Why descaling matters
Any machine that heats water builds limescale. The deposit is calcium carbonate, and it forms when the bicarbonate dissolved in tap water is heated and falls out of solution onto hot metal. It coats the boiler and narrows the water paths, so the machine runs cooler and slower than it was built to.
Descaling dissolves that deposit with an acid. Keep to a schedule and your shots stay consistent. Skip it and the coffee goes off first. The repair bill comes later.
How often depends on your water, not on a number
Almost every guide says every one to three months, and ours said it too until this update. The number is not so much wrong as averaged across places that have nothing in common.
We can be more specific than that, because we already gathered the data for something else. Our Espresso Water Map reads the published water-quality reports for the 100 largest US cities, and 81 of them publish a hardness figure. The thing that scales a boiler is carbonate hardness, the fraction that precipitates when heated, which we take as the lesser of total hardness and alkalinity. Put those figures against a machine pulling two double shots a day and the intervals come out a long way apart.
Only 27 of the 81 cities land inside the one-to-three-month window that everyone publishes. Fifty-four of them need it less often than that.
San Antonio water runs near 300 ppm, which works out at roughly five weeks. Phoenix comes out at nine. Chicago is sixteen and Los Angeles fourteen. New York, Boston, Seattle and Portland are soft enough that the model runs into its twelve-month ceiling.
Philadelphia is the case that shows why total hardness on its own misleads. Its water measures 98 ppm hard, which reads as middling, but its alkalinity is lower, and alkalinity is the part that turns into scale. The interval lands near nine months.
Two limits on all of that, and they matter. The interval is built to compare cities rather than to prescribe for your machine: it assumes two doubles a day, and both your manual and your machine's own descale alert outrank it. If you already run a tank filter or filtered water, you have changed the number, which is what the last section here is about.
The step-by-step routine
1. Empty the drip tray and take any water filter out of the tank. Descaler will wreck a filter cartridge.
2. Mix the solution at the ratio on the bottle, or at the ratio in your manual, and fill the tank.
3. Put a jug under the group head. Run the solution through in stages, pausing a minute or two between them so the acid has time to work, until roughly half the tank has passed through.
4. Run the rest through the steam wand and the hot-water outlet if your machine has them. Scale forms wherever water gets hot, not only behind the group.
5. Refill with fresh water and rinse. Two full tanks through every outlet is the floor, not the target. Acid left sitting in a boiler is a worse problem than the scale you just took out.
6. Put the filter back, then pull one shot you intend to pour away.
If yours is a bean-to-cup or super-automatic, use its own descaling program instead of this routine. De'Longhi, Philips and Jura machines meter the solution through internal circuits you cannot reach by hand, and stopping the cycle partway can leave acid standing in them.
Descaling and cleaning are different jobs
These two get confused constantly, and the machines do not help, because plenty of them signal both with one light.
Descaling is an acid and it works on the water side: the boiler, the pipes, the parts you never see. Cleaning, or backflushing, uses an alkaline detergent on the coffee side, the group head and the shower screen, where the residue is oil rather than mineral.
Neither substitutes for the other. A machine backflushed every week can still be packed with scale, and a freshly descaled machine can still pull bitter shots because the screen is filthy.
Doing both in one session, clean first and descale second, so the long rinse that ends the descale is also the last thing to pass through the machine.
What to use
A dedicated descaling solution is the safe default. It is what the manufacturer tells you to use, which matters while the machine is under warranty, and the bottle carries its own mixing ratio.
Citric acid is the active ingredient in many of those bottles and costs less bought loose. Published home ratios vary widely enough that we are not going to print one: the safe concentration depends on what the boiler is made of, and aluminium tolerates less of it than stainless. Your manual is the authority.
Vinegar is a common suggestion and a weak one. It can attack seals in machines with aluminium boilers, and the smell carries into the next few shots. Use it only where the manual says you can.
Baking soda does not work, and the reason is worth stating plainly. Limescale is a carbonate, and carbonates are dissolved by acids. Baking soda is alkaline. It is a mild abrasive and a deodoriser, so it earns its place cleaning a carafe, but no chemistry takes it from there to removing scale from a boiler.
Slowing scale down in the first place
Descaling treats the symptom. Your water sets the frequency. Filtered water, a tank filter, or bottled water blended for coffee will slow the build-up a lot, and many warranties quietly assume you use one of them.
One caution: do not put distilled or zero-mineral water in a machine with an electronic water-level sensor. Some of those sensors need minerals present to detect water at all. Lightly filtered is the sweet spot for most home machines.
If flow stays slow or steam stays weak after a descale, scale may have set harder and deeper than a tank of solution can reach. A proper service will do more at that point than another cycle will.