Cold water sources and water hardness

Where the water comes from

Mains water comes from two kinds of source.

• Surface water: rivers, reservoirs and lakes (in Scotland also lochs and burns).
• Groundwater: water held underground in water-bearing rock, called an aquifer. It comes to the surface at springs or is pumped up from boreholes and wells.

The balance changes from region to region. About a third of England's public supply is groundwater, mainly from the Chalk and from Permo-Triassic sandstones, and in south-east England groundwater provides over 75% of the supply. Some properties are not on the mains at all. About 1% of people in England and Wales use a private water supply, usually from a borehole, spring or stream, and it is regulated by the local authority rather than a water company.

Why some water is hard

Rain is naturally soft. Water becomes hard when it runs through or over rock such as chalk or limestone and dissolves calcium and magnesium from it.

• Groundwater drawn from chalk or limestone is usually hard.
• Water from upland reservoirs, moorland rivers and lochs has picked up little calcium or magnesium and is usually soft.
• Lowland rivers and lakes can be hard as well: Thames Water takes 65% of its water from rivers, yet all of it is hard because it has passed through chalk.

So the rock the water has travelled through matters more than whether it is surface water or groundwater.

Temporary and permanent hardness

• Temporary (carbonate) hardness comes from calcium and magnesium carbonates and bicarbonates. Heating or boiling turns it into limescale, so it is the hardness that scales kettles, elements and hot water pipes.
• Permanent (non-carbonate) hardness comes mainly from calcium and magnesium sulphates. Boiling does not remove it and it does not form scale when heated.
• Total hardness is the two together.

Measuring and describing hardness

Hardness is given as the equivalent amount of calcium carbonate in milligrams per litre (mg/l as CaCO3), which is the same number as parts per million (ppm). Appliance manuals sometimes use degrees instead: multiply mg/l by 0.07 for Clark (English) degrees, by 0.1 for French degrees or by 0.056 for German degrees. So 200 mg/l is 14 degrees Clark.

There is no single agreed set of names. The Drinking Water Inspectorate (DWI) uses:

• soft: up to 100 mg/l
• slightly hard: 100 to 150 mg/l
• moderately hard: 150 to 200 mg/l
• hard: 200 to 300 mg/l
• very hard: over 300 mg/l

Several water companies split the DWI's soft band into soft (up to 50 mg/l) and moderately soft (51 to 100 mg/l). Work from the number, not the name, and get the figure for the address from the local water company.

Hardness, health and the rules

• There is no drinking water standard for hardness, calcium or magnesium, and the DWI says there are no health-based guidelines for it.
• There is a standard for sodium: 200 mg/l at the consumer's tap in England. A salt softener swaps hardness for sodium, which is why softened water is kept off the drinking tap. Do not confuse this 200 with the 200 ppm hardness figure below.
• In England, Approved Document L treats water with a total hardness above 200 ppm as CaCO3 as hard. In hard water areas it expects suitable measures to treat the feed water to water heaters and to the hot water circuit of combination boilers, to reduce limescale. The 2026 edition, which takes effect on 24 March 2027, keeps the same wording.
• Boiler makers may give their own advice: one bases it on temporary hardness above 200 ppm rather than total hardness, so read the appliance instructions as well.

What hardness does to a system

• Scale builds up on immersion heater elements and cylinder coils, at the base of cylinders and calorifiers, in the hot water circuit of combination boilers, in hot pipes and in thermostatic mixing valves. It shortens element life, lowers efficiency and can restrict the flow.
• Hotter water deposits more scale, especially above 60 °C. One unvented cylinder maker allows 70 °C but advises 60 to 65 °C in very hard water. Stored hot water must still reach at least 60 °C for Legionella control, so set 60 °C rather than hotter.
• Very soft water, 100 mg/l or less, and artificially softened water are more corrosive and can dissolve lead, copper and brass. Harder water tends to lay a thin protective scale inside pipes. Where old lead pipes are a risk, water companies add orthophosphate to cut plumbosolvency (lead dissolving into the water).

Softening and scale reduction

An ion-exchange (base-exchange) softener takes calcium and magnesium out of the water and puts sodium in, and is regenerated with salt. When one is fitted:

• keep an unsoftened supply to the drinking water tap over the kitchen sink by taking that branch off the supply pipe before the softener. The Building Regulations allow softened water whose sodium is above the drinking water limit at basins, baths and showers, but not at drinking points or food-preparation sinks, and the DWI advises keeping the kitchen tap unsoftened;
• notify the water undertaker before fitting it, because it uses water to regenerate and produces waste water;
• fit a servicing valve on its inlet and backflow protection rated for at least fluid category 2, which is how the official guidance classes a domestic salt-regenerated softener; a single check valve carries that rating;
• run its waste to a drain through an air gap, never by a direct connection;
• check the boiler instructions before filling a heating system with softened water: one maker forbids salt-softened fill water and asks for an untreated filling point or a bypass at the softener;
• look after it: keep it topped up with salt and backwashed. Water Regs UK lists inspection every 2 months and routine maintenance every 6 months, and a softener left unused in an empty property needs servicing before use.

Softened water should not be used to make up baby feeds. The DWI notes that softening only the water going to the cylinder or boiler is a cheaper way to stop scale than softening the whole supply.

Scale reducers, such as magnetic units and chemical scale inhibitors, aim to slow scale or stop it sticking; they do not soften the water. The DWI suggests asking for evidence that a magnetic unit works in a home, where water can stand still for hours overnight.