Water Hardness / Hard Water

What is water hardness?

Water hardness is caused by dissolved calcium and magnesium. Depending on local geology the hardness of the water supply will vary. The hardness of water, expressed in mg/l CaCO3 (calcium carbonate), can be classified as shown below:

Table 1 Hardness of Water


Water Hardness (mg/l CaCO3)


up to 100

Slightly hard

100 -150

Moderately hard

150 – 200


200 – 300

Very hard

More than 300

This map shows typical hardness of water supplies in England and Wales. For an accurate measurement ask your water company directly because there are areas with local variation not represented by the map.

Hard water causes scaling in hot water systems, kettles, electric irons and domestic appliances. Scaling is defined as deposits of calcium and magnesium that build up on heating elements over time. Scaling of heating elements shortens their life and makes appliances less efficient. Hard water produces less lather from soap, washing up liquid and washing powders. It also leaves ‘tide marks’ on basins, sinks, baths and toilets and a scum on the surface of hot drinks, especially tea brewed in the cup with a teabag (due to the air and oils in the tea).

Water hardness and health

The Drinking Water Directive and the UK drinking water quality regulations do not specify standards for hardness, calcium or magnesium. The World Health Organisation Guidelines 2017 identified that water with a hardness of value of 200 mg/l or higher (measured as CaCO3 calcium carbonate) will produce scale and soft water with a value of 100 mg/l (as CaCO3 calcium carbonate) or less will be more corrosive to pipework.

Where water companies artificially soften water before putting it into supply, it is recommended that they maintain a minimum total hardness of 150 mg/l (as CaCO3 calcium carbonate). This is because there is some limited evidence of a relationship between water hardness and cardiovascular health which may be related to the beneficial properties of magnesium and calcium in the diet.

Softening your water

Softening can be achieved by lime-soda softening, where the addition of lime (Ca(OH)2) and sodium carbonate (Na2CO3) to the water causes the hardness compounds to precipitate. An alternative method, common in domestic water softeners, is ion-exchange (base exchange), whereby the calcium and magnesium ions in the water are replaced by sodium ions. Where water is softened in this way by base exchange softening it is important to provide an unsoftened outlet for drinking purposes. Installation of a softener just before the hot water tank or boiler is a more economical method for preventing precipitation of hardness salts (limescale) than softening the whole supply.

Some devices are sold on the basis that they produce a magnetic field which reduces scaling by altering the shape of the crystals from needle like to rhomboid – which means they are less adherent to the heating elements in boilers. These devices do not soften the water. The science behind them is based on continuous water flow and they were designed originally for large industrial water systems, not for the household situation, where water tends not to flow for up to eight hours in 24 hours (at night). If you are considering purchasing such a device it is recommended you do so only on a sale or return basis, and that you request data on performance in the home setting.

Some activated carbon jug filters also contain ion exchange resin beads which alter the “temporary” hardness of water so that the filtered water has a lower tendency to form scum on the top of hot drinks like tea and they may minimise scale build up in kettles. These devices do not alter the permanent hardness of the water. “Temporary hardness” is water hardness due to the presence of calcium and magnesium carbonates and bicarbonates. These can be removed from the water by boiling, forming scale.

Should I use a water softener?

This is a matter of personal choice. If you live in a hard water area, then a softener will improve the efficiency and increase the life of domestic appliances. It will make lathering easier and reduce tide marks on sanitary ware.

If you do install a water softener, it is very important that you make sure that it is correctly installed, and you do not soften the water to the tap in your kitchen which is used for drinking and cooking. This is because most water softeners work by replacing the hardness with sodium. Too much sodium can be a problem for premature babies because their kidneys are not good at filtering it out of the blood, and for people who are on a low sodium (low salt) diet. Artificially softened water may also be aggressive to plumbing causing leaching of copper and lead.

When purchasing a water softener, we advise that you buy it from a reputable supplier. It should be installed only by a qualified plumber who is a member of a recognised Trade Association, such as the Institute of Plumbing. We also advise that you consider putting in place a maintenance contract to ensure the softener continues to operate correctly and does not become a hygiene hazard.

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