How to Descale a Coffee Machine, and Why Vinegar Is a Bad Idea

Use the descaler your machine's manufacturer recommends — usually a product based on citric or lactic acid — run the machine's own descaling cycle, and rinse it thoroughly with fresh water afterwards. Do it when the machine asks, or when brewing slows down and the coffee comes out cooler than it should. And do not use vinegar in an espresso machine: at least one major manufacturer says it damages the machine and voids the warranty.

What scale is

Scale is limestone. Tap water carries dissolved calcium and bicarbonate, and when that water is heated, they combine and precipitate as calcium carbonate, a hard white crust that builds up wherever the water gets hot.

In a coffee machine that means the heating element, the boiler or thermoblock, and every narrow pipe and valve the hot water passes through. The harder your water, the faster it forms. The water guide explains which minerals in water help coffee taste good and which ones form scale — they overlap more than people expect.

Why it matters

Scale is an insulator and an obstruction:

• It slows heating. A crusted element transfers heat poorly, so the machine takes longer and uses more energy.
• It lowers brewing temperature. Coffee comes out cooler, which under-extracts it and makes it taste flat or sour — see the brewing temperature guide.
• It slows the flow. Narrowed pipes mean longer brew times and inconsistent shots.
• It breaks things. Flakes of scale lodge in valves, pumps and sensors, and a machine that is never descaled eventually fails.

The signs

• The machine prompts you — many do, based on how much water has passed through them.
• Brewing takes noticeably longer.
• The coffee is cooler than it used to be.
• You see white, chalky deposits around outlets, in the reservoir or in the kettle.
• The machine is noisier — gurgling or straining.

What to use

The manufacturer's descaler is the safe default, because the wrong product can void your warranty. Most dedicated coffee machine descalers are built on relatively mild organic acids such as citric acid or lactic acid, which dissolve limestone without being harsh on seals and metals.

Not vinegar. Philips is unusually explicit: its descaling guidance says not to use products containing sulfuric, hydrochloric, sulfamic or acetic acid (vinegar), because they damage the machine and void the warranty. Even where vinegar does no harm, it leaves a smell and taste that takes several rinses to clear.

How to descale: the general method

Always follow your machine's manual first; the steps below are the general pattern.

1. Empty the machine. Remove coffee, empty the drip tray and the water tank.
2. Take out any water filter. Descaler can damage a filter cartridge, and the filter would absorb some of the descaler.
3. Mix the descaler with water exactly as the instructions say. More concentrated is not better.
4. Run the descaling cycle. Many machines have a dedicated programme. For a basic drip machine, run the solution through as if brewing, pausing partway so it can sit in the system for a while before finishing.
5. Rinse, then rinse again. Run at least two full tanks of fresh water through, including through the steam wand on an espresso machine. Taste the water; it should taste of nothing.
6. Put the filter back and reset the descaling counter if your machine has one.

Kettles

The same chemistry applies to an electric kettle: fill it with a citric-acid descaler solution, bring it to the boil, let it stand, then empty it and boil fresh water once or twice before using it. Since your kettle's water becomes your coffee, a scaled kettle is worth keeping on top of.

Espresso machines with boilers

Machines with boilers — heat exchangers, dual boilers and saturated-group designs — need more care. Heavy scale loosened by acid can break away in flakes and lodge in valves and small passages. For these machines, preventing scale with the right water matters more than descaling. Follow your manual — some say not to descale at home at all — and if a boiler machine is heavily scaled, have it serviced rather than flushing acid through it.

Prevention beats descaling

The cheapest descaling is the one you never need.

• Filter or soften your water. A filter jug or an in-machine filter removes some of the scale-forming minerals. Philips notes that its in-machine filter reduces how often descaling is needed.
• Set your machine's water hardness, if it has the option, so its descaling prompts match your water.
• Know your water. Your water supplier probably publishes hardness figures, and inexpensive test strips measure hardness and alkalinity at home.

Descaling is not cleaning

It is easy to confuse the two. Descaling removes mineral deposits from the water side of the machine. Cleaning removes coffee oils and residue from the coffee side — the brew basket, group head, carafe and steam wand — which go rancid and make coffee taste stale. Most machines need both, using different products, on different schedules.