a-z of air conditioning

These definitions are for guidance of users and we appreciate engineers could argue that some descriptions could be more accurate. If you think we may have seriously misled anyone, please e-mail us with your argument or correction and we will add or edit. The terms are to help lay persons understand air conditioning terminology without getting too technical or theoretical.

absolute as in absolute freezing point which is minus 273 degrees Kelvin – the Kelvin temperature scale developed by Lord Kelvin in the 1800′s is also known as the absolute temperature scale. It is a scale mainly used by scientists and the zero point is considered the lowest temperature of anything in the universe. 0ºK is equivalent to -273.16º Celsius.
acrib acronym for air conditioning refrigeration industry board
air we all need it to breathe, sometimes it is dirty and polluted, sometimes it is clean and fresh
air conditioner term used to describe a packaged (complete) air conditioning unit in one box, as in mobile, portable, window, console or rooftop
air flow as in volume air flow rate needed to transfer heat
ammonia a very toxic, slightly flammable substance that can be used as a refrigerant.
anemometer instrument used to measure air velocity (speed)
ari a standard test condition but does not relate to UK design conditions – see nominal duty
biocide used in your drip tray to keep condensate trays and drains clean – also sometimes referred to as tabs, Comfort use them for maintenance contract customers.
Btu a measurement of heat, the British thermal unit, as heat flow rate = Btu/hr. Put simply, one Btu is the amount of heat required to raise the temperature of 1 lb. of water 1 degree Fahrenheit. Heat flow rates are now measured in watts or kilowatts.
cassette a fan coil unit that fits mainly in the ceiling void with only a diffuser plate visible, diffuses conditioned air in one, two, three or four directions
Celsius the correct metric unit of temperature measurement 0 degrees Celsius is freezing point and 100 degrees Celsius is boiling point for water
centigrade see Celsius, it’s effectively the same
cfc a chlorofluorocarbon that is now not allowed to be used as a refrigerant, typically R11 or R12
cfm an imperial measurement of air flow rate – cubic feet per minute
chiller a machine that uses refrigeration to produce chilled water
cibse acronym for the chartered institute of building services engineers – UK authority on design matters
close control commonly means air conditioning to precise control requirements of air temperature, humidity and cleanliness – very expensive to own and run and wastes energy but is sometimes necessary for machines, processes and historical archives or artefacts
comfort our namesake but what we all strive to achieve, air conditioning provides this by altering the environment served
comfort cooling often derided by some engineers (who should know better) as not being air conditioning, is a common term to describe cooling for comfort as opposed to close control. Modern comfort air conditioning systems control temperature very accurately but not humidity, as it would be an unnecessary waste of energy to do so, purely for human comfort.
compressor at the heart of most air conditioning systems there has to be a refrigeration compressor. With current technology without a compressor, there would not be enough cooling for most applications
condenser a device used to condense a refrigerant thereby rejecting the heat to another source, typically an air cooled or water cooled condenser
condensing unit a machine comprising a compressor, condenser and if air cooled, a fan, used to reject heat
condensate water, or the moisture removed from the air when the air is cooled
conductivity the property or power of conducting heat.
cooler a machine, commonly using water, a fan and a coil or mesh to provide evaporative cooling – very effective and cheap to run in hot dry climates, but not so effective in humid climates, such as the UK. It is now illegal to refer to them as air conditioners.
cooling a process of which there are many to remove either sensible or latent heat, or both, which is then referred to as total heat
cooling tower a device, commonly used in larger buildings to reject heat from a chiller to the atmosphere via water and forced air. They have to be regularly cleaned and maintained, particularly to prevent legionella bacteria forming within the water. It is probably preferable to use air cooled condensers now.
cop the coefficient of performance, used to determine how efficiently a heat pump operates. It is now common for modern systems to exceed 3 to 1, whereby for every 1kw of electrical power used by the unit 3kw of heat will be provided. Latest systems can exceed 4 to 1, but note that the COP is related to external and internal temperatures.
