Scale is what is left behind when hard water is heated. It builds inside the boiler and the pipes, slows the flow, throws the brew temperature out, and eventually blocks the machine entirely. Descaling dissolves it. It is the single piece of maintenance that decides whether a machine lasts three years or fifteen.
How often
Water hardness decides the interval, not the calendar. As a rough guide for Australian mains water, descale a home machine every two to three months in a hard water area, and every four to six months in a soft one. A cafe machine on filtered water is on a service schedule instead, which is the point of the filter.
Signs you have left it too long: the shot runs slower than it used to at the same grind, the steam wand spits rather than steams, the machine takes longer to come up to temperature, or the pump sounds like it is straining.
What to use
Use a descaler made for coffee machines. It is an acid, usually citric or lactic, in a dose that dissolves scale without attacking the seals and the metal inside. Do not use vinegar: it works, but the smell lingers in the boiler for weeks and it is harsher on gaskets.
The one exception: plumbed and heat exchanger machines
Descaler is for machines with a tank you fill yourself. If your machine is plumbed in, or it is a heat exchanger or dual boiler machine with a copper or brass boiler, do not pour descaler through it without checking the manual. Some manufacturers void the warranty for it, and on those machines scale is managed with a water filter and a technician instead. A La Marzocco Linea Mini is descaled by a service agent, not by you and a bottle.
Step by step, tank machines
- Empty the drip tray and remove the water filter from the tank if there is one, or the descaler will simply be absorbed by it.
- Mix the descaler with water in the tank at the dose on the pack. Do not guess, and do not double it.
- Put a large jug under the group head and another under the steam wand.
- Run about a third of the tank through the group head, then open the steam wand and run a third through that. Scale builds in both paths.
- Turn the machine off and let the rest sit in the boiler for around 20 minutes. This is the part that does the work.
- Run the remainder through, then rinse: two full tanks of clean water through both the group and the wand, minimum. Any descaler left behind will taste of it.
Descaling is not cleaning
Descaling removes mineral scale from inside the water path. It does nothing about the coffee oils that build up in the group head and the basket, which is what backflushing and a group head cleaner are for. Both jobs need doing, on different schedules. We cover the other one in how to clean and backflush an espresso machine.
Prevention beats cure
Filtered water slows scale down enormously, and in a cafe it is not optional. A filter cartridge and head on the mains line is cheaper than one boiler replacement. At home, filtered water in the tank helps, and emptying the tank rather than leaving water standing in it helps too.
If you are choosing a machine and want to know what a serviceable one looks like, read how to choose a commercial coffee machine for a cafe.
