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HVAC TERMS GLOSSARY

Need to know what an HVAC (Heating, Ventilation & Air Conditioning) Term means?  Use our handy glossary.

AC (Alternating Current):  Electrical current where the directional flow of the current reverses at regular intervals.

Accumulator: A storage tank that receives liquid refrigerant from the evaporator, preventing refrigerant from flowing back into the compressor.

Acoustical:  Relating to sound, the science of sound, or the sense of hearing.

AFUE: Annual Fuel Utilization Efficiency. AFUE is a rating of how efficient a gas furnace is.  An AFUE of 80 means 80% of the fuel used provides heat to the home.  The remaining 20% of the heat is vented out with the exhaust.

AGA:  American Gas Association, Inc.

AHRI: The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is the trade association representing manufacturers of: HVAC Systems, Refrigeration equipment  and water heating.

Air Cleaner: A device that removes allergens, pollutants and other undesirable particles from air.

Air Conditioner: Central air conditioning is the most common home cooling method in the U.S.  Residential HVAC Systems are a “split” system.  This means:

  • part of the cooling equipment is outside (condenser)
  • part of the cooling equipment is inside the furnace or ductwork (evaporator coil).
  • the (indoor) furnace or air-handler is also part of the HVAC System.

Air Diffuser: Air distribution outlet or grill designed to direct and assist in balancing air flow.

Air Filtration System: A device that removes allergens, pollutants and other undesirable particles.

Air Flow: The amount of air moving through an HVAC System.  Central Air conditioners are move about 400 cubic feet per minute (cfm) for each ton of AC capacity

Air Handler Unit: (AHU):  A Central HVAC System consists of heating and cooling components, plus a fan to move air through the system.  A furnace or air handler moves air through the home’s ductwork.

Allergen: A substance that causes an allergic reaction. Allergens include: dust & dust mites, lint, pollen, pet hair & pet dander, mildew, fungus, tobacco smoke, cooking grease, bacteria and other particulates.

All-In-One System: An HVAC System where all components are located in one cabinet.

Ambient: Normal atmospheric conditions of temperature & pressure.

ASHRAE:  American Society of Heating, Refrigerating and Air-Conditioning Engineers. ASHRE and its members focus on: building, heating & cooling systems, refrigeration equipment, energy efficiency, indoor air quality, and sustainability within the industry.

Boot: The metal transition piece connecting ductwork on one side to a grille or register on the other side.

British Thermal Unit (BTU): A measurement for the quantity of heat required to be raise or lower the temperature of 1lb. of water 1°F.

Burner Orifice:  The opening in the burner where gas or fuel passes through on its way to combustion.

Burner:  The device that facilitates the combustion of air and gas to create heat.

Capacity:  Capacity refers to the maximum ability of an HVAC System to heat or cool a  space. Heating capacity is expressed in BTUs, while cooling capacity is expressed in tons (1 ton = 12,000 BTU’s). The higher the capacity, the more heating or cooling the system can supply.

Carbon Monoxide: An odorless, colorless gas that is harmful and deadly. Carbon Monoxide is a by-product of fuel combustion.

Cascade System: A cooling system with two or more refrigerant circuits working in series to provide the designated level of cooling.  When the primary cooling circuit cannot keep up with cooling demand, an additional circuit(s) turns on until cooling demand drops to the capacity of the primary circuit.

Celsius:  A temperature scale that registers the freezing point of water as 0° and the boiling point as 100° under normal atmospheric pressure.

Certified Matched System:  AHRI puts heating and cooling equipment through  certification testing to ensure matched systems (of indoor and outdoor equipment) are capable of delivering the rated performance.  If the outdoor and indoor components of an HVAC System are not matched, rated performance levels may not be achievable.

CFCs: Chlorofluorocarbons (CFCs) have been widely used as refrigerants. The manufacture and use of such compounds has been phased out because they harm the ozone layer.  CFCs havebeen replaced with non ozone-depleting products, such as Hydrofluorocarbons (HFCs).

CFM:  Cubic Feet per Minute, a standard of airflow measurement. A typical air conditioning system produces around 400 CFM per ton of air conditioning.

