Conditioner
- A system that cools and dehumidifies indoor air by moving heat outdoors through a refrigerant cycle.AC
- Air Filtration System (Air Cleaner)
- Whole-home or portable equipment that captures airborne particles to improve air quality and protect HVAC components.Indoor Air QualityAir Purification
- Air Flow Volume
- The quantity of air a system moves, typically measured in cubic feet per minute (CFM); proper airflow is essential to comfort and efficiency.Ventilation System
- Air Handler
- The indoor component that circulates conditioned air through ductwork; it houses the blower and, in cooling, the evaporator coil.ACHeat Pump
- ASHRAE
- American Society of Heating, Refrigerating and Air-Conditioning Engineers — sets widely used standards for ventilation, efficiency, and indoor air quality.
B
- BTU (British Thermal Unit)
- The amount of heat required to raise one pound of water by one degree Fahrenheit; HVAC capacity is rated in BTUs.
- BTUH (BTU per Hour)
- A measure of heating or cooling capacity expressed as BTUs delivered or removed per hour.
- Burner
- The furnace component where fuel mixes with air and ignites to produce heat.Heating
- Burner Orifice
- A precisely sized opening that meters the flow of gas into a furnace burner for clean, safe combustion.Heating
- Burner, Sealed Combustion
- A high-efficiency burner that draws combustion air directly from outdoors and vents exhaust outside, improving safety and efficiency.Heating
C
- CADR (Clean Air Delivery Rate)
- A rating of how much filtered, clean air an air purifier delivers; higher CADR means faster removal of airborne contaminants.Air PurificationIndoor Air Quality
- Capacity
- The heating or cooling output of HVAC equipment, usually expressed in BTUs per hour or tons.
- Celsius
- A temperature scale where water freezes at 0° and boils at 100° at sea level.
- Certified Matched System
- An indoor and outdoor unit combination tested together and AHRI-certified to deliver its rated efficiency and capacity.ACHeat Pump
- CFM (Cubic Feet per Minute)
- The volume of air a blower or fan moves per minute — a key measure of system airflow.Ventilation System
- Charge
- The amount of refrigerant in a system; correct charge is critical to efficiency and equipment longevity.AC
- Comfort Coil (Evaporator / Indoor Coil)
- The indoor coil that absorbs heat from household air during cooling.AC
- Compressor
- The heart of an AC or heat pump: it pressurizes and circulates refrigerant through the system.ACHeat Pump
- Condensate
- The water that forms as warm, humid air passes over a cold evaporator coil; it drains away through the condensate line.ACHumidity Control
- Condenser Coil (Outdoor Coil)
- The outdoor coil that releases heat collected from indoors to the outside air.AC
- Condenser Fan
- The fan in the outdoor unit that pulls air across the condenser coil to expel heat.AC
- Contactor
- An electrically controlled switch that starts and stops the compressor and condenser fan.AC
- Crankcase Heater
- A heater that keeps compressor oil warm in cold weather, preventing refrigerant from migrating into the oil and damaging the compressor.Heat Pump
- CSA
- CSA Group — a testing and certification body that verifies the safety of gas and electrical HVAC appliances.
D
- Damper
- A movable plate inside ductwork that regulates or redirects airflow, often used in zoning systems.Ventilation System
- dB (Decibels)
- A unit of sound level; quieter HVAC equipment carries a lower decibel rating.
- DC (Direct Current)
- Electrical current that flows in one direction, used by variable-speed ECM motors for efficiency and precise control.
- Defrost
- A heat pump cycle that briefly reverses operation to melt frost from the outdoor coil in cold weather.Heat Pump
- Degree Day
- A measure of heating or cooling demand based on how far the average daily temperature deviates from 65°F.
- Dehumidifier
- Equipment that removes excess moisture from indoor air to improve comfort and prevent mold — especially valuable in humid climates.Humidity ControlIndoor Air Quality
- Diffuser
- A supply outlet that distributes conditioned air into a room in a controlled pattern.Ventilation System
- DOE
- U.S. Department of Energy — sets minimum efficiency standards for HVAC equipment.
- Downflow Furnace
- A furnace that pulls air in at the top and discharges heated air downward, common in homes with ductwork below the living space.Heating
- Drain Pan
- A tray beneath the evaporator coil that collects condensate and channels it to the drain line.AC
- Dry-Bulb Temperature
- Air temperature measured by a standard thermometer, independent of humidity.