dBA a unit for measuring sound power or pressure, deciBel on the A scale
de-humidify to remove moisture from the air, achieved by mechanical cooling
dew point the temperature at which the moisture in air condenses from a vapour back to a liquid. As in the condensation that forms on a cold surface relative to air temperature, i.e. glazing or un-insulated ducts or pipes.
diffusion the art or science of delivering air to achieve cooling or heating with good air distribution without causing discomfort or draughts, uses a diffuser
diffuser a device forming part of an air conditioning unit or duct system to deliver and diffuse air efficiently without causing discomfort or cold draughts
drip tray a component of most air conditioning units and fan coil units to collect the condensate water removed from the air whilst cooling
dry bulb the dry air temperature – relative to the wet bulb temperature – see wet bulb.
dry cooler similar to a condenser
duct a pipe or tube made from various materials either rectangular, circular, oval, flexible, vapour sealed or and insulated used to convey air.
eer energy efficiency ratio – a factor calculated to determine how efficient a unit/system is.
energy labelling a system commonly used from 2003 to show how energy efficient a unit or system is.
enthalpy a term used to determine the amount of moisture held by a specific volume of air at a certain condition.
cooling effect caused by air blowing across a moisture laden surface – see cooler – but also is the effect you feel if air is blown across your skin. Try it now, you have moisture on your skin and by blowing on it you will feel the cooling effect.
evaporator commonly the room unit of an air conditioning unit, that gives off cooling by typically evaporating refrigerant from a liquid to a gas. This action causes a refrigerated cooling effect and the heat within the room air is transferred to the refrigerant. Also a component part of a chiller.
fan a device used to move or circulate volumes of air
fan coil a device commonly having a fan and a coil located within a room to provide local cooling or heating
Fahrenheit the imperial measurement of temperature
filter a device of varying types and qualities used to remove impurities from air, gases or liquids. Always keep your filters clean! Dirty air conditioning filters can be the largest cause of unnecessary breakdowns!
freezing the process of converting a liquid or gas to a solid state
gas common term used to refer to some refrigerants, which are at sometimes in a gaseous state – otherwise any substance that is freely moving but is neither a solid or a liquid.
grille not to be confused with a diffuser, a grille is device supplying or returning air, commonly a grille is never used for supplying air from a ceiling, as it cannot diffuse the air. It should only be used in a sidewall application or for vertical discharge.
gwp global warming potential – is a measurement of how much effect any given refrigerant will have on global warming in relation to carbon dioxide. This is usually measured over a 100 year period. The lower the value of GWP the better the refrigerant is for the environment.
hydrocarbon, amongst other applications also a highly flammable refrigerant – propane
hcfc hydrochlorofluorocarbon – a refrigerant, typically R22 – see refrigerants
heat pump a machine similar to a condensing unit but can reverse it’s refrigeration cycle and extract heat from an external source, air or water and then pump it back in to a building to provide heating. Very efficient because even when it is -15ºC outside there is still heat in the air and a heat pump can extract this heat, upgrade it and use it.
hfc hydrofluorocaron – a refrigerant, typically R134A, R407C, R410A – see refrigerants
heat gain the heat a building or room gains causing possible discomfort, this can be from many sources including solar gain (heat radiated from the sun), conduction gain, (heat transmitted to a space due to the difference between the inside and outside temperatures), lights, people, machines, fresh air etc.
heat loss The heat a building or room loses causing possible discomfort, this can be primarily due to the difference between internal and external temperatures, but also the amount of cold air used for ventilation.
hi-wall a term used to describe a room fan coil unit mounted at high level on a wall
humidistat a control switch used to operate (turn on and off) another device based on pre-set humidity conditions
humidity or relative humidity, the measurement of how much water vapour is contained within the air – i.e. 50% is considered acceptable comfort, 20% would be an uncomfortable dry condition and 100% means it’s probably raining!
hvca acronym for the heating and ventilating contractors association
hygrometer an instrument used to measure the relative humidity
ice solid water at 0ºC, -32ºF
internal heat gain the heat produced by internal rather than external sources, can be people, lights, machines, computers, electronics, cooking, TV’s, hot meals, etc.
inverter an electrical device that allows the frequency of the electric supply to be varied from it’s standard 50Hz, thereby allowing motors (compressors) to be run at variable speeds. This in turn allows more precise delivery of energy to match the required load at any one time, rather than a constant delivery, which may not match. The result is an extremely controllable and energy efficient system that is not only cheaper to run but also needs a smaller power supply.
