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Air Change – The amount of air required to completely replace the air in a room or building; not to be confused with re-circulated air.

Air-conditioning – A form of air treatment whereby temperature humidity and air cleanliness are all controlled within limits determined by the requirements of the air-conditioned enclosure.

Air Cooled – Uses a fan to discharge heat from the condenser coil to the outdoors.

Air Diffuser – Air distribution outlet or grille designed to direct airflow into desired patterns.

Air Handler – The portion of the central air conditioning or heat pump system that moves heated or cooled air throughout the ductwork. In some systems a furnace handles this function.

Artificial Water Systems – Any water system that has been constructed and does not occur naturally such as a hot water system.

Audits – The inspection and verification of an existing Legionella risk management system. This would normally involve the review of existing Legionella risk assessment reports and other documentation to ensure all critical risks are addressed and action plans are operational.


Biofilm – A community of bacteria and other micro-organisms, embedded in a protective layer with entrained debris, attached to a surface.


Calorifier – An apparatus used for the transfer of heat to water in a vessel by indirect means, the source of heat being contained within a pipe or coil immersed in the water. Usually cylindrical design, manufactured from steel / copper and mounted in the vertical / horizontal plane. Usually 750 – 2500 litres storage.

Central Air Conditioner System – System in which air is treated at a central location and carried to and from the rooms by one or more fans and a system of ducts.

Chilled Water System – A type of air conditioning system that has no refrigerant in the unit itself. The refrigerant is contained in a chiller, which is located remotely. The chiller cools water, which is piped to the air conditioner to cool the space.

Chlorination – The process by which cold water storage tanks are cleaned of all dirt and debris. Chlorination should be carried out in accordance with BS6700 specification for the design, installation, testing and maintenance of services supplying water for domestic use and their curtilages.

Cold Water Service – Installation of plant, pipes and fitting in which cold water is stored, distributed and subsequently discharged.

Condenser – A device that transfers unwanted heat out of a refrigeration system to a medium (either air, water, or a combination of air and water) that absorbs the heat and transfers it to a disposal point. There are three types of condensers: air-cooled condensers, water-cooled condensers, and evaporative condensers. The evaporative condenser uses a combination of air and water as its condensing medium. Most residential systems have an air-cooled condenser.

CWS – Cold Water Storage, usually in reference to a Cold Water Storage tank which can be found in a variety of situations from domestic homes to industrial plant. The water is supplied by the mains cold water service and held in the tank to be distributed via pipework to the relevant sources. Not a source of drinking water. Under ACoP (L8) cold water in the tank should be stored at less than 20ºC.

Cylinder – Normally a copper domestic style cylinder mounted in the vertical plane. Usually electric immersion or hot water coil to heat the stored water. Volume normally 300 – 750 litres.


Damper – Found in ductwork, this movable plate opens and closes to control airflow. Dampers are used effectively in zoning to regulate airflow to certain rooms.

Dead end – A length of pipe closed at one end through which no water passes.

Deadlegs – Pipes leading to a fitting through which water only passes when there is draw-off from the fitting.

Decontamination – The process of removing or neutralising pathogenic bacteria in a water or other system.

Defrost Cycle – The process of removing ice or frost buildup from the outdoor coil during the heating season.

Dehumidification – The reduction of water vapor in air by cooling the air below the dew point; removal of water vapor from air by chemical means, refrigeration, etc.

Dipslides – A dipslide is a means of testing the microbial content of liquids. It consists of a plastic carrier bearing a sterile culture medium which can be dipped in the liquid to be sampled. It is then incubated to allow microbial growth. The resulting microbial colonies are estimated by reference to a chart.

Direct Vent – Pulls outside air for combustion and vents combustion gases directly outside.

Disinfection – A process which destroys or irreversibly inactivates micro-organisms and reduces their number to a non-hazardous level.

Domestic Water Services – Hot and cold water intended for personal hygiene, culinary, drinking water or other domestic purposes.

