A brewed coffee is roughly 98% water by weight, so the water is not a neutral carrier — it is most of the drink. Two properties matter: how hot it is, and what is dissolved in it.
Temperature
Temperature controls how fast compounds dissolve. Too cool and the extraction stalls, leaving the cup sour and thin. Too hot and it strips bitter compounds out of the grounds faster than the sweet ones.
| Light roasts | 92–96 °C |
|---|---|
| Medium roasts | 92–94 °C |
| Dark roasts | 88–92 °C |
| AeroPress (dark) | 80–85 °C |
| Espresso | 92–94 °C at the group |
A variable-temperature gooseneck kettle holds a set point within a degree or two. A stovetop kettle needs a thermometer and attention, and boiling water poured straight onto coffee is usually well above the useful range.
What is dissolved in it
Total dissolved solids (TDS) is a measure of how much mineral content the water carries. Water that is too soft tastes flat and extracts poorly; water that is too hard deposits scale and tastes chalky. The useful middle is narrow, and most tap water sits outside it in one direction or the other.
| Water | Effect on the cup | Effect on the equipment |
|---|---|---|
| Very soft / distilled | Flat, hollow, under-extracted | Can be aggressive to metal over time |
| Moderate hardness | Balanced extraction | Minimal scale with regular descaling |
| Very hard | Chalky, dull, muted acidity | Rapid scale in boiler and valves |
A simple filter jug improves most tap water enough to be worth the counter space. If you descale regularly and the water is still hard, filtering will reduce the frequency — see the descaling guide.
Why scale matters more than it looks
Calcium carbonate deposits on the heating element and inside the water passages. The consequences arrive in this order: temperature becomes less stable, flow slows, pressure drops, and eventually a passage blocks. None of that announces itself as a water problem — it shows up as coffee that has quietly got worse.
Descaling solution is an acid. Wear gloves, ventilate the room, never mix it with bleach or another cleaner, and flush the machine thoroughly with clean water before you drink from it again. Keep the bottle out of reach of children.
A practical routine
Filter the water
A jug filter, changed on schedule. It is the cheapest improvement available.
Set the temperature per roast
Higher for light, lower for dark. Change it one step at a time.
Taste the water
If the water tastes bad on its own, the coffee will too. This is a fast, free test.
Descale before the symptoms
On a schedule set by your water hardness, not by the machine misbehaving.
The free upgrade
Before buying anything, taste your tap water at room temperature. If it tastes of chlorine or chalk, filtering it will improve your coffee more than most equipment changes.
This guide covers hot water, steam, pressurised equipment and, in places, alcohol. Follow the instructions supplied with your own equipment, ventilate when you clean or descale, keep spirits out of reach of children, and drink responsibly.