Change of state: A change from one phase to another – solid, liquid or gas.

Charging a System:  Adding refrigerant to an HVAC system.

Checking the charge:  Determining how much refrigerant is in the system.

Evaporator Coil:  This coil is located inside or above the furnace, and can’t be seen without removing a panel.  As warm indoor air passes through the evaporator coil, temperature and humidity are reduced.

Compressor The compressor is responsible for moving refrigerant through the system.

Condensate:  As warm air moves across the cool evaporator coil, humidity is removed.  The coil becomes wet with water / condensate which is  drained away from the equipment to outside.

Condenser Coil:  The outdoor coil that removes heat from the refrigerant.

Condenser Fan:  A fan that creates movement of air through the condenser coil.

Condensing Unit: Every split system cooling system contains two parts: the (indoor) cooling coil & furnace / air handler — and the (outdoor) condensing unit. The condensing unit contains the compressor and condenser coil.

Convection: The transfer of heat by changes in air-density. Warm air moves upward and cool air moves downward.  Convection is typically obvious when a bathroom door is opened after a shower. Cool air rushes in at the floor while warm air moves outward at the top of the door opening.

Damper: A movable plate used in ductwork that opens and closes to control airflow.

DC: Direct Current. Electrical current that flows in only one direction.  As compared, alternating current reverses direction on a regular basis.

Degree-Day:  Calculated by subtracting the average outdoor temperature from 65º F. This measurement is used to estimate the amount of heating or cooling a home or building will need.

Dehumidification: Removing moisture from the air.

Dew Point:

  • HVAC: The pressure/temperature where refrigerant gas changes from gas to liquid.
  • Weather: A measurement representing the amount of humidity in outdoor air. The Dew Point is the temperature at which dew will form on outdoor surfaces due to the humidity in the air.

Diffuser: A grille over an air supply duct with vanes that control the direction of air.

DOE: United States Department Of Energy.

Downflow:  An furnace / air handler that discharges air downward.

Drain Pan:  Also called condensate pan. The drain pan collects the condensate and directs it to the drain line.

Drop-in Refrigerant: A refrigerant that can be used in a cooling system designed for another type of refrigerant.  EXAMPLE: R422D can be used as a drop-in refrigerant replacement for R22 / Freon in many applications.

Dry Bulb Temperature:  The measured temperature without consideration for humidity.

Dual-Fuel: An HVAC System that can generate heat with either electricity (using the heat pump) and gas (using the gas furnace).

Ducts:  Deliver and remove air within a room or space.

Ductwork: The air distribution system in your home is known as ductwork.  Inside ductwork, air is delivered to rooms and returned from rooms to the HVAC System.

Ductless Mini-Split: A split-system heat pump that has no ducts. The blower and evaporator coil are in the indoor unit, which is mounted on a wall or ceiling in the room.  These systems provide cooling in zones, allowing the user to cool or heat only the space(s) they are currently using.  To stop cooling / heater, the unit on the wall or ceiling is turned off.

EC Fan: Electroncially Commutated (EC) fans use DC (direct electrical current) motors.  They include electronics to convert the AC electricity current to DC current.

EER: Energy Efficiency Ratio (EER) is a measure of HVAC System efficiency.  It is  calculated by dividing the BTU Cooling Output by the Electricity Requirement in watts. EXAMPLE: 10,000 BTU / 1,000 watts = EER-10.

Electronic Air Cleaner: Electronic air cleaners filter out particles and contaminants in indoor air.

Energy Star®:  A program by the Environmental Protection Agency (EPA) designed to assist consumers in identifying items that are more efficient.  ENERGY STAR® heating equipment is up to 15% more efficient than standard equipment. ENERGY STAR® cooling equipment is up to 7% more efficient.

EPA:  The United States Environmental Protection Agency is an agency of the US federal government that creates & enforces regulations & laws that are created to protect the environment.

Evaporator Coil: Located inside or above the furnace / air handler. This coil is cold when the a/c is running.  Warm air is blown through the coil, and refrigerant inside the coil absorbs the heat.  Cooled air is then distributed to the home.

