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Limescale is building up in the machine or kettle

White chalky deposits appear inside the kettle or reservoir, heat-up takes longer, or flow through the machine has dropped.

Applies to: Espresso machine, Espresso, Filter coffee

Where limescale comes from

CoffeeHQ explanation

Tap water carries dissolved calcium and magnesium. Part of it, the part paired with bicarbonate and called temporary hardness, stays dissolved only while the water is cool. Heat the water and that part comes out of solution as solid calcium carbonate. It settles wherever the heating happens: on a kettle element, on the walls of a boiler, inside a thermoblock's narrow channel.

Scale does not flavour coffee directly. It harms the machine. It insulates the heating element, so the machine heats slowly and wastes energy. It narrows pipes and valves, so flow and steam weaken. Flakes can break off and block small openings. How fast it forms is set by two things only: how hard the water is and how much of it you heat.

Causes, most likely first

There are 3 likely causes, listed with the most probable first. Work down the list and change one thing at a time, so that when the problem goes away you know which change fixed it.

  1. Cause 1 of 3 · Commonly the cause

    Hard water with no treatment

    How to check
    Do you know how hard your water is?
    What to change
    Test the water hardness or check your supplier's figure, then choose filtration or descaling frequency to match.

  2. Cause 2 of 3 · Commonly the cause

    Descaling interval too long for the water

    What to change
    Shorten the interval on hard water, following the manufacturer's guidance for the product and the machine.

  3. Cause 3 of 3 · Reported, not independently verified

    Filter cartridge past its life

    What to change
    Replace on schedule; performance falls off gradually rather than visibly.

If none of these fixes it

Where a manufacturer advises against descaling, or where flow does not recover after descaling, consult the manufacturer or an authorised technician rather than attempting further chemical treatment.

Work through it for your setup

The causes above are general. These go to the part of a diagnostic that takes this symptom one check and one change at a time.

  1. A white crust in the kettle, or a machine that asks to be descaled oftenThe possibilities, the quickest check and one thing to change first
  2. What builds up: scaleWhat it does to the cup or the machine, and how you would notice

Find out how hard your water is

Everything else follows from this, and it is easy to learn. Your water supplier publishes a quality report for your area, and inexpensive test strips or drop kits give a reading at home. With a hardness figure, the interval your manual suggests can be read against your actual water instead of taken as a calendar rule. A soft-water home may need to descale very rarely; a hard-water home, often.

Ways of treating the water, and what each leaves alone

Tendency, not a guarantee

Jug filter with ion-exchange resin
Reduces temporary hardness, so it slows scale. The cartridge has a limited life and stops working quietly when it is spent.
In-tank softening pouch or cartridge
Works the same way inside the machine. Replace it on the maker's schedule; an exhausted one does nothing.
Carbon-only filter
Removes chlorine and odour and does not reduce hardness. It improves taste and does nothing about scale.
Low-mineral bottled or blended water
Slows scale according to its mineral content. Check the machine's stated water specification, because very pure water is not suitable for every machine.

Descaling is the manufacturer's procedure, not a general one

Some machines are designed to be descaled at home with a named product and a set routine. Others, particularly those with large boilers, are not, and their makers say so, because loosened scale can block the machine and, in one maker's words, descaling chemicals can corrode the metals inside and are hard to rinse out. Read the manual before using any descaler, use the product and quantity it specifies, and treat prevention as the main defence.

Easily confused with

  • Coffee oil build-up — Oil collects where coffee touches the machine and makes coffee taste bitter and stale. Scale collects where water is heated and makes the machine slow and weak. Different places, different products.
  • Weak steam — Scale is one cause of a wand that has lost power, but a blocked tip is commoner and easier to check first.

What not to change at the same time

Do not change your water treatment and your descaling interval at once. Change the water, then watch how quickly scale appears in the kettle over a few weeks before deciding how often the machine needs attention.

How to tell it is fixed

A kettle used with the same water stays free of white crust between washes, and the machine's heating time and steam strength stop declining.

Water suitability helper — Enter your hardness and alkalinity. It treats scale risk and flavour as separate questions, because a water can be gentle on a machine and still flatten a coffee.

Related issues

Work it out

Interactive tools that take you through this one change at a time.

Learn the technique

Step-by-step guides to the skills this problem comes from.

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