Water hardness, lime scaling from hot water use, water softeners, particulate filters and the quality of fill water in closed heating systems (VDI 2035) are everyday plumbing work. Here we gather how water quality affects the installation — and what to do about it.
Introduction
Water is not just water. Its content of lime, oxygen and particles determines whether an installation will scale, corrode or silt up. Water treatment is about adapting the water so it does not damage the system it flows through — both on the drinking water side and in closed heating systems.
Water hardness (°dH)
Hardness is a measure of the water's content of calcium and magnesium and is specified in Denmark in German degrees of hardness, °dH. The harder the water, the more scale can be precipitated. Hardness varies geographically with groundwater: Western Jutland generally has softer water, whilst Zealand and the eastern regions often have hard to very hard water. The local value can be looked up on GEUS' water hardness map or from the water company.
| Designation | Hardness (°dH) |
|---|---|
| Very soft | 0–4 |
| Soft | 4–8 |
| Medium hard | 8–12 |
| Rather hard | 12–18 |
| Hard | 18–30 |
| Very hard | over 30 |
The classification above is the standard °dH classification, which among others GEUS and water companies use to describe Danish drinking water; the specific limit for when water softening is recommended depends on application and local practice. (Source: GEUS — Denmark's water hardness map; drinking water regulation — guidance classification.)
Lime precipitation with hot water
Limescale (calcium carbonate) is more likely to precipitate, the warmer the water is. Therefore, limescale deposits occur precisely on the hot surfaces: in the hot water tank, on the immersion heater, in the exchanger and in the flow-through water heater. The precipitation accelerates at high temperatures, and a layer of limescale on a hot surface acts as an insulator — it reduces heat output, increases energy consumption and can in the worst case clog narrow channels and fittings.
Softening
A softening plant removes hardness by ion exchange: the water is passed through a vessel with ion exchange resin that exchanges calcium and magnesium ions for sodium. When the resin is saturated, it is automatically regenerated with a salt solution (hence the salt tank). The result is softer water that precipitates less lime — beneficial for tank, heat exchanger, and fittings.
- 01Water softening reduces lime, but adds sodium and removes minerals — therefore drinking water is not softened indefinitely. Some waterworks soften centrally to a sensible level rather than completely soft water.
- 02Water softening systems must meet requirements for materials and hygiene (including DS/EN 14743 for ion exchange systems and material compatibility according to DS 439:2024).
- 03A water softening system is an installation with still water and salt — it must be maintained so it does not itself become a hygiene problem.
Filtering
Particles — sand, rust, mill scale — wear on fittings, valves and seals and can lodge in thermostat valves and solenoid valves. A particle/fine filter on the inlet catches them before they reach the installation. Filters must be rinsed or replaced, and fineness (mesh size) is chosen according to what needs protection. On exposed installations, filter and any softening are combined on the inlet.
Members only
The rest of the article is locked
Create a free user account to unlock all articles, calculators, templates and range — and get access to quizzes and classes. It takes less than a minute: name, email and a password. No payment card.
Want to try something first? See what you can use without an account — the junior zone, the encyclopedia, the videos and more.