HVAC Terminology & Definitions

1st Stage / 2 nd Stage Heat or Cool -  On mid sized or larger equipment, for the reason of efficiency and effectiveness, the systems are usually designed to run as a smaller system to meet lighter cooling or heating requirement.  If the cooling or heating requirement are high, both or all stages will then operate.

Accumulator - A storage canister for excess refrigerant on the suction side of the system used in heat pumps.

Air Handler (AHU) – A section of the A/C system that contains the evaporator and blower motor located inside the space.  The other part of the system is the condenser unit located outside the space.

Amperes (AMPS) – Electron or current flow past a given point in a circuit.

A.S.H.R.A.E. – (American Society of Heating, Refrigeration, Air Conditioning Engineers) Non Profit Organization designed to provide H.V.A.C.R information to the industry.

Auxiliary Drain Pan – This pan is usually mounted under the AHU and is a backup to the main drain pan if it stops draining properly.

Bearing - Allows metal to metal parts to spin or move together for a long time.

Belts – Made of a rubberized material and designed to transfer the rotation of a motor to a fan.

Blower Wheel - Is a type of fan that is used in the Air Handler section of the system.

Brazing - method of joining metal parts using nonferrous filler metals with high melting points such as copper, silver, and aluminum alloys. Brazing differs from soldering by using a higher temperature; and unlike welding, the parts are not melted. Brazing is best for dissimilar or thinner metal parts and for parts difficult to weld or solder.

Breaker - Designed to protect the building wiring from an over-amperage condition and catching fire.

BTU - A unit used to measure quantity of heat, defined as the quantity of energy necessary to raise the temperature of 1 lb. of water 1° Fahrenheit

Burner - The device where gas and air mix and burn. Located next to the heat exchanger, the burner creates the hot air that is blown through the heat exchanger.

Circuit of Refrigeration - The entire circular path the refrigerant makes to produce cold air.  Some systems have more than one circuit.

Compressor (Reciprocating “Recip”, Scroll) - This component compresses the gas refrigerant allowing the refrigerant to change state and provide cooling.  Reciprocating is the most common compressor and gets its name from the back and forth motion the pistons move to perform the compression.

Condensate Drain Line (Drain Line) – Piping that allows the water removed at the evaporator to be remove to a designated drain.

Condensate Leak – Water that is removed from the air stream is intended to be contained in the drain pan and flow to a drain.  Leaks occur if the drain line gets plugged by biological growth or debris or the drain pan develops a hole, usually caused by rust.

Condenser Coil - Constructed of copper tubing with fins to dissipate heat and fans to speed the transfer.  This coil is located outside the building and has the function of removing the heat collected from the occupied space inside to the outdoors through the refrigerant.

Condenser Fan - This fan is designed to pull air through the coil and speed the process or removing heat from the refrigerant.

Condenser Unit (CU) – The outdoor portion of an air or heat pump that either collects or expels heat, depending on the time of year.  The main components consist of the compressor, condenser coil, & condenser fan

Contactor - is an on/off for high voltage components controlled by low voltage controls such as thermostats & relays.

Control Board - This is a microprocessor controller that controls various processes and sequences of operation within the system.

Crankcase Heater - Refrigerant will migrate to the coldest spot in a system and during cooler weather this is often the compressor.  If the compressor starts with a lot of refrigerant in the compressor it will, upon startup, mix with the oil and leave the compressor with the oil that is critically needed in the compressor.  This leads to premature compressor failure.   The crankcase heater stops the migration of refrigerant by keeping the compressor warm.

Curb - On down flow roof top package equipment this is attached to the roof and the A/C unit rests on top of this mounting frame.

Cycle the Unit – Confirm the proper sequence of operation.

Damper - Controls the flow of conditioned air in the duct work.

Direct Drive – A fan blade that is directly connected to the shaft of the motor.

Discharge Line - A copper line on the high pressure side of the refrigerant circuit from the compressor to the condenser coil

Discharge Line Dryer - A device mounted in the refrigerant circuit on the liquid side designed to remove moisture and contaminants

Discharge Muffler – Quiets the operation of a compressor and is internal to the compressor.

Disconnect - An on/off switch located on equipment for servicing the unit.

Down Flow Unit - A description of the direction of air flow either from a Roof Top Package unit.

