VDI2035 and how to go about it

Most homeowners never think about the water inside their heating system — and that is entirely understandable. But for heating engineers who care about doing things properly, water treatment is one of the most important elements of any heat pump installation. Get it wrong and you risk corrosion, sludge build-up, reduced efficiency, and potentially significant damage to your system over time. Get it right and your heat pump will run cleanly and efficiently for decades.

At Clean Heat Solutions, we follow the VDI 2035 standard on every system we install — here is what it means, why it matters, and what we do about it.

What Is VDI 2035?

VDI 2035 is a technical guideline developed by the Association of German Engineers that sets out the water quality requirements for closed-loop heating systems. It focuses on preventing two of the most common causes of heating system damage — scaling and corrosion — by controlling the properties of the water inside the system itself, rather than relying on chemical inhibitors to manage the problem after it has started.

Whilst the UK's own standard, BS 7593:2019, has traditionally recommended the use of chemical inhibitors and inline filters, VDI 2035 takes a fundamentally different and more proactive approach. It is already standard practice across Germany, Austria, and Switzerland, and it is now rapidly gaining traction amongst the most technically advanced heat pump installers here in the UK.

Why Does Water Quality Matter in a Heat Pump System?

Corrosion in a heating system is an electrochemical process — it requires the system water to act as a conductor. The higher the mineral and salt content of the water, the higher its electrical conductivity, and the greater the risk of corrosion taking hold. Over time, this leads to the formation of sludge and magnetite deposits that reduce heat transfer, block components, and force your heat pump to work harder than it needs to — directly increasing your running costs and shortening the lifespan of the system.

Scaling is a separate but equally damaging issue, caused by calcium and mineral deposits building up inside pipework and heat exchangers — reducing efficiency and, in serious cases, causing localised overheating and component failure.

Both problems are entirely preventable. VDI 2035 shows us how.

How Does VDI 2035 Work?

VDI 2035 is built around two key principles — controlling conductivity and maintaining the correct pH level in the system water.

Using Demineralised Water Rather than filling your heating system with standard mains water — which in much of West Sussex and Hampshire is notably hard, carrying high levels of calcium and mineral content — VDI 2035 specifies the use of demineralised water. By removing the ions that enable corrosion and scaling, the water inside the system becomes a poor medium for electrochemical reactions, stopping the problem at its source rather than attempting to manage it with chemicals.

Maintaining the Correct pH VDI 2035 specifies that system water should be slightly alkaline, with a pH between 8.2 and 10. Importantly, in most cases the system will naturally self-alkalise to within this range within a few weeks of operation — meaning no chemical additives are typically required to achieve the correct balance.

Monitoring Conductivity Once filled correctly, the system should be monitored using a digital conductivity meter. Target conductivity is below 100 µS/cm. This is a straightforward check that takes minutes and gives a clear, reliable picture of the system water's health.

Why Does This Matter More for Heat Pumps?

Heat pumps are significantly more sensitive to poor water quality than traditional boilers. They operate at lower flow temperatures, contain more sophisticated components, and are designed to run continuously — meaning any inefficiency caused by sludge, scaling, or corrosion compounds over time and has a direct impact on performance and running costs. Many heat pump manufacturers are also increasingly specifying demineralised water and VDI 2035 compliance as a requirement for warranty validity.

As a Heat Geek Elite approved installer, we believe that cutting corners on water treatment is simply not an option — which is why we incorporate VDI 2035 methodology into every installation we carry out across West Sussex and Hampshire.

What Does This Mean in Practice?

Before we commission any heat pump system, we flush the existing pipework thoroughly to remove any existing dirt or sludge. We then fill the system with demineralised water, test conductivity and pH, and document the results. We also advise on ongoing monitoring and, where appropriate, recommend inline demineralisation filters to maintain water quality over the long term — particularly in areas of West Sussex and Hampshire where mains water hardness is high.

The result is a cleaner, more efficient system from day one — and a heat pump that will continue to perform at its best for years to come.

If you have any questions about water treatment or would like to discuss a new heat pump installation, get in touch with our team — we are always happy to help.