Downflow – Refers to a type of precision air conditioning system that discharges air downward, directly beneath a raised floor, commonly found in computer rooms and modern office spaces.

Duct – A pipe or closed conduit made of sheet metal, fibre-glass board, or other suitable material used for conducting air to and from an air handling unit.

Ductwork – Pipes or channels that carry air throughout a building.


Electronic Air Cleaner – An electronic device that filters out large particles and contaminants in indoor air. It then electronically pulls out tiny particles that have been magnetized, such as viruses and bacteria, drawing them to a collector plate.


Filter – A device used to remove dust and other particles from air for the purposes of reducing the load on the respiratory system and to protect the HVAC equipment. Filters vary greatly in particle arrestance; the higher the MERV rating, the better the filter.

Flushing – The process of draining and cleaning a cooling system, including all associated pipework.



Heater – Usually a relatively low storage volume / fast recovery device mounted in the vertical plane. Normally gas fired i.e. manufactures Andrews, A.O. Smith, Hamworthy. Usually 150 – 300 litres storage.

Heat Exchanger – A device for the transfer of heat energy from the source to the conveying medium.

Heat Gain – The normal location for cold water storage tanks in roof spaces and roof top plant rooms makes them liable to extraneous heat gain. In order to minimise heat gain the use of natural ventilation should be considered or mechanical ventilation under exceptional circumstances.

Hot Water Services (HWS) – Installation of plant, pipes and fittings in which water is heated, distributed and subsequently discharged (not including cold water feed tank or cistern).

Humidification – The process of adding moisture to the air within a space.

Humidity – The amount of moisture in the air. Air conditioners remove moisture for added comfort.

HVAC – Heating, Ventilation, and Air Conditioning.





Legionnaires’ Disease – A severe form of pneumonia caused by Legionella bacteria.

Legionella – Type of aerobic bacterium which is found predominantly in warm water environments (singular of legionellae).

Legionellosis – Any illness caused by exposure to legionella.

Low use – Any outlet / appliance that has been identified as being used less than once a week. It is important that all of the domestic hot and cold water outlets and appliances are being used on a regular basis so that satisfactory turnover is achieved. If possible, low use outlets and appliances should be removed and the pipework services should be cut back and capped at the nearest live distribution positions. If the low use outlets and appliances cannot be removed, it must be ensured that they are thoroughly purged on a weekly cycle under a log book regime.


Micro-Organism – An organism of microscopic size including bacteria, fungi and viruses.

MCWS – Drinking water supplied from the Mains Cold Water Service.



Operating Cost – The day-to-day cost of running your air conditioning system, based on daily energy use.


Pasteurisation – Heat treatment to destroy micro-organism usually at high temperature. There is no British standard for pasteurisation but treatments must be carried out by trained personnel who should be closely supervised.

Plume – The visible discharge of air and moisture from a cooling tower or other scooling system. This can include condensation and aerosols and as such represents a potential Legionella risk.

Point-of-use heater (in-line) – A water heater that is served off the mains supply or a down service and without an integral cold water storage tank facility. Normally electrically heated and with a small storage volume i.e. 15 – 50 litres. Normally serves single point or small number of WHBs / sinks.

Point-of-use heater (storage) – A water heater with an integral cold water storage tank facility. Normally at point of use or locally to the area it serves. May serve one or more rooms. Normally electrically heated. Variable storage volume i.e. 25 – 300 litres.

Pontiac Fever – A disease caused by species of legionella, an upper respiratory illness less severe than Legionnaires’ disease.

Purge Device – A device which removes air and water vapor from the refrigerant inside a chiller. A purge is a necessity in negative pressure designs, but is not necessary in positive pressure designs where air and water vapor are kept out of the system by the internal pressure.



Relative Humidity – The ratio of the amount of vapor contained in the air to the greatest amount the air could hold at that temperature. Normally expressed as a percentage.