Expansion Valve:  A valve that meters the levels of refrigerant through a temperature or pressure control.

Fahrenheit:  A temperature scale in which water freezes at 32 degrees and boils at 212 degrees.

Fan:  A device that moves air.  The device consists of a motor and a blower wheel or blade.

Filter:  The air filter prevents airborne contaminants from entering the equipment.

Filter-Drier: A device that removes water from the refrigerant.

Fixed Speed: Fixed-speed fan motors run at one speed.  They are are used in standard-efficiency HVAC equipment. High-Efficiency HVAC Systems typically have variable-speed motors.

Flue:  A vent which removes combustion-byproducts from a fuel burning system (such as a furnace or water heater) and vents them outside.

Fluorocarbon: A refrigerant containing fluorine and carbon. Most of today’s refrigerants are classified as fluorocarbons, that don’t damage the ozone layer.  Older (Freon) refrigerants contained chlorine and carbon.  They damaged the earth’s ozone layer.

Footprint: The amount of space an HVAC System requires.

Furnace: A heating unit that uses either: natural gas, electricity or heating-oil to create heat.

Fuse:  A delicate metal strip connecting two parts of an electrical circuit. This strip works as a circuit protector, and fails (blows) in the event of excess electrical current.

GAMA:  Gas Appliance Manufacturers Association

Geothermal Heat Pump:  A heat pump that moves heat to or from the ground.  Ground source heat pump (GSHP) is a another name.  Most Heat Pumps are Air Source Heat Pumps.  They remove heat from the outside air and transfer it into refrigerant which warms the home’s indoor air.

Grille: A non-operable cover used for return-air.

HCFC: Hydrochlorofluorocarbon (R134): A non ozone depleting & global warming refrigerant.

Heat Exchanger: Transfers heat from one location to another.  In a gas or heating-oil furnace, the heat-exchanger transfers heat from the burners’ fires into the home’s indoor-air.  The purpose of the heat-exchanger is to keep combustion-air separate from indoor air, as combustion-air has carbon monoxide and must be vented outside.

Heat Gain:  The amount of heat added to a designated area.  Heat gain occurs when the outside temperature is higher than the indoor temperature.

Heat Loss:  The amount of heat lost from a designated area.  Heat loss occurs when the outside temperature is lower than the outdoor temperature.

Heat pump: A heat pump absorbs heat from outdoor air and transfers it into the home.  Most Heat Pumps work with air.  Other Heat Pumps use the ground, or water as the heat source.

Heat Transfer:  The flow of heat from one area to another by conduction, convection, and/or radiation.

Heating Coil:  An electric coil that creates heat for an HVAC System.

HEPA Filter: HEPA stands for ‘High-Efficiency Particulate Absorption’.  HEPA filters are high-efficient filters, and can remove even the tiniest particles from the air.

Home Automation: Anything that provides: remote, automatic or voice-control of things around your home.  This may include HVAC, lighting or security system.

Horizontal Flow: A type of furnace (installed on its side) which draws air from one end and discharges air out the other end.

HSPF:  Heating Seasonal Performance Factor. A measure of the energy-efficiency of a Heap Pump (in heating mode) over an entire heating season.  A higher HSPF indicates a more efficient Heat Pump.

Humidification: Adding moisture to air.

Humidifier: A device that adds moisture to air.

Humidistat:  The device that measures humidity, and turns the humidifier on and off as needed.

Humidity: A measure of the amount of moisture in the air.

HVAC:  Stands for: Heating, Ventilation & Air Conditioning.  HVAC systems provide: heating, cooling, air filtration, and humidity removal.

Hybrid HVAC System: An HVAC System with an electric heat pump plus a gas furnace.  This allows the user to choose the less expensive heating option.

IAQ:  Indoor Air Quality.

Ignition:  The process of lighting a fuel.

Indoor Coil:  Provides cooled air.  It is located inside or above the furnace / air handler and can’t be seen without removing a panel.

Indoor/Outdoor System: An HVAC System consisting of components both inside and outside. Also called a Split-System.

