Is softened water bad for your central heating boiler or water heater?

We are regularly told that suppliers of central heating boilers and water heaters advise against using softened water, on the grounds that it may even invalidate the warranty. The crux of this discussion is whether softened water can have a negative effect on water heaters and central heating boilers. The short and simple answer to that is: no.

For those who are interested, here is the more technical answer.

Properties of water

The four most important physical properties of water are:
1. Conductivity, expressed in 522 µS/cm (micro-Siemens per centimetre)
2. TDS (Total Dissolved Solids) expressed in PPM (parts per million)
3. Acidity expressed as pH
4. Hardness expressed in dH (German hardness)

Softened water versus filtered water

In the case of softened water (by means of a water softener) Ca²⁺ and Mg²⁺ are replaced by Na⁺. This naturally alters the composition of the water (no limescale formation), but fortunately this has little or no effect on the physical properties of the water.

Unlike a water softener, water filters do alter the composition of the water through the filtering process (removal of minerals, including Ca²⁺ and Mg²⁺). Minerals and other substances (such as chlorine) are filtered out of the water, and nothing is added in return! When water filters are used, the conductivity, TDS, acidity and hardness all decrease. This is, in itself, quite logical given that minerals and other substances are filtered out.This results in improved drinking water, but for boilers and central heating systems, this water is actually undesirable. This is because the boiler and other metal components, such as the heat exchanger, will rust (corrode) when water comes into contact with oxygen. .

An anode is therefore used in a water heater or central heating boiler to prevent corrosion of the appliance’s metal surface. This anode, often referred to as a sacrificial anode, is usually made of a different metal that corrodes more easily than the metal of the water heater or central heating boiler. The anode is allowed to corrode deliberately in order to protect the metal of the boiler.

Both the electrical conductivity and the acidity (pH) of the water affect how well the anode works and how effectively corrosion is prevented.

Electrical conductivity

Electrical conductivity: The sacrificial anode works by generating an electric current. If the water has low conductivity, the anode’s current generation may be impeded, which can reduce protection against corrosion. Sufficient conductivity is therefore necessary to ensure that the anode can effectively generate current and disrupt the corrosion process.
Acidity (pH): The acidity of the water, expressed as the pH value, can also affect corrosion. Water with a pH that is too low (acidic) can be more aggressive towards metal surfaces, thereby accelerating the corrosion process.

In short, to ensure that the anode in a water heater or central heating boiler works effectively and prevents corrosion, it is important that the water is sufficiently conductive to allow an electric current to pass through it and that the pH of the water falls within a certain range to minimise corrosion.

It is therefore important to prevent the central heating system and boiler from coming into contact with low-mineral water that has a low acidity and low conductivity. However, this is not an issue at all with water softening, and such water is perfectly suitable for use in central heating systems and boilers. Furthermore, softened water naturally prevents that troublesome limescale build-up, which reduces the efficiency and shortens the lifespan of the equipment. So-softened water is actually beneficial because it retains the desirable properties of hard water whilst preventing its undesirable effects.

With filtered water, however, the acidity and conductivity will deteriorate, and this water is therefore not recommended for central heating and water heaters.

Conclusion

Soft water is fine for the central heating system and the boiler, but filtered water isn’t.

Test results:
Unfiltered hard water (Spain) Filtered hard water (Spain):
Acidity: 7.76 pH 6.66 pH
Conductivity: 522 µS/cm, 363 µS/cm
TDS: 260 ppm 181 ppm
Hardness: 12.5dH 3.5dH

Hard water (NL) Softened water (NL):
Acidity: pH 7.25, pH 7.32
Conductivity: 382 µS/cm 383 µS/cm
TDS: 191 ppm 191 ppm
Hardness: 7.81 TP59T 0.01 TP59T