HEAT & ENERGY BRIEFING
Protecting commercial hot water systems from the inside out
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Water is often taken for granted in a commercial hot water system. It comes into the building, is heated and distributed and then either recirculates or disappears down the drain. But the quality of that water can have a significant impact on system performance, reliability, efficiency and service life, as Steve McConnell, director, ICOM, explains
he new ICOM guide ‘Water Quality Consideration of Domestic Hot Water Systems for Commercial Applications – Edition 2’ [2026-Treatment- Conditioning-of-Commercial-Hot-
Water-Systems-Guide26.pdf] highlights why water treatment needs to be considered as an integral part of system design, commissioning and ongoing maintenance, rather than something to be addressed only when problems appear. From the very start of any project, it’s important that commercial heating and hot water engineers understand the water before deciding how to treat it.
Water quality is not one-size-fits-all
Mains water supplied for human consumption is treated to meet regulatory requirements for wholesomeness. That does not necessarily mean it is suitable for every type of commercial water heating equipment. Water quality varies significantly across the UK, with hardness being one of the most important considerations. Hardness can result in scale formation and is a major cause of engineering problems in hot water systems. Conductivity, chlorides, chlorine content and pH can also influence equipment performance and corrosion. This means installers and specifiers should not simply assume that the regional water hardness shown on a map is sufficient information. The guide recommends establishing the actual hardness with the relevant Local Water Authority and assessing the duty and volumetric flow of water through the equipment before deciding whether and which treatment is necessary. Most importantly, manufacturers’ water quality recommendations must always be followed. Incorrect water quality can damage equipment, shorten its life expectancy and potentially affect warranty provision.
Scale is the enemy of hot water
Scale remains one of the biggest threats to commercial hot water equipment. As water temperature increases, the solubility of calcium carbonate decreases, making scale-forming compounds more likely to deposit on heat transfer surfaces. The effect applies across different types of water heaters. For a commercial system, that can have serious consequences. Scale on a heat exchanger reduces heat transfer and can affect flow and
temperature performance. Over time, deposits can contribute to equipment failure and increase maintenance requirements. The appropriate response, however, is not automatically to install a water softener or chemical treatment system. Treatment should be selected following analysis of the water supply, assessment of system design and consideration of the materials used and the way the system operates. Where treatment is required, available technologies include water softeners, reverse osmosis, electrolytic scale reduction and polyphosphate dosing. In many UK installations using mains water, no further ongoing treatment will be required. The important point is that this should be established through risk assessment and appropriate specialist advice, rather than assumption.
Corrosion needs equal attention
Scale may be the most prevalent problem, but corrosion is another important consideration, particularly where systems contain combinations of different metals. Corrosion can be influenced by water chemistry as well as system design and operating conditions. Pipe layout, flow velocities, installation practice, maintenance and system controls all have a role to play. Dissimilar metals also need to be considered because galvanic corrosion can lead to premature failure of less noble metals. Where corrosion is a concern, treatment options include orthophosphate, polyphosphate and sodium silicate, with or without zinc. Sacrificial or powered anodes can also provide corrosion protection in appropriate storage applications. Again, there is no universal chemical solution. Equipment manufacturers and water treatment specialists should be consulted before a treatment regime is selected.
Don’t overlook microbiological risk
Water treatment is not solely about protecting equipment. Commercial hot water systems also have an important health and safety responsibility. Biofouling, including the potential for Legionella, is the most critical health consideration in hot water system design. Non- domestic systems need to be managed with appropriate reference to HSG274 and the responsibilities of the relevant duty holders. There is an important distinction around
biocide use. Routine biocide additions should not generally be necessary for hot water systems, particularly those supplied from mains water. Where bacterial contamination is identified, physical treatment technologies such as ultraviolet light or ultrafiltration may be considered. Biocide additions, where appropriate, should be used for off- line disinfection during commissioning or before returning systems to service following maintenance. This reinforces the importance of risk assessment rather than automatically applying chemical treatment.
Commissioning is the starting point
Water treatment should form part of the commissioning process, not be bolted on afterwards.
In the guide, we recommend giving particular consideration to water treatment during commissioning because of the potential for scale deposition on hot surfaces. Manufacturers’ commissioning guidance should be followed, with system water quality tested after commissioning to ensure it meets the relevant equipment requirements. Where different manufacturers specify different water
10 BUILDING SERVICES & ENVIRONMENTAL ENGINEER SEPTEMBER 2026
Read the latest at:
www.bsee.co.uk
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