HVAC Glossary

The Terms, in Plain English

Every trade has its jargon. Here's what the words on your estimate, thermostat, and equipment labels actually mean — so you can make decisions with confidence.

A

A-Coil: A heat exchanger (evaporator coil) consisting of two diagonal coils that are joined together in a manner that looks like the letter “A”.
AC: AC, A.C. or Ac may refer to: Air conditioning, technologies for altering the temperature and humidity of air; Alternating current, a type of electrical current in which the current repeatedly changes direction.
AC or DC: Abbreviation for equipment capable of operating on alternating or direct current.
ACCA: Air-Conditioning Contractors of America.
Acoustical: Of or pertaining to sound.
AFUE: Annual Fuel Utilization Efficiency — a measure of a gas furnace's efficiency in converting fuel to heat. The higher the rating, the more efficient the furnace. Heating services
AGA: Abbreviation for American Gas Association, Inc.
AHRI: Air Conditioning, Heating & Refrigeration Institute
Air cleaner (also Air Filtration System): Removes airborne allergens from your home.
Air Conditioner: Any device that can change the temperature, humidity or general quality of the air. More specifically, an air conditioner makes your home cooler, by drawing heat energy out of the house and transferring that heat to the outdoors
Air filtration system (air cleaner): Equipment that removes dust, pollen, and other particles from the air as it circulates through your HVAC system. Indoor air quality
Air flow volume: The amount of air the system circulates through your home, expressed in cubic feet per minute (cfm). Proper air flow depends on the outdoor unit, the indoor unit, the ductwork and even whether the filters are clean.
Air handler: The indoor unit that moves conditioned air through your home — typically containing the blower, the indoor coil, and the air filter.
ASHRAE: American Society of Heating, Refrigeration and Air Conditioning Engineers- ASHRAE is an international voluntary organization for people involved in heating, ventilation, air conditioning, or refrigeration (HVAC&R). The society promotes the general sciences of HVAC&R. The main Society headquarters is located in Atlanta, Georgia with local chapters located across the United States and throughout the world. ASHRAE publishes a four-volume Handbook that serves as a resource for members, with one of the four volumes updated each year. ASHRAE also publishes a well recognized series of standards and guidelines relating to HVAC Systems. ASHRAE Standards are often referenced in other model building codes and in the US Green Building Council’s Leadership in Energy Design (LEED) green building rating system.

B

BTU: British Thermal Unit — the amount of heat needed to raise one pound of water by 1°F. Heating and cooling capacity is measured in BTUs per hour (BTUh).
BTUh: British thermal units per hour. 12,000 BTUh equals one ton of cooling.
Burner (sealed combustion): A burner that obtains all air for combustion from outside the heated space.
Burner Orifice: The opening through which gas flows to the air/gas mixing chamber of the burner.

C

CADR Clean Air Delivery Rate (CADR): provides the number of cubic feet of clean air that a unit delivers each minute. Efficiency of the system times the air flow of the system.
Capacity: The output a heating or cooling system can deliver, usually stated in BTUs per hour (or tons for cooling). Proper sizing matches capacity to your home's load.
Celsius: The metric temperature scale in which water freezes at zero degrees and boils at 100 degrees, designated by the symbol “C”. To convert to Fahrenheit, multiply a Celsius temperature by 9, divide by 5 and add 32 (25 x 9 equals 225, divided by 5 equals 45, plus 32 equals 77 degrees Fahrenheit).
Certified Matched System: The Air Conditioning, Heating and Refrigeration Institute (AHRI) puts heating and cooling equipment through rigorous certification processes to ensure systems deliver the promised performance at certain test conditions.
CFM: Cubic Feet per Minute — the measure of airflow volume a system moves through the ductwork.
Charge: To add refrigerant to a system. This is refrigerant contained in a sealed system or in the sensing bulb to a thermostatic expansion valve.
Comfort™ coil (also evaporator coil or indoor coil): Part of an indoor air unit, it functions with the air conditioner or heat pump outdoors to cool and condition indoor air that flows over it by drawing out heat and moisture.
Compressor: The pump at the heart of an air conditioner or heat pump. It pressurizes refrigerant and keeps it moving between the indoor and outdoor coils.
Condensate: The water that forms on the indoor coil as air is cooled and dehumidified. It drains away through a condensate line — a common clog point worth keeping clear. Maintenance
Condenser coil (outdoor coil): The coil in the outdoor unit that releases the heat collected from your home. Dirty condenser coils are a leading cause of lost cooling capacity.
Condenser fan: The fan that circulates air over the air-cooled condenser.
Contactor: An electrical switch in the outdoor unit that energizes the compressor and fan when the thermostat calls for cooling or heating.
Crankcase heater: This is the electric resistance heater installed on compressor crankcases to boil off liquid refrigerant that may have combined with compressor oil. Many newer cooling systems do not require crankcase heaters, however heat pumps do require crankcase heaters.
CSA: Canadian Standards Association.