Interconnection Agreement: A link between Power Grids that enables one grid to purchase reserve-capacity electricity from another grid.  This occurs when a grid cannot produce enough electricity to meet its current demand.  A power grid must always have matched supply and demand.  Without that balance, the grid will fail and shut down.

If a power grid cannot meet its electricity demand:

  • The power grid purchases (and imports) electricity from another power grid (if available).
  • In times of electricity demand in excess of supply, a temporary fix is to create  “brown-outs” — a reduction in electricity voltage.  This allows the power grid to be able to meet demand when electricity generation is at its maximum.  Brown-outs can damage air conditioning equipment, as it’s forced to run at a lower voltage than it’s designed for.
  • A more extreme measure can be “rolling blackouts”.  In this case, electricity supply is temporarily shut off to certain areas within the power grid.  After a pre-determined time, power is restored to the first area, and shut off to the next area.  This continues until electricity demand drops to the point of matching the grid’s capacity to supply electricity.

Kilowatt (kW):  1,000 watts.

Kilowatt Hour (kWh): 1,000 watts of electricity used for a period of 1 hour.

Line set: The 2 refrigerant lines that connect the condensing (outside) unit to the evaporator (inside) coil. The uninsulated tube is the (liquid) refrigerant line. The insulated tube is the (gas) refrigerant line.

Load Calculation: Determining how much heat a house gains or losses through the building’s “envelope” (all its outside edges).

Manufacturer Approved System:  When replacing both indoor and outdoor HVAC System, the system installed is manufacturer-approved as a matched system.  A matched system: is more energy efficient, operates at its maximum capacity, and will last longer.  A mismatched system has weaker components which do not have the capacity of stronger components.  This limits the entire HVAC system to operate at only the capacity of the weakest component.

  • EXAMPLE: A new a/c unit with more capacity to cool than the furnace accommodates (because the furnace cannot move enough air).  This limits the ability of the a/c to that of the furnace.  This reduces energy-efficiency, limits cooling capacity to that of the furnace’s ability, and can stress the a/c, causing it to wear out faster.

Matched System: An HVAC System where all components are matched in capacity and efficiency. This ensures the HVAC System can perform at its best and highest ability.  For this reason, matched systems are more energy-efficient and last longer.

Media Air Filter: Made of materials (media) that do a better job of catching small particles entering the HVAC System.  Media air filters can have large negative effect on the System’s ability to move air, because the filter media pores (where the air passes through) are smaller.  For this reason, high MERV rated filters must be monitored and changed more frequently.

Media:  The fine material of a filter that traps dirt, dust, mildew and bacteria.

MERV: Minimum Efficiency Reporting Value:  A Filtering Efficiency Rating for filters used in HVAC systems. The higher the MERV Rating, the more airborne particulates it catches.  Also, the higher the rating, the more the filter reduces air flow through the HVAC System.

Micron: Microns are the unit of measure used to define the efficiency of a filter. The smaller the micron size, the more particles the filter  will be able to remove from indoor air.

Mineral oil: A lubricant commonly used with refrigerants, with a low affinity for moisture.

NATE:  North American Technician Excellence is the nation’s largest non-profit certification organization for heating, ventilation, air conditioning and refrigeration technicians. NATE is the only technician certification organization governed, owned, operated, developed and supported by the HVACR industry.

NEC:  National Energy Council or National Electrical Code

NEMA:  National Electrical Manufacturing Association

ODP:  Ozone Depletion Potential, a measure of damage a refrigerant can cause to the ozone layer.

Oil Separator: A device designed to separate oil from refrigerant.

Outdoor Coil:  Located in the outdoor unit, this coil dissipates heat from the refrigerant.

Outdoor Unit: Used interchangeably with Outdoor Coil.

Package System / Unit:  An HVAC System where all components are located in one cabinet located outside.

Particulates:  The fine liquid or solid particles contained in combustion gases. The quantity and size of particulates emitted by cars, power and industrial plants, and other sources are regulated by the U.S. Environmental Protection Agency.

Plenum: The metal ductwork attached to the furnace / air handler.  The main / trunk line is attached to the plenum, and all other ducts are attached to the trunk line.

Programmable Thermostat:  A thermostat that allows the user to program a pre-set schedule of times and temperatures for the HVAC System to run.