Kelvin see absolute
kcal a measurement of heat used in continental Europe, as 1000 calories – yes it’s the same unit those diet people keep referring to.
kw standard measurement of heat or power, 1kw = 1000 watts = 3412Btu/hr = 860kcal
kilowatt same as kw above
Kyoto protocol – we suggest you research this on the Internet, it covers the recommendations to reduce global warming and is a very complex subject. Air conditioning manufacturers and refrigerant producers are working hard to produce new and energy efficient products. Make sure you read it all though, don’t believe everything you hear on the news or read.
joule a measurement of energy
latent heat the heat that does not change the temperature but does change the state of a substance when added to or removed from it.
liquid a substance that is neither solid nor a gas
low ambient term used to refer to a low external temperature, often an air conditioner or system will require low ambient controls to enable it to keep on cooling with low external temperatures
legionella it’s full name is Legionnaires’ Disease (Legionellosis) and is a very uncommon form of pneumonia caused by a bacterium that when inhaled may have serious consequences. It is only possible to catch this via a water based aerosol and apart from natural sources the most likely way a single individual can catch this is from a little used shower head. Cooling towers that have not been correctly maintained more often causes larger public outbreaks.
mobile as in a mobile air conditioner, you simply wheel it in, plug it in and turn it on, firstly having decided where you will put the condenser or how to discharge the flexible hose to external air. You can also have a split mobile where the room unit is connected by an umbilical to an external condenser.
monobloc term used to describe a single packaged mobile or portable air conditioner, it will be a one piece unit normally with a flexible discharge hose, not a remote condenser.
Montreal protocol covers the subject of substances that deplete the ozone layer.
mouth organ slang term used to describe a horizontal ceiling suspend room fan coil unit, now you know why, it is similar to a mouth organ having slots on both sides through which air is blown. They normally do not make music or any noise if properly designed.
m3/hr a measurement of air volume flow rate – cubic metres per hour
m3/s a measurement of air volume flow rate – cubic metres per second
m/s a measurement of air velocity or speed – metres per second
multi-split a term used to describe a system where there is one outdoor unit connected to a number of room fan coil units
nitrogen a constituent part of air – oxygen free nitrogen (OFN) an inert and safe gas is used to pressure test refrigeration pipework and is passed through the pipework when it is being installed. But only reputable companies do this.
nominal duty the standard means of measuring the capacity of an air conditioner or unit, system etc., relates to conditions which mean nothing to UK
odp ozone depleting potential – new refrigerants have zero odp and therefore, if released do not deplete the ozone layer. However, they still must not be released to atmosphere.
ofn oxygen free nitrogen – used in brazing copper refrigeration pipes to prevent moisture and oxides collecting and remaining in the pipework during manufacture or on site installation. Failure to use it can have disastrous effects later, causing expensive failure of compressors.
oil used to lubricate the compressor, can be mineral oil, but increasingly synthetic with new refrigerants
oxygen we need it to oxygenate our bloodstream and live, therefore we have to ensure there is enough in the air we breathe.
ozone as in that layer above earth that protects us from ultraviolet rays and other nasty things happening to our world. Some refrigerants are ozone depleting, which if they were released could possibly damage the ozone layer. New refrigerants are not able to deplete ozone.
physchrometrics the art or science of the relationships between air, gas, liquids used to calculate air conditioning requirements
portable as for mobile air conditioner
propane or R290, although a naturally occurring substance that can be used as a refrigerant it is also a highly flammable gas and must be carefully used.
pressure the changing pressure of a substance can affect it’s state and hence properties. A compressor is used to change the pressure of a refrigerant and hence it’s ability to absorb or transfer heat. The pressure a compressor, fan or pump is capable of producing will also affect how far a gas or liquid can be moved. The resistance to pressure a pipe or duct causes will also determine how far a gas or liquid can be moved.