D

Damper: A movable plate inside ductwork that adjusts or blocks airflow — the working part of a zoning system.
DB (Decibels): A unit to measure sound.
DC: Direct current electricity. This type of electricity (as opposed to Alternating Current, or AC) flows in one direction only, without reversing polarity.
Defrost: A heat pump's automatic cycle that melts frost off the outdoor coil in cold weather. Steam rising from the unit during defrost is normal. Heat pumps
Degree-day: A degree-day is a computation that gauges the amount of heating or cooling needed for a building. A degree-day is equal to 65 degrees Fahrenheit minus the mean outdoor temperature.
Dehumidifier: Equipment that removes excess moisture from indoor air, improving comfort and discouraging mold.
Diffuser: A grille over an air supply duct having vanes to distribute the discharging air in a specific pattern or direction.
DOE: Department of Energy
Downflow furnace: A furnace that intakes air at its top and discharges air at its bottom.
Drain pan: This is also referred to as a condensate pan. This is a pan used to catch and collect condensate (in residential systems vapor is liquefied on the indoor coil, collected in the drain pan and removed through a drain line).
Dry bulb temperature: Heat intensity, measured by a dry bulb thermometer.
Dry bulb thermometer: An instrument that measures air temperature independently of humidity.
Dual Compressors: An outdoor unit featuring two compressors, one for everyday cooling and a second larger one for extreme temperature days. Provides superior durability designed for cost-saving efficiency and low sound levels.
Ductless mini-split: A heating and cooling system that serves individual rooms without ductwork — an outdoor unit paired with one or more wall-mounted indoor heads. Ideal for additions, garages, and older homes that never had ducts. Ductless mini-splits
Ductwork: The network of channels that distributes conditioned air through your home. Leaky or undersized ducts waste a large share of the energy a system produces.
DX: Direct expansion; a system in which heat is transferred by the direct expansion of refrigerant.

E

Ecologic: Maytag’s designation of green heating and cooling equipment.
EER: Energy Efficiency Ratio — cooling efficiency measured at one fixed outdoor temperature (95°F), unlike SEER which averages a whole season.
Electronic Air Cleaner (EAC): Electronically filters out magnetized airborne particles from contaminants in indoor air, sending them to a collector plate.
EPA: Environmental Protection Agency
Evaporator coil (indoor coil): The coil inside your home where refrigerant absorbs heat from passing air, cooling it. A frozen evaporator coil usually points to airflow or refrigerant problems. AC repair
Expansion valve: The metering device that regulates how much refrigerant enters the evaporator coil, controlling the cooling process.

F

Fahrenheit: The temperature scale on which water freezes at 32 degrees and boils at 212 degrees, designated by the letter F. To convert Fahrenheit to Celsius, subtract 32 from the Fahrenheit number, multiply by 5 and divide by 9 (77–32 equals 45, times 5 equals 225, divided by 9 equals 25 degrees Celsius).
Fan: Any device that creates air currents.
Filter: The screen that catches dust and debris before air passes through the system. Change standard filters every 1–3 months — the single easiest way to protect efficiency.
Flue: The vent pipe that carries combustion gases from a gas furnace safely out of the home.
Furnace: Equipment that heats air by burning fuel (usually natural gas) or with electric elements, then circulates it through the home's ductwork. Furnace repair
Fuse: A metal strip in an electrical circuit that melts and breaks the circuit when excessive current flows through it. The fuse is designed to break in order to save more expensive electrical components.

G

GAMA: Gas Appliance Manufacturers Association
Gas furnace heat exchanger: Located in the furnace, the heat exchanger transfers heat to the surrounding air, which is then pumped throughout your home.

