Page 2 - INDEX Winter 2024

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WA TER B Y DE SIGN

“In place of chemicals to treat water in a closed
loop system it is far better to fill the system with
demineralised water and then to maintain it in
this non-corrosive state using electro-chemistry”
writes David J Whitfield, EngTech LCIBSE, one of
the contributors to CIBSE's Code of Practice on
Heat Networks and managing director of IWTM
UK Limited.

One of the most significant changes included
in CIBSE's revised Code of Practice on Heat Networks (CP1 (2020)) is the inclusion of a reference to the technology
advocated within VDI 2035.

VDI 2035: Prevention of damage in water heating installations is published by the Association of German Engineers
(Verein Deutscher Ingenieure) and sets out how corrosion in closed loop heating systems can be prevented without
the use chemical inhibitors or biocides. Many major HVAC manufacturers reference systems being filled to VDI2035
standards in their manuals and/or warranty documents.
Chemical-free Water Treatment (CFWT) methodologies, employed effectively since the 1970s and affirmed by VDI2035,
are centered on the control of three factors crucial to the corrosion process:

• Dissolved oxygen (DO), pH and conductivity

When these three factors are effectively controlled a non-corrosive state can be established and maintained within
closed-loop hydronic systems.

VDI 2035 promotes the use of demineralised water and in part, electrochemical water treatment. These include the use of
ion exchange resins and electrochemistry to maintain the system in a non-corrosive state.

Corrosion occurs in heating and cooling systems when they are filled (and kept topped up) with untreated tap water
water. This contains minerals, salts and gases that can react with the metal components that make up a system. The
resulting reaction can lead to the formation of sludge deposits, limescale and rust.

Corrosion reactions in hydronic systems are primarily determined by the presence of oxygen, which is why it is important
that the oxygen concentration should be as low as possible. Oxygen enters the system during filling; 1 litre of raw water
contains approximately 8-11mg of dissolved oxygen. Generally, in a sealed system the dissolved oxygen from the initial
fill will dissipate through limited corrosion with the metal pipework and components in a short period of time, usually
without significant damage.

Systems with closed diaphragm expansion vessels are recommended because they help limit the entry of air. If a
system is not sealed effectively, or if the system requires frequent top-ups, or if oxygen can diffuse through permeable
seals and pipes, then corrosion will continue to be a problem for mild steel elements, such as pipework, where it will
form iron oxide, FE3O4, which can manifest as sludge.

To help limit corrosion, sludge and scale formation, the conventional approach in the UK has been to dose systems with
chemicals. One of the problems with this is that some corrosion inhibitors raise the conductivity of system water and
can, in the event of over-dosing or under-dosing, lead to additional corrosion. Corrosion inhibitors can also result in
the formation of biofilms, which can facilitate bacterial corrosion. It is well documented that some chemicals provide
a source of nourishment for bacteria. Ironically the proposed solution to this is often to add yet more chemicals to the
cocktail, which can add to the problem if not handled correctly.

In addition to being an ongoing expense, chemical protection regimes require a range of health and safety
procedures to be followed relating to the use, storage and disposal of chemicals. Should a leak or pipe failure occur
in a district heating system for example, the chemically treated water can present a pollution threat or leach into the
water table.