- Dry-Bulb Thermometer
- A standard thermometer that measures dry-bulb (ambient) air temperature.
- Dual Compressors
- Two compressors in one unit that allow staged operation for better efficiency and humidity control.AC
- Ductwork
- The network of channels that distributes conditioned air throughout a building; leaks or poor design waste energy.Ventilation SystemDuct Sealing
- DX (Direct Expansion)
- A system in which refrigerant expands directly in the indoor coil to cool the air, as opposed to a chilled-water system.AC
E
- EcoLogic
- A general term for energy- and environment-conscious HVAC design and equipment choices.
- EER (Energy Efficiency Ratio)
- Cooling output (BTU/h) divided by power input (watts) at a fixed condition; higher EER means more efficient cooling.AC
- Electronic Air Cleaner (EAC)
- An air cleaner that electrically charges particles so they can be captured on collector plates, improving indoor air quality.Air PurificationIndoor Air Quality
- EPA
- U.S. Environmental Protection Agency — regulates refrigerants and certifies technicians who handle them.
- Evaporator Coil (Comfort / Indoor Coil)
- The indoor coil where refrigerant absorbs heat and moisture from household air during cooling.AC
- Expansion Valve
- A metering device that controls refrigerant flow into the evaporator coil to optimize cooling.AC
F
- Fahrenheit
- A temperature scale where water freezes at 32° and boils at 212° at sea level; standard for U.S. thermostats.
- Fan
- A device that moves air across coils or through ducts to distribute conditioned air.Ventilation System
- Filter
- A replaceable media that traps dust and particles to protect equipment and improve air quality; should be changed regularly.Indoor Air Quality
- Flue
- The vent that safely carries combustion gases from a furnace to the outdoors.Heating
- Furnace
- A heating appliance that warms air by burning fuel or using electric elements, then distributes it through ductwork.Heating
- Fuse
- A safety device that breaks an electrical circuit if current exceeds a safe level, protecting HVAC components.
G
H
- Heat Exchanger
- A component that transfers heat between two fluids or between combustion gases and air without mixing them.Heating
- Heat Gain
- The amount of heat a space absorbs from sources like sun, occupants, and equipment, which the AC must remove.AC
- Heat Loss
- The amount of heat a space loses to its surroundings, which the heating system must replace.Heating
- Heat Pump
- A system that moves heat rather than generating it, providing both heating and cooling by reversing its refrigerant flow.Heat Pump
- Heat Transfer
- The movement of thermal energy from a warmer to a cooler area — the principle behind all HVAC operation.
- Heating Coil
- A coil that adds heat to passing air, used in electric furnaces, heat pumps, and supplemental heating.HeatingHeat Pump
- HEPA Filter
- A High-Efficiency Particulate Air filter that captures at least 99.97% of particles 0.3 microns in size, used for superior air cleaning.Air PurificationIndoor Air Quality
- Hertz (Hz)
- A unit of electrical frequency; North American power runs at 60 Hz.
- Horizontal Flow
- A furnace or air handler installed on its side so air moves horizontally, common in attics and crawlspaces.Heating
- HSPF (Heating Seasonal Performance Factor)
- A rating of heat pump heating efficiency over a season; higher HSPF means more heat per unit of electricity.Heat Pump
- Humidifier
- Equipment that adds moisture to dry indoor air for comfort and health.Humidity ControlIndoor Air Quality
- Humidistat
- A control that switches humidification or dehumidification equipment on and off to maintain a target humidity level.Humidity Control
- Humidity
- The amount of water vapor in the air; high humidity makes spaces feel warmer and can foster mold.Humidity Control
- Humidity, Absolute
- The actual mass of water vapor present in a given volume of air, regardless of temperature.Humidity Control
- Humidity, Relative
- The ratio of current water vapor to the maximum the air can hold at that temperature, expressed as a percentage.Humidity Control
- HVAC
- Heating, Ventilation, and Air Conditioning — the systems that control indoor temperature, airflow, and air quality.ACHeatingVentilation System
I
K
- Kilowatt (kW)
- A unit of power equal to 1,000 watts, used to express electrical demand of HVAC equipment.
L
- Latent Heat
- Heat absorbed or released when moisture changes state (e.g., condensing) without a change in temperature — central to dehumidification.Humidity Control
M
- Media
- The filtering material inside an air filter or cleaner that captures particles.Indoor Air Quality
N
- NATE
- North American Technician Excellence — a leading certification program for HVAC technicians.