H

Heat exchanger: The furnace component that transfers heat from combustion to the passing air while keeping exhaust gases separate. A cracked heat exchanger is a safety issue — replace, don't run.
Heat gain: Heat added to the conditioned space by infiltration, solar radiation, occupant respiration and lighting.
Heat gain / heat loss: The heat a home absorbs on a hot day (gain) or loses on a cold one (loss). Load calculations quantify both to size equipment correctly.
Heat loss: The rate of heat transfer from a heated space to the outdoors.
Heat pump: A refrigeration system that can run in reverse — moving heat out of your home to cool it, or into your home to heat it. One system for both seasons, and a strong fit for San Diego's mild climate. Heat pump services
Heat transfer: The movement of heat energy from one point to another. The means for such movement are conduction, convection, and radiation.
Heating coil: Any coil that serves as a heat source.
HEPA filter: High-Efficiency Particulate Air filter — captures at least 99.97% of particles down to 0.3 microns, the standard for serious air cleaning. Indoor air quality
Hertz: In alternating current (AC electricity), the number of cycles per second.
Horizontal flow: Often used in attics/crawl spaces, this furnace takes air from its side, heats it and sends warm air out the other side.
HSPF: Heating Seasonal Performance Factor — the heating-season efficiency rating for heat pumps. Higher HSPF means lower winter operating costs.
Humidifier: A machine that adds water vapor to the air to increase humidity.
Humidistat: A control that measures and manages indoor humidity, the moisture-side counterpart to a thermostat.
Humidity: The presence of water vapor in the air.
Humidity, absolute: Weight of water vapor per cubic foot of dry air, expressed as grains of moisture per cubic foot.
Humidity, relative: The amount of moisture in the air expressed as a percentage of the maximum amount that the air is capable of holding at a specific temperature.
HVAC: Heating, Ventilation, and Air Conditioning — the whole system that keeps indoor air comfortable and healthy year-round.
Hybrid (dual-fuel) system: A heat pump paired with a gas furnace. The system automatically uses whichever heat source is most economical for the conditions.
Hybrid heat (also Hybrid System): Provides energy-efficient comfort for moderate heating conditions. Can use either gas or electricity.

I

Ignition: The mechanism that lights a gas furnace's burners — modern furnaces use electronic ignition instead of a standing pilot light.
Indoor coil (also Comfort™ Coil or evaporator coil): Part of an indoor air unit, it functions with the air conditioner or heat pump outdoors to cool and condition indoor air that flows over it by drawing out heat and moisture.
Indoor/Outdoor system (also split system): The combination of an outdoor unit (air conditioner or heat pump) with an indoor unit (furnace or air handler). Split systems must be matched for optimum efficiency.
Inverter compressor: A variable-speed compressor that ramps output up and down instead of cycling on and off — quieter, more efficient, and better at holding a steady temperature.

K

Kilowatt (kW): 1,000 watts

L

Latent heat: A type of heat, which when added to or taken from a substance, does not change the temperature of the substance. Instead, the heat energy enables the substance to change its state.
Load calculation: The engineering math (Manual J) that determines exactly how much heating and cooling a specific home needs. The right way to size a system — never by square footage alone. System installation

M

Media: The material in a filter that traps and holds the impurities.
MERV: Minimum Efficiency Reporting Value — the 1–16 scale for how effectively a filter captures particles. Higher MERV filters catch more, but must be matched to your system's airflow.

N

NATE: North American Technician Excellence — the leading independent certification for HVAC technicians.
NEC: National Energy Council / National Electric Code
NEMA: National Electrical Manufacturing Association

O

OEM: Original Equipment Manufacturer
Operating cost: Cost of running your home comfort system, based on energy usage
Orifice: An opening or hole; an inlet or outlet
Outdoor coil (also Condenser coil): In a heat pump, it takes in heat to warm your home. In an air conditioner, it dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid to cool your home.

P

Packaged system (or Packaged unit): A single cabinet, typically installed outside, that houses both heating and cooling components.
Packaged unit: A system with all components in a single outdoor cabinet, common on light-commercial rooftops, instead of the split indoor/outdoor arrangement. Commercial HVAC
Payback analysis: Number of years required before your monthly energy savings offset the original HVAC investment.
PSI: Pounds per square inch.
PSIA: per square inch, absolute.
PSIG: Pounds per square inch gauge.
PVC: Polyvinyl chloride; a type of plastic.

