Water is the primary medium in almost every heating system – it transports heat from the boiler or heat pump to radiators, underfloor heating, or the domestic hot water storage tank. Other heat transfer fluids are rarely used. However, not all water is equally suitable for this purpose. Demineralized water is increasingly used in modern heating systems, but it's a double-edged sword if the demineralization station proves too effective for our needs. Improperly used demineralized water can also cause problems and lead to defects in some components of the heating system.
Demineralized water (also known as demineralised or demineralised) is water from which almost all mineral salts – such as calcium, magnesium, sodium, iron, or chlorides – have been removed. It is obtained through processes like reverse osmosis, ion exchange, or distillation. As a result, this water is almost completely free of the components that cause scale deposition and corrosion.
Why is this done? To remove from the water the elements responsible for its hardness. As we know, hard water is the cause of scale, so it is undesirable. However, demineralized water can also be harmful if too many minerals are removed from it - and if it reaches too low a pH. So when can it be used to avoid causing problems?
First, there are several positive aspects of demineralized water:
✅ Here, however, we must note that extremely demineralized water with a water pH below 7 becomes acidic and starts to be aggressive towards metals. Does it really provide protection against corrosion? Then, quite the opposite: Overly demineralized water can also be a cause of corrosion, attacking especially steel, heat exchanger solders, or welds on other devices (also steel ones).
✅ To actually protect against corrosion instead of causing it, one should not overuse demineralization stations, choose ion exchange filters with resin that does not require the use of salt, and use corrosion inhibitors.
Although demineralized water has many advantages, its improper application can also lead to problems and very often is the cause of premature defects in steel or cast iron devices, shortened lifespan of heat pump or boiler components. Very often, ion exchange filters are used without real justification, in places with only slightly elevated water hardness, while overly demineralized water can cause bigger problems than hard water and degrade equipment much faster.
Due to its very low mineral salt content, such water has a greater capacity for dissolving metals (to achieve ionic balance) which can accelerate corrosion or cause leaks in installation components – especially when the water pH is too low (in particular, the process of metals being attacked by demineralized water accelerates below pH = 7, i.e., when dealing with acidic water). Very often, the victims of this property of low pH water are heat exchangers, as devices with thin layers of steel.
Additionally, water completely devoid of ions can affect the electrical conductivity in the system, hindering the proper operation of some sensors and safety devices. To avoid these threats, it is recommended to maintain an appropriate pH level (usually 8–9) and periodically check the water quality in the installation according to the VDI 2035 standard.
Furthermore, it's worth remembering properly selected corrosion inhibitors. Many professional practitioners are directly opposed to using demineralized water and treat it as a last resort, or assume that the selection of other system components must be adapted to it. In most cases, replenishing corrosion inhibitors is forgotten, leading to system leaks and failures of devices operating within it.
More and more boiler and heat pump manufacturers require the use of water with specific parameters – sometimes this is precisely demineralized or softened water. Failure to meet these recommendations may result in the loss of the device's warranty. Therefore, before filling the installation, it is always worth checking the water parameters recommended by the manufacturer. Most often, however, this does not refer to water that is too effectively stripped of minerals, but simply to water that will not have a harmful degree of hardness. Certainly, in such situations, the preferential pH of the water is specified, which cannot be acidic.
Therefore, these two issues should be distinguished, as they are not the same. The problem is rather the overly intensive demineralization of water - achieving a medium devoid of ions. Sometimes the reason for this is oversizing (i.e., improper selection) of the devices used for this purpose, and sometimes using them in places where there are no real indications, meaning the water is not extremely hard. Unfortunately, many failures of hydraulic devices in installations with this type of water are recorded.
Demineralized water is obtained using devices designed for this purpose:
Filters with ion exchange resins vary, and there are different resins as well. Some require the use of salt. It's good to know that this salt certainly does not positively affect the lifespan of delicate devices, such as heat exchangers. It is worth choosing, if you decide on an ion exchange filter - a solution that does not require the use of salt.
Using demineralized water in heating systems - if done thoughtfully - is an investment in the durability, efficiency, and safety of the entire system - but it is a solution that often radically changes the conditions in the heating system, so it should be carefully considered and motivated by real needs. Demineralized water protects installation components from scale and corrosion, and also ensures compliance with the requirements of modern heating equipment manufacturers - but it can also be the cause of problems greater than hard water, which, although it damages installations, does so slowly.
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