- NEC
- National Electrical Code — the standard governing safe electrical installation, including HVAC wiring.
- NEMA
- National Electrical Manufacturers Association — sets standards for electrical motors and enclosures used in HVAC.
O
- OEM
- Original Equipment Manufacturer — the company that built a piece of equipment; OEM parts are made to original specifications.
- Operating Cost
- The ongoing cost to run HVAC equipment, driven largely by its efficiency rating and local energy prices.
- Orifice
- A precisely sized opening that meters the flow of gas or refrigerant within HVAC equipment.
- Outdoor Coil (Condenser Coil)
- The outdoor coil that releases heat to the outside air during cooling.AC
P
- Packaged System (Packaged Unit)
- An all-in-one HVAC unit housing both heating and cooling components in a single outdoor cabinet, common where indoor space is limited.
- Packaged Unit (Packaged System)
- A single-cabinet system containing all heating and cooling components, often rooftop- or pad-mounted.
- Payback Analysis
- A comparison of a higher-efficiency system's added cost against the energy savings it delivers over time.
- PSI
- Pounds per Square Inch — a unit of pressure used when measuring refrigerant and gas.
- PSIA
- Pounds per Square Inch Absolute — pressure measured relative to a perfect vacuum.
- PSIG
- Pounds per Square Inch Gauge — pressure measured relative to local atmospheric pressure.
- PVC
- Polyvinyl chloride — plastic piping used for condensate drains and high-efficiency furnace venting.
R
- Reciprocating Compressor
- A compressor that uses pistons to pressurize refrigerant; durable but less efficient than scroll or inverter types.AC
- Reclaiming
- Recovering used refrigerant and processing it to original purity standards for reuse.
- Recycling
- Cleaning recovered refrigerant for reuse in the same or a similar system.
- Refrigerant Charge
- The precise amount of refrigerant a system requires; over- or under-charging reduces efficiency and damages components.AC
S
- SEER / SEER2
- Seasonal Energy Efficiency Ratio — cooling output over a season divided by energy used; higher SEER2 means lower cooling bills.AC
- Self-Contained System
- A packaged unit with all heating and cooling components in one cabinet.
- Sensible Heat
- Heat that changes air temperature (and that you can feel), as opposed to latent heat tied to moisture.
- Sensor
- A device that detects temperature, humidity, or other conditions and reports them to HVAC controls.
- Setpoint
- The temperature or humidity level you set on a thermostat or humidistat for the system to maintain.
- Spine Fin Coil
- An all-aluminum outdoor coil design with spiny fins that resists corrosion and improves heat transfer.
T
- Thermostat
- The control that senses temperature and signals the HVAC system to heat or cool; smart models add scheduling and remote control.
- Thermostatic Expansion Valve (TXV)
- A precision metering device that regulates refrigerant flow into the evaporator based on load, improving efficiency.AC
- Ton
- A unit of cooling capacity equal to 12,000 BTU/h — roughly the cooling once produced by melting a ton of ice in 24 hours.AC
- Two-Stage Compressor
- A compressor that runs at low capacity for most conditions and high capacity on demand, improving efficiency and humidity control.AC
U
- U-Factor
- A measure of how well a building element (like a window) conducts heat; lower U-factor means better insulation.Insulation
- UL
- Underwriters Laboratories — an independent body that tests and certifies the electrical safety of equipment.
- Upflow Furnace
- A furnace that draws air in at the bottom and discharges heated air upward, common in basement installations.Heating
V
- Vacuum
- Evacuating air and moisture from refrigerant lines before charging, ensuring clean, efficient operation.
- Volt
- The unit of electrical potential; residential HVAC typically uses 120V or 240V circuits.
- Voltage
- The electrical pressure that drives current through HVAC components.
W
- Watt
- The basic unit of electrical power; 1,000 watts equals one kilowatt.
- Wet-Bulb Thermometer
- A thermometer with a moistened tip used to measure humidity through evaporative cooling.Humidity Control
Z
- Zoning
- Dividing a building into independently controlled comfort areas using dampers and multiple thermostats.Ventilation System
- Zoning System
- Equipment — dampers, controls, and thermostats — that delivers different temperatures to different areas of a building.Ventilation System
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