BrewWater
What water hardness causes limescale in an espresso machine?
Short answer
Scale forms when heated water deposits insoluble carbonates. The WHO calls water below 60 mg/L calcium carbonate soft, 60 to 120 moderately hard, 120 to 180 hard, and above 180 very hard. Espresso boilers scale fastest in the hard bands, well above the 40 mg/L alkalinity coffee brewing targets.
| Measure | Value | Unit | Source |
|---|---|---|---|
| Upper bound of soft water; above this, water is at least moderately hard | 60 | mg/L as CaCO3 | World Health Organization · verified |
| Upper bound of moderately hard water; above this, water is classified as hard | 120 | mg/L as CaCO3 | World Health Organization · verified |
| Concentration above which water is classified as very hard | 180 | mg/L as CaCO3 | World Health Organization · verified |
| Taste threshold range for the calcium ion in drinking water | 100–300 | mg/L | World Health Organization · verified |
| Target total alkalinity for brewing water | 40 | mg/L as CaCO3 | Specialty Coffee Association of America · verified |
How is water hardness classified?
The four bands in the table are the WHO classification, expressed as milligrams of calcium carbonate equivalent per litre. They are a description of water, not a machine specification, so treat them as a way to locate your supply rather than as a pass or fail line. The useful comparison is against the brewing alkalinity target, which sits below even the soft band.
Why does heating water cause scale?
Carbonate hardness is soluble while the water is cold and stops being soluble when it is hot. The WHO document cited here describes scale deposition in heated water applications, where insoluble metal carbonates form, coat surfaces and reduce the efficiency of heat exchangers. The hottest surface collects the deposit first, which in a machine means the element and the boiler wall.
Can water be too soft for a machine?
Yes, and this is the half of the problem that gets ignored. The same document states that soft water which has not been stabilised has a great tendency to corrode metal surfaces and pipes, and that demineralised waters are highly aggressive. Chasing zero hardness to avoid scale trades a slow, visible, descalable problem for a faster and less visible one.
Does hardness change how coffee tastes?
Separately from what it does to the machine, calcium becomes directly detectable in water at a concentration well above the brewing target. The WHO gives a taste threshold range for the calcium ion, which is listed in the table. Below that range hardness is a machine question; above it, it is also a flavour question.
Common questions
- How often should I descale?
- It depends on your water and your throughput, so no single interval is honest. The useful signal is your alkalinity figure: the further above the soft band it sits, the faster carbonate accumulates on the heating element.
- Does a filter jug stop limescale?
- A plain carbon jug filter removes chlorine and does little to hardness. Jugs with an ion-exchange resin do reduce hardness, but they exhaust with use, so protection falls off as the cartridge ages.