Return Air – Air drawn into a heating unit after having been circulated from the heater’s output supply to a room.

Reversing Valve – A device in a heat pump that reverses the flow of refrigerant as the system is switched from cooling to heating.

Risk Assessment – Identifying and assessing the risk from legionellosis from work activities and water sources on premises and determining any necessary precautionary measures.


Scale – Crystalline deposits that form on system surfaces or pipework. Scale normally results from a build up of unwanted minerals, usually calcium carbonate.

Scale Inhibitors – Chemicals used to control scale. They function by holding up the precipitation process and/or distorting the crystal shape, thus preventing the build-up of a hard adherent scale.

Sentinel Taps – For a hot water services – the first and last taps on a recirculating system. For cold water systems (or non-recirculating hot water systems), the nearest and furthest taps from the storage tank. The choice of sentinel taps may also include other taps which are considered to represent a particular risk.

Setpoint – The temperature to which a thermostat is set for desired comfort level.

Sludge – A general term for soft mud-like deposits found on heat transfer surfaces or other important sections of a cooling system. Also found at the base of calorifiers and cold water storage tanks.

Spa Pools (Spa Bath/Hot Tub/Jacuzzi) – A spa pool is a self-contained body of warm water designed for sitting in (not whole body immersion). It is intended for a small number of people to use at one time. The water is re-circulated and kept between 30ºC – 40ºC and is usually not drained between use and is continually filtered and cleaned. A hydro-jet circulation, with or without an air induction bubble system, is also used to agitate the water.

Split System – A central air conditioner consisting of two or more major components. The system usually consists of a compressor-containing unit and condenser, installed outside the building and a non-compressor – containing air handling unit installed within the building. This is the most common type of system installed in a home.

Stagnation – The condition where water ceases to flow and is therefore liable to microbiological growth.

Stratification – Stratification can occur in any calorifier or cylinder and there will be some temperature gradient across the vessel depending on the heat input and rate of draw. Temperature stratification can be overcome by the use of small pump units, which can circulate water from the top to the bottom of the hot water storage vessel.


Temperature Gauge – A suitable temperature gauge should be sited near to the calorifier / cylinder water supply outlet so that a quick visual indication can be made of the operating temperature. The temperature gauges should be checked and re-calibrated on a periodic basis.

Thermal Disinfection – Heat treatment to disinfect a system.

Thermidistat – Highly sophisticated programmable thermostat that senses the outdoor temperature, indoor air temperature, and indoor relative humidity. A microprocessor communicates with the heating and cooling equipment to determine the most efficient way to achieve perfect comfort. When used with a variable speed blower motor, this cite can reduce humidity even when cooling isn’t necessary (as when the indoor temperature is 70ºC but relative humidity is high, eg, when it’s raining).

Thermostat – A temperature control device, typically found on a wall inside that consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system

Thermostatic Mixing Valves – The hot water distribution temperatures that are required for the control and prevention of Legionella can lead to discharge temperatures in excess of 50º C. To prevent scalding injury to young children, the elderly and other vulnerable persons, action will be needed to limit water discharge temperatures.

Thermostatic mixing valves can be installed to reduce hot water temperatures to a safe level. The following maximum safe hot water temperatures are derived from the, “NHS Estates health Guidance Note ‘safe’ hot water and surface temperatures” 1998.

Showers 41º C
Wash basins and sinks 41º C
Baths 44º C



Ventilation – The process of supplying or removing air, by natural or mechanical means, to or from any space. Such air may or may not have been conditioned.

Ventilator – A ventilator captures heating or cooling energy from stale indoor air and transfers it to fresh incoming air.


Water Cooled System – A type of air conditioning system that uses freon as a refrigerant and water as a condensing medium. Typically, the water-cooled condenser is located inside the air conditioner with the rest of the refrigeration components. Water is piped to the unit from a cooling tower or other suitable source.



Year-Round – Air Conditioner which uses gas or oil for heating.


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