How Scale Forms Inside an Espresso Machine

Scale is what's left behind when hard water gets hot. The minerals that were dissolved come out of the water as a solid, and they stick to whatever is nearby.

The short chemistry

Hard water carries calcium and magnesium. When it's heated, the dissolved minerals can drop out. Wikipedia says this precipitation, meaning an insoluble solid forming, is primarily caused by the thermal decomposition of bicarbonate ions.¹

You don't need to remember that sentence. What matters is the result: heat makes the water give up some of its minerals. That's why scale shows up where water gets hot.

Wikipedia describes the solids as calcium and magnesium carbonates, which tend to be deposited as off-white solids on the inside surfaces of pipes and heat exchangers.¹ That pale crust is limescale.

Where it collects in a machine

Think about every place the water gets hot or moves slowly.

• The boiler or heating block, where the water is warmed.
• The narrow pipes and tubes that carry hot water out to the brew head and steam wand.
• Valves and small openings, where a thin crust can matter most.

That list is a general picture, not a parts diagram for your model. Machines are built differently, and the maker's manual is the guide to yours. Where to find it is covered elsewhere on this site.

What the buildup does

The general rule from engineering is clear. In boilers, deposits impair heat transfer to the water.¹ A crust works a bit like a blanket between the heater and the water.

Wikipedia also says the build-up of scale restricts the flow of water in pipes.¹ Inside a small machine, a narrow passage can't take much of a crust.

One maker puts the effect in its own words. Hard water can contribute to mineral buildup inside your espresso machine over time, it says.² Notice the soft wording. It says "can" and "over time." It doesn't say "always."

How makers deal with it

Makers don't tell you to ignore scale, and they don't expect you to scrape it off. They point you to descaling with their own method and products. Breville's support article says to descale the machine according to its maintenance instructions.²

How often depends on your water. De'Longhi says the hardness setting on its machines determines how regularly you will need to descale.³ Harder water means a shorter gap, which is the logic behind those settings.

The same Breville article says replacing the water filter as recommended may reduce how frequently the machine needs to be descaled.² The filter page explains what that cartridge does.

What not to do

Don't make up your own descaling mix from the kitchen cupboard. Use the method and products your maker names, and follow the steps for your model. If you can't reach the inside, or something's leaking, stop. Opening the case and any electrical work is a job for the maker or a qualified technician.

Knowing your water first helps you judge how much attention scale will need. Start with the hardness units page if the numbers on your report look unfamiliar.