More
Heat pump installations are increasing rapidly across the UK, driven by changes to permitted development rules, the Boiler Upgrade Scheme and the government's target of 600,000 annual installations by 2028. As the market grows, so does the importance of getting water treatment right from the outset.
Heat pump systems operate differently to traditional boilers in ways that directly affect water quality requirements. Lower flow temperatures, different system materials and the presence of glycol antifreeze in many installations all create a set of considerations that need to be addressed at design or commissioning stage, not resolved on site after the fact.
This blog sets out why heat pump systems need particular attention when it comes to water treatment, what BS 7593:2019+A1:2024 now requires, and what specifiers and installers should be accounting for.
.png)
The most significant difference between a heat pump system and a conventional boiler is the operating temperature. Heat pumps typically deliver space heating at flow temperatures of 35 to 55 degrees Celsius, compared to 60 to 80 degrees for a gas boiler. This lower operating range has direct implications for water treatment.
Higher temperatures in traditional systems provide a degree of thermal protection against microbiological activity. Heat pump systems, operating at lower temperatures, do not benefit from this in the same way. The risk of microbiological fouling, biofilm formation within pipework and components, is therefore higher, and this needs to be specifically addressed through the choice of water treatment products.
The materials used in heat pump systems also differ. Many installations incorporate aluminium heat exchangers alongside copper pipework and steel radiators. A multi-metal system of this kind requires an inhibitor formulated to protect all of these
materials simultaneously. Not all inhibitors are designed for this purpose, and selecting an appropriate product is an important part of both the specification and commissioning process.
Heat pump heat exchangers also tend to have narrower internal passages than boiler equivalents, making them more sensitive to debris and contamination. Even a relatively small accumulation of magnetite or non-magnetic particles can restrict flow and reduce efficiency in ways that would be less significant in a boiler system.
Many air source heat pump systems use a glycol and water mixture in the external circuit to prevent freezing in the outdoor unit during cold weather. The presence of glycol affects water treatment in several important ways.
Standard corrosion inhibitors are formulated for water-only systems. In the presence of glycol, their performance characteristics change, and the required dosing concentration may differ. It is important to confirm that the chosen inhibitor is compatible with the specific glycol product in the system, and that dosing is adjusted accordingly.
Glycol also affects the viscosity of the system fluid, which in turn influences flow rates and the performance of in-line filters. Filter selection should account for the glycol concentration in the system to ensure that debris capture remains effective across the range of operating conditions.
Where glycol is present, its concentration should be checked regularly using a refractometer and recorded as part of the annual service record. This is particularly important because glycol degrades over time, and a reduction in concentration below the specified level compromises its protective function.
BS 7593:2019+A1:2024 now explicitly includes heat pump systems within its scope. This was one of the significant developments in the 2019 revision, and it means that the same framework of best practice that has long applied to boiler installations now applies equally to air source heat pump systems.
The standard's core recommendations apply in full. Systems should be chemically cleaned and flushed before commissioning, whether the installation is new or an existing system being converted to a heat pump. A permanent in-line filter should be installed, capable of capturing both magnetic and non-magnetic debris. Corrosion inhibitor should be dosed correctly at commissioning, with compatibility with any glycol in the system confirmed.
Because heat pump systems operate at lower temperatures, the standard also recommends the use of a biocide in these circuits to prevent microbiological fouling. This is a step that can be overlooked in installations where the focus is on the heat pump technology itself, but it is an important element of the overall water treatment approach.
An annual water quality test should be carried out at every service interval, checking inhibitor levels, pH, glycol concentration and system cleanliness. Inhibitor should be re-dosed at five-year intervals, or a laboratory water test carried out to confirm that ongoing protection remains effective.
Water treatment has a measurable impact on heat pump performance. Research by Synysav France found that correct water treatment can deliver energy savings of up to 27% in heat pump systems, compared to up to 17% in boiler-based systems. The difference reflects the greater sensitivity of heat pump systems to system condition because heat pumps rely on efficient heat exchange, any reduction in thermal transfer has a proportionally larger impact on performance.
For building operators and asset managers, this matters beyond the immediate energy cost. Heat pumps are typically installed as part of a long-term decarbonisation strategy, and their performance over time depends on the condition of the system in which they operate. A correctly specified and maintained water treatment programme is one of the most cost-effective ways to protect that investment.
From a specification perspective, this reinforces the importance of addressing water treatment explicitly at design stage. Leaving it as a site decision introduces the risk of inconsistent outcomes, products that are not suited to the system, and performance shortfalls that may not be immediately visible but will compound over time.
A specification that accounts properly for heat pump water treatment should reference BS 7593:2019+A1:2024 explicitly and note that its scope includes heat pump systems. It should name an appropriate inhibitor, one that is compatible with multi-metal systems and glycol solutions, and specify the requirement for a permanent in-line filter sized for the system's flow characteristics.
Where glycol is part of the system design, the specification should define the required concentration and confirm that the inhibitor selection is compatible. The requirement for biocide treatment in low-temperature circuits should also be stated clearly rather than left to site judgement.
Ongoing maintenance obligations, annual water testing, inhibitor re-dosing at five-year intervals, filter cleaning and glycol concentration checks, should be set out as part of the project handover documentation. Completion of the Benchmark commissioning checklist provides a formal record that the system has been installed and commissioned in line with recognised standards.
The growth of the heat pump market brings with it a corresponding need for water treatment knowledge and practice to keep pace. These systems are more sensitive to system condition than traditional boilers, operate in ways that require specific product choices, and are being installed at a scale and pace that makes consistent, high-quality commissioning more important than ever.
For specifiers, addressing water treatment at design stage is the most reliable way to ensure that the systems being designed will perform as intended throughout their operational life. For installers, understanding the additional considerations that heat pump systems introduce, glycol compatibility, lower-temperature biocide requirements, multi-metal inhibitor selection, is a fundamental part of delivering a compliant and effective installation.
