Hard water and limescale: testing, descaling and softening options

If you live in a hard-water area, you already know the signs. There's a white crust around the base of the tap, a kettle that rattles when you pick it up, cloudy glasses out of the dishwasher and a shower head that sprays in three directions. In a kettle, limescale is mostly a cosmetic irritation. In a combi boiler or a washing machine element, it shortens the life of expensive parts. Here's how to find out how hard your water is, where scale costs you, how to shift it safely and how the main treatment options compare.

What hard water actually is

Rain is naturally soft. As it soaks through ground containing chalk or limestone, it dissolves calcium and magnesium, mainly in the form of bicarbonates. That dissolved mineral load is what we call hardness. Water from granite, peat or upland catchments tends to be soft. Water drawn from chalk and limestone aquifers tends to be hard.

Hardness comes in two kinds:

• Temporary hardness comes from calcium and magnesium bicarbonates. When the water is heated, they break down and drop out as calcium carbonate, the chalky solid we call limescale.
• Permanent hardness comes from other salts, such as sulphates, that stay dissolved when heated.

Temporary hardness is the one that blocks things up, which is why scale builds fastest wherever water gets hot: kettle elements, boiler heat exchangers, immersion heaters and washing machine elements.

Hard water is not a health problem. Plenty of people prefer the taste, and it's perfectly fine to drink. The issue is what it does to equipment.

Finding out how hard your water is

• Ask your water supplier. Most publish water quality data by postcode or supply zone, and hardness is almost always included. Watch the units. Hardness may be given in milligrams per litre as calcium carbonate, or in degrees, and German, French and English (Clark) degrees are three different scales. Don't compare a figure in one unit with advice written in another.
• Use test strips. Inexpensive hardness strips change colour when dipped in a glass of cold tap water. They're approximate, but more than good enough to tell soft from moderately hard from very hard. Dishwashers and softeners sometimes come with one in the box.
• Read the house. A kettle element furred up within a few weeks, white tidemarks on a glass shower screen, soap that won't lather and a grey ring round the bath all point the same way.

On a private supply such as a well or borehole, hardness can vary more through the year, and a laboratory test alongside other water-quality checks is sensible.

Where scale does the damage

Kettles. Scale insulates the element, so boiling takes a little longer, and flakes end up in your tea. Easy to fix, and a good early warning of how hard your water is.

Shower heads. Scale narrows the tiny outlets until jets block, spray sideways or dribble. A badly scaled head can make a perfectly good supply feel weak.

Tap aerators. The small mesh insert inside the spout traps scale and grit. When it clogs, the flow drops and the stream sputters. A scaled aerator is one of the first things to check when one tap seems feeble; our guide to low water pressure causes covers the others.

Tap cartridges and washers. Scale makes ceramic cartridges stiff and stops washers seating properly, and the result is a tap that drips however hard you turn it. If that's where you've ended up, see our leaking tap repair guide.

Combi boilers. In a combi, mains water runs through a compact plate heat exchanger every time a hot tap opens. Scale narrows the passages inside it. Typical symptoms are hot water that swings between hot and lukewarm, a kettle-like rumbling or banging, and a longer wait for hot water. The heating side is a sealed circuit that doesn't scale in the same way, though it has sludge problems of its own; our hub on radiator cleaning and combi boiler care deals with those.

Hot water cylinders. Immersion heater elements fur up, get noisy and eventually fail. The base of the cylinder collects loose scale.

Washing machines and dishwashers. Elements scale over time. Dishwashers have their own small built-in softener that runs on dishwasher salt, and it only works properly if the hardness setting in the menu matches your local water.

Descaling safely

Two mild acids do most household descaling: citric acid, sold as a powder or as a branded descaler, and white vinegar. Both are gentle on people but not harmless to every surface. A few ground rules:

• Never mix a descaler with bleach or any other cleaning product.
• Wear rubber gloves, keep the room ventilated and rinse everything well afterwards.
• Follow the appliance manufacturer's instructions where they exist. They know their seals and finishes.