PSIPound per Square Inch is a unit of pressure resulting from the force of one pound applied to an area of 1 square inch.

PVC: PolyVinyl Chloride: A type of plastic (usually white).  PVC vent pipes are used with High-Efficiency gas furnaces (90% or higher) for venting of exhaust gases.  This is possible because HI-E furnaces remove most of the heat from the exhaust gases before they are vented outside.

Refrigerant Types:

  • Chlorofluorocarbons (CFCs): Chemical compounds containing chlorine, fluorine and carbon atom.  CFCs are degraded in the stratosphere. This releases chlorine radicals that attack ozone molecules.
  • Hydrofluorocarbons (HFCs): Chemical compounds that contain hydrogen, fluorine, and carbon atom.  HCFC chemicals decompose photochemically before they reach the stratosphere.  This makes HFC’s non-damaging to the ozone layer.
  • R22 / Freon (CFC Refrigerant) R22 is the refrigerant in older HVAC systemsR22 is a ChloroFlouroCarbon (has chlorine) which has ozone depleting potential. The use of R22 for maintenance or repair was banned on January 1, 2015
  • R410A / Puron (HFC Refrigerant): R-410 replaced R22/Freon in HVAC equipment.  R-410 has no ozone depletion potential.
  • R12 / Freon (CFC Refrigerant) R22 is the refrigerant in older automotive air conditioning systemsR12 is a ChloroFlouroCarbon (has chlorine) which has ozone depleting potential. The use of R22 for maintenance or repair was banned on January 1, 2015
  • R134a (HFC Refrigerant): R134a is an automotive a/c refrigerant which replaced R12 (CFC Refrigerant) automotive a/c refrigerant. R134a has no ozone depletion potential.

Radiant floor:  A type of heating system where the building’s floor contains channels or tubes through that hot fluids circulate through.  The heat then radiates upward into the room from the heated floor.

Radiation:  The transfer of heat through matter or space by means of electromagnetic waves.

Reciprocating Compressor:  Also known as a piston compressor.  A type of compressor used in cooling systems to moving refrigerant using a piston.

Reclaim: The re-processing of refrigerant by filtering, drying, distillation and chemical treatment of the recovered refrigerant.  Also known as recycling the refrigerant.

Recovery: Moving used refrigerant out of a cooling system into storage tanks for reclamation. This eliminates refrigerant being released into the environment and is performed by an EPA Certified Technician.

Recycle: To improve the quality of recovered refrigerant before re-use. This is to clean refrigerant by oil separation, distillation and passes through filter-driers to remove moisture, acidity and particulate matter. Also called reclaiming the refrigerant.

Refrigerant:  A substance that produces a cooling effect. It’s used in air conditioning and refrigeration systems.

Refrigerant Charge:  The total amount of refrigerant in a system.

Refrigerant: The fluid used for the heat-transfer in a cooling system. The refrigerant absorbs heat (at low temperature & pressure) and releases heat (at high temperature & pressure).

Refrigeration cycle: The cycle that allows a cooling system to remove heat at one location and release the heat at another location.

Register: A cover for air-supply vents. It has operable louvers that allow control of air flow.

Relative Humidity: The amount of water vapor present in air (expressed as a percentage) in relation to the amount needed for saturation at the same temperature.

Retrofit: To remodel the refrigerant system to improve its performance.  EXAMPLE: A retrofit may include installing new refrigerant gas & adding energy-saving components..

Return Air: The side of the duct system that pulls air from the house into the furnace or air handler. Return-air vents don’t have the ability to control the airflow like supply-air vents do.

Reversing Valve: Allows a heat pump to do both cooling and heating.  This valve allows the heat pump to run in “reverse” to make heat.  It runs normally when cooling.

Scroll Compressor: An a/c compressor with 2 scrolls (one fixed and one oscillating). Scroll compressors have fewer moving parts. The refrigerant is compressed as it passes between the scrolls.

SEER: Seasonal Energy Efficiency Ratio. A rating of the energy efficiency of an air conditioner or heat pump (in cooling mode).  The higher the SEER, the more energy-efficient the unit is.

