Closed Water System FAQs

Our team of closed water system experts have developed a good resource of answers to some common questions asked by our customers. If you have a question on workplace compliance, please email us at

    What are closed water systems?

    Closed water systems are sealed, recirculating water loops with no exposure to the outside air. These are commonly used for heating and cooling within a building, often they act as a transfer between heating and cooling plant (e.g. boilers and chillers), and terminal units where the heating or cooling is needed (e.g. radiators, fan coil units, or chilled beams). This method of heat transference allows larger, sometimes noisy plant equipment to be kept away from front of house areas of the building, additionally it allows a central plant to provide heating or cooling to many different areas of the building. Common systems within an office building or similar are low temperature hot water (LTHW) systems, chilled water (CHW) systems, condense systems, and air handling unit heat recovery systems. If you have any of these systems on site, then a closed water system management review will help you to understand how these are managed.

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    When should I undertake a closed water system management review?

    If you have treated closed water systems on site that are being maintained by a maintenance and electrical contractor or by an in-house maintenance team, then you may wish to know if these systems are being managed effectively in line with current guidance. Typically, the maintenance team will subcontract the water treatment requirements of your closed systems to a water treatment company, often the addition of multiple parties can result in a lapse of communication and understanding. A closed water system management review will help you, as the responsible person; to understand your system, see the recent history of the system, and get an overview of where management controls are strong and where they need further attention. The review could be a tool to determine if the management system already in place is working well, or it could be used in a building you have recently taken on to help set up a robust regime moving forwards.

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    What are the problems that can occur within closed water systems?

    There are many waterborne bacteria that can grow and thrive within closed water systems when internal conditions are favourable, bacterial growth in a system can lead to biofouling and corrosion. Biofouling is caused when a bacterial slime coats the internal surfaces of the system, this will prevent chemical corrosion inhibitors from having regular contact with these surfaces, leading to corrosion. Biofilms can also restrict flow within pipework and plant equipment, particularly in heat exchangers where narrow bore pipework is located. This can cause reduced energy efficiency and in extreme cases the poor flow can cause pressure build-up and damage to plant equipment. Corrosion in a system will damage the internal surfaces, wearing them thin and causing leaks which reduces the life span of equipment. Corrosion will also cause excess suspended solids and debris deposits in the system, this can again impact flow, and clog and damage plant equipment. Replacement plant equipment is expensive, and damages could also result in expensive running costs and loss of service for your clients so prolonging its lifespan is paramount to generating healthy profit when it comes to building management.

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    What guidance is available to help me manage my closed water systems?

    BSRIA documents BG29/2021 ‘Pre-Commission Cleaning of Pipework Systems’ and BG 50/2021 ‘Water Treatment for Closed Heating and Cooling Systems’ give guidance on water treatment requirements and concerns for closed water systems prior to, during, and after they are commissioned. The latter is intended for use by facilities managers, maintenance staff, and water treatment staff responsible for looking after completed systems once a building is handed over. Additionally British Standard 8552/2012 ‘Sampling and Monitoring of Water from Building Services Closed Systems’ contains guidance on issues of sapling heating and chilled water systems in buildings from construction to pressure testing, to commissioning, to routine operation. Between these documents there is guidance on common issues, chemical and bacterial testing methods and regimes, appropriate parameters, or chemical and bacterial testing.

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    Is bacteria in my closed water system a health and safety risk?

    As closed water systems are sealed there is very little risk of exposure to the water within. Bacteria such as Pseudomonas which have health concerns in open water systems are limited to more operational concerns within closed water systems due to the biofilms these bacteria create. While bacterial contamination in a closed water system is not a direct health and safety risk, poor management of the closed water systems can cause equipment failure or reduced efficiency in plant equipment such as calorifiers which supply hot water to the domestic water services in a building. Low hot water temperatures in calorifiers could cause Legionella growth in the domestic hot water system that is a health and safety concern.

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    Is a Variable Refrigerant Flow (VRF) system a closed water system?

    No. While VRF systems are sealed, recirculating loops with no exposure to outside air these systems solely contain refrigerant, contrary to a closed water system which contains treated water. Therefore, they do not require the same maintenance and water treatment monitoring as a closed water system.

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    Why does my water treatment contractor keep quoting closed systems dosing, what is this for?

    Water treatment companies will provide remedial dosing quotes for closed systems based on the results of their regular chemical and bacterial sampling. If results show that inhibitor levels are low, then they will recommend and provide a quote to top up the system with additional inhibitor to keep levels within recommended parameters and prevent corrosion within the system. If results show elevated general bacteria levels or indicator species such as Pseudomonas, nitrite reducing bacteria (NRB), and/or sulphate reducing bacteria (SRB) then they will recommend and quote for biocide dosing to be carried out to remove the contamination. Heavy bacterial growth could lead to poor system performance and corrosion. If you are receiving multiple remedial dosing quotes for the same thing on a consistent basis it is important to understand the trend and identify why this is happening. For example, consistently low inhibitor levels could indicate there is an unidentified leak on the system which is diluting water treatment chemicals, or regular recommendations for biocide dosing could be due to areas of low turnover such as deadlegs or seasonal isolations which area allowing areas of the system to harbour bacterial growth where biocides will not affect them. A closed water system management review may be a useful exercise in these situations to help identify these trends and give you the confidence to challenge your water treatment contractor.

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    How do I know what all the tests and results mean on the results certificates for my closed water systems provided by my water treatment contractor?

    Closed water system chemistry is complex and detailed, you should understand what the results are saying but it is the job of your expert water treatment contractor to be interpreting these results for you in a way that you and your engineering team can understand and action if needed. Chemical and bacterial analysis of your closed water systems should be presented with clear parameters, good commentary, and easily understood recommendations, this not only helps you to manage the systems, but it also helps your engineers to carry out remedials where required, and it helps the water treatment contractor to be heard correctly. Unfortunately, results are commonly supplied in the form of laboratory certificates with no supporting information, these are very difficult to interpret without prior knowledge of closed system chemistry, if this is what your results certificates look like then maybe a Closed Water System Management Review will help you to instruct your water treatment contractor to supply more useful information moving forwards.

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    How often should my water treatment contractor attend site to carry out chemical or bacterial analysis of my closed water systems?

    There is no regulatory requirement to undertake any chemical or bacterial analysis from the closed water systems, however, without internal monitoring of water quality there is no way of knowing if corrosion is occurring and the impact could be a significant reduction in the lifespan of your system and associated plant equipment. Guidance in BG 50/2021 ‘Water Treatment for Closed Heating and Cooling Systems’ suggests that the monitoring regime for closed water systems should fluctuate dependant on operation characteristics. During the first two months after handover, during and/or after remedial cleaning, or after any uncontrolled water losses the recommended frequency of chemical and bacterial analysis is every two weeks. In systems where there is evidence of deteriorating water quality, or evidence of water losses, or during periods when external contractors such as fit-out contractors are present the recommended frequency is every month. If the system has a good record of water quality with minimal losses, then the recommended frequency is quarterly. It is also important to make sure that sufficient samples are being taken – making decisions based upon just one sample per system may lead to incorrect decisions being taken, or such results could give false reassurance on the operation of the system as a whole.

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