Kettle

Half-fill with water, add a tablespoon or two of citric acid powder, bring it to the boil and leave it for half an hour. Pour away, rinse, then boil a full kettle of clean water and discard that too.

Shower head

1. Unscrew the head from the hose or arm.
2. If it has flexible rubber nozzles, rub them firmly with your thumb first. A lot of scale flicks straight out.
3. Soak the head in warm citric acid solution for 30 minutes to an hour.
4. Scrub the face with an old toothbrush, and clear stubborn holes with a pin.
5. Rinse, refit, and run the shower for a minute before using it.

For a fixed head you can't remove, fill a plastic bag with solution, pull it up around the head and secure it with an elastic band.

Aerators and chrome taps

Unscrew the aerator, soak it in citric acid solution and brush it clean. On the tap itself, vinegar or citric acid works on chrome, but keep contact short: wipe it on, leave it a few minutes, then rinse and dry. Long soaks can dull plating. Brushed, black, gold-coloured and other coated finishes are more delicate still, and many manufacturers say mild soap and water only. Keep acids off natural stone such as marble and limestone around the basin, because they etch it.

Washing machines and dishwashers

Run an empty hot cycle with citric acid or an appliance descaler, following the manufacturer's dose. Clean the dishwasher filter while you're there.

Boilers and cylinders

Not a DIY job. Chemically cleaning or replacing a combi's plate heat exchanger means opening a gas appliance, which needs a properly registered or licensed engineer under your country's rules. The same applies to unvented cylinders.

Treatment options compared

Which approach suits your home

• Scale is mostly a nuisance in the kettle and on taps. Regular descaling and a filter jug for the kettle may be all you need.
• You have a combi boiler in a hard-water area. A scale inhibitor on the cold feed to the boiler is common practice, and in some countries installation guidance calls for one on new combis in hard-water areas. Ask your installer what applies locally.
• Scale is everywhere: showers, glass screens, appliances, cylinder. If you own the home and have space under the sink or in a utility room, an ion-exchange softener is the only option here that actually removes hardness.
• You rent. A whole-house softener needs the landlord's permission and plumbing work. Point-of-use filters, a shower-head filter and a descaling routine are realistic alternatives.
• You're tempted by a magnetic or electronic device. It's inexpensive to try and won't do harm, but don't rely on it to protect a boiler.

Softened water and drinking

An ion-exchange softener adds sodium to the water in proportion to the hardness it removes. For most healthy adults on a normal diet the extra amount is small, but anyone on a low-sodium diet should ask their doctor, and public health advice in several countries says not to use softened water for making up infant formula. The usual arrangement is to leave one kitchen tap on the unsoftened mains supply for drinking and cooking. Installers also commonly leave the outside tap unsoftened, since the garden doesn't need it.

Some people find softened water feels slippery in the shower. That's normal: soap rinses off less readily when there's no hardness to react with, so you can use less of it.

Salt, regeneration and upkeep

A softener's resin gradually fills with calcium and magnesium. During regeneration, usually at night, the unit rinses the resin with brine from its salt reservoir and flushes the waste to a drain. Metered units regenerate based on how much water you've used, which wastes less salt and water than older timer-only models.

• Use the right salt. Block or tablet salt made for softeners, as the manufacturer specifies. Not table salt, and never road salt.
• Check the level monthly and top up before it runs low.
• Watch for salt bridging. A hard crust can form above the water level, so the reservoir looks full but no salt is dissolving. Break it up gently with a broom handle.
• Test the softened water with a strip every few months to confirm it's still doing its job.
• Know where the bypass valve is, so you can isolate the unit during a fault or service.
• Have it serviced as the manufacturer recommends. The resin lasts many years but can be fouled by iron or other contaminants.

The drain connection needs an air gap so waste water can never be drawn back into the supply. That's part of a proper installation, and one more reason to have a softener fitted by someone who knows the local water regulations.