R22 an hcfc refrigerant which from 2002 can only be installed new as small heat pumps
R407C an hfc refrigerant that is non-ozone depleting, zero odp, commonly used in
small air conditioning systems
R410A an hfc blended refrigerant that is non-ozone depleting, zero odp, but operates at higher pressures than other hfc refrigerants.
R417A the zero odp replacement, suitable for new equipment and as a drop-in replacement for existing R22 systems.
refcom the register of companies assessed as competent to handle refrigerants
refrigerant a substance used to absorb and transfer heat, can be air, water, gas or chemical.
refrigeration the process of mechanical cooling
remote controller a device that allows the remote operation or setting of another device, can be “wireless” as infrared or “hard wired” with a cable connecting – allows you to control your air conditioning
sensible heat the heat which affects the temperature of a substance. e.g. there is no moisture change, such as heat from machines, lights or the sun etc.
sick building syndrome often blamed on air conditioning, but is primarily due to insufficient or poor ventilation and also the chemicals used in manufacture of furniture, carpets and other office equipment or processes
sling physchrometer looks like a football rattle (remember those!) but comprises two thermometers, one to measure dry bulb, the other wet bulb temperatures. has to whirled around to get air temperature and a correlation between the two temperatures gives us the relative humidity
solar gain the heat gained by a building or room from the suns heat radiation.
sound most air conditioning systems are relatively quiet and do not cause noise disturbance to users or the environment. It is common to illustrate equipment sound levels as either sound power, or sound pressure in dBA (deciBel on the A scale). This can be very confusing and the resultant room noise level is relative to many other factors, resulting in a room noise level, often expressed as a NR or NC level.
split system an air conditioner that has been split, very common and ideal for most buildings, the room unit or evaporator, fan coil is split (separated) from the condensing unit, external or heat pump and connected by means of small bore pipes and cables.
tabs see biocide
temperature a measurement of how hot or cold a substance is. A comfortable summer internal temperature would be 24ºC, in Winter 21ºC
thermometer an instrument for measuring the temperature
thermostat a device for controlling or switching on or off another device based on a pre-set temperature.
ton an imperial measurement of refrigeration heat – 1 ton = 12000Btu/hr or 3.517kw
total heat the sum total of sensible and latent heat
u value a calculated value used to further calculate the rate of heat flow through a structure, e.g. the heat conducted through a wall, roof or window
vav variable air volume – a type of a/c system sometimes used in very large buildings
velocity speed
vrf variable refrigerant flow – a common type of air conditioning used in many buildings today, same as vrv-
vrv variable refrigerant volume, a protected trademark for the system developed by Daikin and now copied by many others who have to refer to it as vrf
vwt variable water temperature, a water pipe loop system having both boilers to raise the temperature and dry coolers or cooling towers to lower the temperature, small heat pumps are commonly connected to the loop to either reject local heat to or extract heat from the loop.
vvt variable volume and temperature – a small ducted air system that can vary both the volume of air and temperature to achieve either heating or cooling matched to the required capacity
water we drink it, swim in it, pollute it and sometimes use it for air conditioning
water cooled heat rejected from a building is transferred to water. Not advisable if you intend to use a cooling tower, unless you regularly maintain the system and ensure water treatment is maintained but can be very efficient if using ground water in conjunction with a heat pump, inverters and a vrf system
wet bulb the air temperature measured with a wet bulb – a small fabric sleeve around the bulb of a thermometer and wetted
x factor many x factors are used to calculate such things as the suns altitude, azimuth and angle of incidence to work out solar gains on buildings.
y factor used with x factors as above.
z factor if you have read the whole of this document you deserve to get some zzzzzzzzz!