Draft Motor (Ventor motor, Induced draft motor) – this is present in gas furnaces, designed to ventilate accumulated unburned gas and help draw the heat through the heat exchanger.

Drain Tablets (Pan Tabs) – Biocide time release tablets designed to minimize condensate leaks and help eliminate unwanted biological growth. e.g. molds, bacteria, alge

Drain Pan - Located below the Evaporator coil to catch, contain, and direct water removed from the air stream (condensation) to a drain line.

Duct – A delivery system of conditioned air

Economizer - Designed to provide low energy cooling when curtain conditions exist outdoors and indoors.  If cool enough outdoors the RTU will not energize the compressor when there is a cooling demand indoors.  Newer economizers have enthalpy control which helps minimize bringing in excessive moisture.

EER (Energy Efficiency Rating) - the most commonly used measure of efficiency for commercial air conditioning systems

EMS (Emergency Management System) – Computerized systems designed to control A/C systems, lighting systems, and /or security systems.  Intended to help reduce energy costs and other issues in a building.

EPA - Environmental Protection Agency. A part of the US federal government that enforces environmental laws and provides information and guidance to policy makers

Evaporator Coil - A heat transfer device in the refrigeration circuit consisting of copper tubing with aluminum fins to aid in heat transfer that refrigerant flows through.  This coil removes heat from the occupied space of a building.

Evaporator Fan - A fan designed to blow air across the evaporator coil to speed the transfer of heat.

Fiber Glass Filters – Filters that use fiber glass (a composition of material consisting of glass fibers and resin) as the main filtering agent

Filter Rack - Designed to hold filters in the return air stream.

Fins - Thin strips of aluminum designed to help dissipate heat quickly

Fire Dampers – Designed to help control building fires by closing off air flow with in a building if the sensor senses a possible fire.

Flame Sensor - A switch activated by heat.  Helps ensure gas is not accumulating in a un burned condition.

Flue - Allows the controlled escape of unburned and dangerous gases on furnaces.

Furnace – Self-contained appliance designed to supply heated air through ducts to spaces remote from the appliance location

Fuses – An electrical safety device consisting of strip of fusible metal in a circuit which melts when a circuit is overloaded.

Gas Regulator - Controls the gas pressure.

Gas Valve - Controls the flow of gas to the burner

Gauges - An instrument for measuring or testing.  Compound gauges are used to test refrigerant circuit operating pressures.

Grille (Register) –  Connects to the end of each duct to aid in air distribution.

Ground (Short) – Internal windings of a motor or compressor touching metal casing.

Hail Guards - Protects the condenser coils from hail damage.  Moderate to heavy hail damage causes the compressors to run longer and with higher head pressures. Increasing the failure rate of compressors.

Head Pressures – Pressure that exists in condensing side of refrigerating system

Heat Exchanger - On gas heat systems hot air is forced through this device located in the air stream

Heat Strips - Electric heat elements (as apposed to a gas burner) designed for the units primary heat or secondary heat as with a heat pump. 

Hermetic Compressor - A sealed refrigerant compressor.

Horizontal Flow or Side Discharge Unit - A description of the direction of air flow either from a Roof Top Package unit or a Furnace/Air Handler unit.

H.V.A.C. – (Heating, Ventilating, & Air Conditioning) acronym

Identification Plate - This gives manufacturer unit information, motor, compressor, and heater specifications

Igniter - Electronic ignition of a gas burner.

Indoor Air Quality (IAQ) – The status of indoor air as measured by numerous factors: temperature, humidity, airflow, pollutants, occupants, etc.

Kilowatts - the power rating of the system. One kilowatt is one thousand watts.

King Valve -  Liquid receiver service valve.

Leak Check Methods – Soap bubble method -squirts of a solution similar to liquid soap that bubbles when sprayed over a leak on a pressurized system.  Electronic leak detector- detects the chemical patterns of different refrigerants. This method can be challenging in windy conditions.  Fluorescent dye injected into a system then later scanned with a special light.

Liquid Line - A copper line (tubing) on the high pressure side of the refrigerant circuit from the condenser coil to the metering device.  The liquid line dryer is also in this line

Low Ambient Control – Designed to keep the evaporator coil from freezing when cooling is needed indoors and when it is cool outdoors as well.