R

Reciprocating compressor: A compressor whose piston or pistons move back and forth in the cylinders.
Reclaiming: Returning used refrigerant to the manufacturer for disposal/reuse.
Recycling: Removing, cleaning and reusing refrigerant.
Refrigerant: The working fluid that absorbs heat indoors and releases it outdoors. Systems are sealed — 'low refrigerant' means a leak that needs professional repair, not a top-off.
Refrigerant charge: The precise amount of refrigerant in a system. Both under- and over-charging hurt efficiency and can damage the compressor.
Refrigerant lines: Two copper lines connecting the outdoor condenser to the indoor evaporator coil.

S

SEER: Seasonal Energy Efficiency Ratio — the season-long cooling efficiency of an air conditioner or heat pump. The higher the SEER, the less electricity used for the same comfort. SEER savings calculator
SEER2: The updated SEER standard (2023) measured under more realistic duct conditions. SEER2 numbers run slightly lower than old SEER for the same equipment — compare like with like.
Self-contained system: A refrigerating system that can be moved without disconnecting any refrigerant lines; also known as a package unit.
Sensible / latent heat: Sensible heat changes the temperature you feel; latent heat is bound up in moisture. Cooling systems handle both — that's why dehumidification matters to comfort.
Sensible heat: That heat which, when added to or taken away from a substance, causes a rise or fall in temperature.
Sensor: Any device that reacts to a change in the conditions being measured, permitting the condition to be controlled.
Setpoint: The target temperature you dial into the thermostat — the number the system works to hold.
Spine-Fin™ coil: All-aluminum outdoor coil that features the patented Spine Fin™ design. It provides greater heat-exchanging capabilities (meaning higher efficiencies) and is more resistant to corrosion than a traditional copper/aluminum coil.
Split system: The most common residential arrangement: an outdoor condensing unit paired with an indoor coil and air handler or furnace, connected by refrigerant lines.
Static pressure: The resistance airflow meets as it moves through ductwork and filters. High static pressure strains the blower and starves rooms of air — a key diagnostic measurement.

T

Thermostat: The control that senses room temperature and tells the system when to run. Smart thermostats add scheduling and remote control; dead batteries are a classic 'no-cool' culprit. HVAC Troubleshooter
Thermostatic expansion valve (TXV): A precision expansion valve that continuously adjusts refrigerant flow to match the load, improving efficiency across conditions.
Ton: The unit of cooling capacity: one ton equals 12,000 BTUs per hour. Typical homes need 2–5 tons, determined by a load calculation — not guesswork.
Two-stage (multi-stage) operation: Equipment that can run at a lower output most of the time and step up only when needed — longer, gentler cycles with better humidity control and lower bills.
Two-stage compressor: Compressor composed of a high and low stage to increase efficiency and comfort while reducing operation noise.
Two-stage heating/Two-stage cooling: Two-stage heating and cooling is considered to be more efficient, because it operates at low speed most of the time. However, on days when more heating or air conditioning is required, it switches to the next stage for maximum comfort.

U

U-Factor: The factor representing resistance to heat flow of various building materials.
UL: Underwriters Laboratories
Upflow / downflow / horizontal furnace: Furnace configurations named for the direction air moves through the cabinet — chosen to fit where the unit is installed (closet, attic, garage, or crawl space).
Upflow furnace: A furnace in which air is drawn in through the sides or bottom and discharged out the top.

V

Vacuum: A pressure below atmospheric pressure. A perfect vacuum is 30 inches of mercury (periodic symbol “Hg”)
Volt: The unit of measure used to describe a difference in electrical potential, abbreviated by the symbol “v.”
Voltage: The force that pushes electrical current along wires and cables.

W

Watt: The unit of electrical power equal to the flow of one amp at a potential difference of one volt.
Wet Bulb Thermometer: A thermometer whose bulb is covered with a piece of water-soaked cloth. The lowering of temperature that results from the evaporation of water around the bulb indicates the air’s relative humidity.

Z

Zoning: A method of dividing a home into different comfort zones so each zone can be independently controlled depending on use and need; an air conditioning system capable of maintaining varying conditions for various rooms or zones.
Zoning system: Motorized dampers plus multiple thermostats that let different areas of a home hold different temperatures independently — no more fighting over one setpoint.

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