water treatment for closed heating & cooling Systems

The treatment of water in modern closed heating and cooling systems is essential for the avoidance of microbiological fouling (biofouling) corrosion and scale. 

These problems can result in energy wastage, poor system performance, and the need for early replacement of plant and components. The consequences or inappropriate or non-existent water treatment can sometimes be disastrous.

The objectives of a water treatment programme in closed heating and cooling systems are to:

Fill Water

Closed heating and cooling systems are usually filled with mains water, though borehole water is is sometimes used. Mains water complies with drinking water quality standards and should therefore be free of contamination from dirt and debris. However, there are other characteristics that may render the water undesirable for use within the system. Pre-treatment should be considered for any fill water that does not comply with the recommendations of BG 29.


Pre-treatment systems available can broadly be classified as ‘Water Conditioning‘ and ‘Microbiological Control’


Where the available water is particularly hard, a softening plant may be temporarily installed to fill the system with reduced hardness water. It is not recommended to install a permanent softening plant solely for fill water as that will be underused and may become a source of bacterial contamination.

The fill water should not be fully soft (complete removal of calcium) as such waters are non-scale forming and, in theory, can be more corrosive than non-softened waters, particularly for copper. This risk can be offset by appropriate corrosion inhibitors e.g. azoles for copper protection.

Base-exchange softening replaces scale forming calcium and magnesium ions with sodium ions. If base-exchange softened water is used to fill the system, it is important to check that all inhibitors are suitable for use with the softened water.

Deionisation (using resin beads) is similar to base-exchange softening but removes both the scale forming and corrosive ions from the water and thus reduces the total dissolved solids (TDS) and conductivity. Deionisation of fill and make-up water is more expensive than base-exchange softening and is generally restricted to high temperature and/or high heat-transfer heating systems. Deionised water is rarely used to fill a closed heating and cooling system but could be helpful where there is a high sulfate or chloride concentration in the raw water. Again, the water treatment package should be tailored to the use of deionised water.

Softened and deionised water have reduced buffering capacity so chemical pH buffers may need to be added as part of the water treatment package to maintain the pH at the required level.  

Softening is not normally required for make-up water for closed systems as there should be minimal make-up during normal operation and the consequent addition of scale formers should be insignificant compared to the volume of the system. Some water treatment practitioners do however favour the use of blended softened water for both filling and topping up where it is already available on-site (usually for hot and cold water services).

Filtration & Reverse osmosis

In principle filtration of mains water should not be necessary but strainers or coarse filters are routinely used as protection against large debris and scale for pumps and control valves.

Mains water is not sterile but contains a wide range of bacteria that are harmless to people but may become implicated in corrosion processes. Low pressure membrane filtration is increasingly being used to provide a physical barrier to bacteria and fungi entering the system from the water supply. This is claimed to reduce the need for biocides for subsequent microbiological control.

Certain types of high pressure reverse osmosis (RO) membrane will also reduce the concentration of scale forming ions as an alternative to softening or deionisation. This may be useful on borehole supplies of hard water. RO softened water may need the addition of pH buffers to maintain pH control as for other softened water.  

ultraviolet (uv) disinfection

Ultraviolet treatment of the fill water is also used as a precautionary measure for water supplies with high bacteria counts. Ultraviolet treatment does not provide an absolute barrier to bacteria but can significantly reduce the number of bacteria entering the system.

Conventional UV treatment cells are not recommended for treating the make-up water for closed systems as make-up should be infrequent and insignificant in volume. 

Automated proportional dosing

Automated proportional dosing of water treatment chemicals and biocides may be used for the initial fill water and as part of pre-commission cleaning. 

Automated proportional dosing is not recommended for treating makeup water for closed systems as the makeup should be infrequent and insignificant in volume. Maintenance additions of water treatment chemicals and biocides should be added via dosing pots or dosing pumps under manual control.

Solids Removal & Filtration

Chemical Water Treatment

The chemical dosing facility should be sized relative to the system volume to allow sufficient amounts of treatment chemicals to be added efficiently. 

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