M.E.R.V. – Minimum Efficiency Reporting Value) Is a rating of filters based on A.S.H.R.A.E 52.2 test method.  The higher the rating the more effective the filtration of a filter.

Metering Device (Automatic Expansion Valve- “AEV” or “AXV”) - Pressure difference between the high side and low side and the amount of refrigerant allowed into the evaporator is controlled by low side pressure.

Metering Device (Capillary Tubes - “Cap” Tubes) - Pressure difference between the high side and low side and the amount of refrigerant allowed into the evaporator is controlled by very small tubes that reduce the pressure.

Metering Device (Thermostatic Expansion Valve- “TEV” or “TXV”) - Pressure difference between the high pressure side and the low pressure side and the amount of refrigerant allowed into the evaporator is controlled by temperature of the leaving evaporator temperature.

Migration – When an air conditioner is off the refrigerant accumulates in the coldest area within the refrigerant circuit.  This can be damaging if the coldest spot is in the compressor.  Crankcase heaters are designed to keep the compressor warm.

Negative Pressure - Pressure less than that of the ambient atmosphere.  This pressure status will bring in unconditioned and unfiltered air into the building from many areas.  Slightly positive building pressures are desired.

Non Condensibles – Contaminants in a refrigerant circuit that affect the characteristics of the refrigerant.

Oil - Designed to lubricate the metal to metal moving parts to ensure a long life.

Oil Separator – Device used to remove oil from gaseous refrigerant.

Outside Air Dampers - Control the amount of outside air introduced into the air stream.  Some outside air is desired for air quality reasons.

P Trap – A shape of the condensate drain line designed to stop air from being sucked back into the evaporator area.  This works by retaining a small amount of water in the drain line, the water acts as a barricade to stop air flow in the drain line.

Package Unit (RTU) – An air conditioner unit where all components are located in a single outdoor unit that may be located on the ground or roof, usually the roof.

Panel - A metal access door for the use of servicing the system

Pleated Filters – Extended surface filter achieve with an accordion pattern.  High quality filter.

Plenum - A transition box, usually metal, connected to the leaving air side of a system that supply ducts are connect to.

Polyester Filters – Medium grade filters consistent with a low density sponge type material.

Positive Pressure – A pressure greater than atmospheric

Pressure Switch - Can be either high pressure or low pressure and is designed to turn off a component if a pressure is reached in a refrigeration circuit.

Pull A Vacuum – The removal process of all refrigerant and noncondensibles achieved by connecting a vacuum pump to a refrigeration circuit.  By pulling a vacuum and holding it the refrigeration circuit can be confirmed to not have a leak at the present time.

R22 - Chlorodifluoromethane or difluoromonochloromethane is a hydrochlorofluorocarbon (HCFC) with chemical formula C H Cl F 2 and CAS number 75-45-6. It is better known under its code names of HCFC-22, R-22 , Halocarbon R22 or Freon 22, and is commonly used in air conditioning applications, such as rooftop units and window air conditioners.

Receiver - A canister in the liquid line near the condenser for excess refrigerant.

Recover Refrigerant – Remove refrigerant from a circuit.  If the refrigerant is contaminated or chemically altered it can not be reused in the system.

Refrigerant – The liquid used to absorb and transfer heat from one area to another.  Usually from the inside area to the outside.

Refrigerant Lines – Copper lines designed to allow the travel of refrigerant through the system to the various components.  Discharge and Liquid lines are under high pressure, smaller and range from hot to warm.  Suction line is low pressure and cold often insulated to minimize sweating.

Relay - An electromagnetic mechanism moved by a small electrical current in a coil.

Return Air - Air returned from conditioned or refrigerated space.

Roof Top Unit (RTU) - see package unit.

Run Capacitor – Used to shift the phase in a single phase motor to the effectiveness of a 2 phase motor.  This helps provide more torque.

Safeties – Designed to help protect components if certain conditions develop.  Such as, high pressure, low pressure, high temperature, low temperature, etc.

Schematic - A structural or procedural diagram, especially of an electrical or mechanical system

Sealed Bearings – non-oilable bearing.

SEER (Seasonal Energy Efficiency Ratio) - the most commonly used measure of the efficiency of consumer central air conditioning systems.  This method factors seasonal effects

Semi-Hermetic Compressor – Larger reciprocating compressor contained in a cast iron frame like a car motor.

