Why Do Kettles Get Limescale?

If you live in a hard-water area, you’ll know the chalky crust that builds up inside a kettle. It’s limescale, and it comes from the water, not the kettle.

How limescale forms

As rainwater soaks through rocks such as limestone and chalk, it dissolves calcium and magnesium. Water with a lot of these minerals is called hard water. The USGS classes water above about 120 mg/L of hardness as hard and above 180 mg/L as very hard.

Much of the calcium in hard water is dissolved as calcium bicarbonate. When the water is heated, carbon dioxide is driven off and the calcium bicarbonate turns into calcium carbonate, which doesn’t dissolve. It comes out of the water as solid particles that stick to the hottest surfaces, especially the heating element or base plate. Each time you boil the kettle, a little more is added.

The type of hardness that boiling removes this way is called temporary hardness. Permanent hardness, mainly from calcium and magnesium sulfates, doesn’t form scale in the same way when water is boiled.

How to tell if your water is hard

• Look in the kettle. White or off-white crusts and flakes are the giveaway.
• Soap test. Shake a few drops of pure liquid soap with water in a bottle. Hard water gives little foam and cloudy water; soft water foams well.
• Ask your water supplier. Most publish the hardness for your area, or use a hardness test strip.

Is limescale bad for you?

No. The World Health Organization does not set a health-based guideline value for water hardness, because hardness is not of health concern at the levels found in drinking water. It treats hardness as a matter of taste and acceptability, not safety. Drinking water can even be a small source of calcium and magnesium in the diet.

Flakes of scale in a cup of tea aren’t harmful, but they’re unpleasant, and scale can give water and tea a chalky taste or a film on the surface.

How to descale a kettle

Limescale dissolves in mild acids. Always check your kettle’s manual first, rinse thoroughly afterwards, and boil and discard a kettle of fresh water before using it again.

Citric acid (best for most kettles)

1. Fill the kettle about half full with water and boil it.
2. Switch off, unplug, and add one to two tablespoons of citric acid powder (sold in supermarkets or as kettle descaler).
3. Leave for 15 to 30 minutes, or until the scale has dissolved.
4. Pour away, rinse well, then boil fresh water once and pour it away.

White vinegar

1. Fill the kettle with equal parts white vinegar and water, enough to cover the scale.
2. Bring to the boil, then switch off and leave for an hour or so.
3. Pour away, rinse thoroughly and boil fresh water once or twice to remove the vinegar smell.

Lemon juice works the same way as citric acid but is weaker.

Commercial kettle descalers

Follow the label. Many are based on citric or other food-safe acids.

Don’t scrape the element or base with metal tools; you can damage the coating.

Does limescale make a kettle less efficient?

It can. Scale on the heating element acts as a layer of insulation, so heat passes into the water a little less easily and the element runs hotter. Over time, heavy scale can shorten the life of an electric kettle and make it noisier. Regular descaling helps the kettle last.

How to prevent limescale

• Only boil as much water as you need, and empty the kettle after use rather than leaving water standing in it.
• Descale little and often, before scale builds up thick layers. In very hard water areas that may be every couple of weeks.
• Use a kettle with a built-in scale filter at the spout. It doesn’t stop scale forming, but it stops flakes reaching your cup.
• Use a filter jug. Many filter jugs with ion-exchange cartridges reduce temporary hardness and slow scale build-up.
• Install a water softener if hard water is a problem throughout your home. Some people prefer to keep an unsoftened tap for drinking water, since softeners add sodium.

See also what are the different types of kettles?