Sensible Cooling: The amount of heat removed from the air without changing the humidity.

Sensible Heat:  Heat added or removed, causing a temperature change.

Sensor: A device that reacts to a change in conditions.

Single-Speed: A single-speed motor runs at one speed all the time.

Single-Stage: Most VAC Systems are single-stage. High-Efficiency systems often run at two stages (low & high), OR in a variable-stage format (they speed up & slow down).

Smart Home: A smart home System offers remote or automatic control of the systems within the home. This can include: HVAC system, lighting, security system or more.

Split System: A Split HVAC System consists of two parts: Outdoor Condenser unit, and Indoor unit – the furnace or air-handler.  Additionally, the evaporator coil (cools the air) is located inside the furnace and can’t be seen without removing a panel.

Thermostat:  A temperature-control device located on a wall inside the home. It consists of a series of sensors and relays that monitor and control the functions of  the HVAC System. Thermostats can be: Manual, Programmable, or Smart.

Thermostatic Expansion Valve:  A device that ensures a consistent evaporator-coil temperature by regulating refrigerant flow. This prevents the evaporator coil from freezing up.

Ton:  Ton is the way to describe the capacity of an air conditioner or heat pump (when it’s in cooling mode).  A ton is equal to 12,000 BTU’s, meaning the a/c can remove 12,000 BTU’s of heat from a home in 1 hour.  1 ton defined: The capacity to melt one ton (2,000 pounds) of ice in 24 hours.  EXAMPLE: A 3 ton A/C is 36,000 BTU’s.

Trunk: The main & largest duct that is attached directly to the furnace plenum (the large metal box attached to where the furnace blows out heat – typically the top).  The main trunk line is where the smaller branch ducts go out to the vents in individual rooms.

Turning Vane: Turning vanes are devices inside ductwork used to smoothly direct air  where there is a change in direction.  Turning vanes reduce air resistance and turbulence.

Two-Speed:  Two-speed fan motors in the outside box and inside the furnace / air handler. The fan motors operate in low speed if that will satisfy the cooling need at the time. When necessary, the fans shift to high speed to meet a larger cooling need / demand.

Two-stage Cooling / Heating:  Two-stage cooling and heating operates at the low (energy-saving) speed most of the time. On days when cooling / heating demand is greater, the System switches to the high stage for maximum cooling / heating ability.

Upflow Furnace:  A furnace that intakes air in at the bottom and discharges air at the top.

UPS:  Uninterruptible Power Supply is an electrical device that provides emergency power when the primary power source fails.

UV Light Systems: UV light kills airborne bacteria, viruses and mold spores to help ensure high Indoor Air Quality.  UV Light also reduces odors and Volatile Organic Compounds (VOC’s) + the odors associated with them.  New paint smell is an example of a VOC.

Vacuum Test:  A test to check that refrigerant lines are sealed before charging with refrigerant.  This is done by creating a vacuum on the lines.

Vapor-Barrier / seal: A vapor-barrier prevents moisture infiltration into a space. A vapor barrier (typically a plastic sheet) prevents: air & moisture leaking into a space through very small openings in the: walls, floor, and ceiling.  It also covers (or is sealed to) items penetrating walls, such as plumbing.

Variable Speed: Variable-speed blower motors adjust blower speed to match current heating and cooling demands.

Ventilation:  The process of moving air in or out of an interior space.

  • Mechanical Ventilation System does this with blowers.  These are installed in newer homes because they are built so air-tight they do not allow leak enough outside air to ensure good Indoor Air Quality.

Volt:  A unit used to measure the force of an electric current.

Watt: A method of measuring the rate of energy transfer.

Wet Bulb Temperature:  The lowest temperature that can be reached by evaporating water into the air.  The wet bulb temperature will always be less than or equal to the air temperature.

Wet Bulb Thermometer:  Measures the relative humidity in the air.

Zoning: Allows control of the heating and cooling delivered to specific area in a house.  In large homes, each zone will have a separate HVAC System.  In small to mid size homes, zoning may be possible with 1 HVAC System plus electrically-operated dampers that are controlled by separate thermostats.

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