Sheave (pulley) - A wheel with a grooved rim like the one used in a pulley block to guide the rope or cable

Short Cycling – Refrigerating system that starts and stops more frequently than it should

Shrader Cap – A screw on cap that helps minimize refrigerant leaks through the shrader valve.

Shrader Valve – A service valve that opens when depressed to allow checking pressures and charging refrigerant circuit.  Same type of device as the valve on a bicycle tire.

Side Discharge Unit – indicates the direction the supply air is directed after leaving the air conditioner system.  This term in more common with roof top package units

Silver Solder (SILFLOSS) - Solder composed of silver, copper and zinc. Has a melting point lower than pure silver, but higher than lead-tin solder.

Site Glass - A glass section in piping to enable visual monitoring of the condition of the fluid or its height; also called a Sight Gauge

Slugging Refrigerant / Slugging Oil - Slugging is what happens when liquid refrigerant (or liquid refrigerant and oil) enters the compressor's cylinder during an on cycle.

Smoke Detector – A fire safety device used to sense smoke.  Once sensed it sends a signal to either a fire control panel of a building or shuts an A/C system off .

Split System - With a typical split system, the condenser and compressor are located in an outdoor unit; the evaporator is mounted in the air handling unit (which is often a forced air furnace) is located indoors.  Common home A/C system.

Squirrel Cage – A type of fan.  It looks similar to a hamster wheel contained in a metal housing.

Start Capacitor - Allows a shift in phase on single phase system motors to ease the start of the motor.

Subcool – Cooling of liquid refrigerant below its condensing temperature

Suction Line – Tube or pipe used to carry refrigerant gas from evaporator to compressor.

Suction Line Dryer – Designed to clean non condensable out a refrigerant circuit.  This dryer is often used in addition to a liquid line dryer when a compressor has grounded.

Super Heat – 1 – Temperature of vapor above its boiling temperature as a liquid at that pressure 2 – The difference between the temperature at the evaporator outlet and the lower temperature of the evaporating in the evaporator.

Supply Air - Cooled or warmed air leaving the evaporator delivered to the interior of the area. Supply air is sometimes called delivery-air.  The conditioned air you can feel coming from a supply grille.

Sweating (Moisture Condensation) – Condensation of moisture from air on a cold surface when the dew point is reached.

Sweating (Welding Process) – Method of soldering in which the parts to be joined are first coated with a thin layer of solder.  Heat is used to melt the solder and upon cooling bonds surfaces together.

Terminal - A position in a circuit or device at which a connection is normally established or broken.

Thermal Overload -  Detects an over heated motor or compressor and disables the motor until it has cooled sufficiently.

Thermostat- Digital -  Electrical sensor designed to run an A/C system within a defined temperature range.

Thermostat- Mercury -  Mechanical sensor that uses mercury to make electrical connection designed to run an A/C system within a defined temperature range.

Thermostat- Programmable -  Electrical sensor that controls time and temperature of the A/C.  Primary design is to help save energy usage.

Ton of cooling – Unit that removes same amount of heat in 24 hours as melting of one ton (2000 pounds) of ice . One ton is equal to 12,000 BTU's

Transformer – Electromagnetic device that transfers electrical energy from the primary circuit into variations of voltage in a secondary circuit.  Often transforming high voltage to a low voltage for the controls of an A/C system.

Tube Sheet – The metal frame at both ends of a coil that give the coil rigidity.

Unloader – A device that allows for easier compressor start-up by temporarily reducing high-side pressure at the cylinder head.

Vacuum – Pressure lower than atmospheric pressure

Variable Air Volume Terminal (VAV) – Device having electronic components used to regulate the volume of air in a distribution system

Volts – 1) Term used to indicate the electrical potential or electromotive force in an electrical unit. 2) Voltage or electrical pressure which causes current or flow 3) Electromotive force

Watts - A measure of the amount of work done by a certain amount or amperage of electric current at a certain pressure or voltage

Welding - To join (metals) by applying heat, sometimes with pressure and sometimes with an intermediate or filler metal having a high melting point.

Wet Switch – A switch that sense water and turn off a system.  Used in a secondary drain pan to sense a drain problem with the A/C system.

Wiring Diagram – Is a map of wires and components indicating what is connected to what and in what sequence.