How does boiling reduce water hardness?

Heating water above about 80 degrees Celsius drives off dissolved carbon dioxide and tips calcium bicarbonate into insoluble calcium carbonate, which falls out as a chalky precipitate. This removes temporary, or carbonate, hardness and the alkalinity that comes with it.

Can boiling remove all the alkalinity?

No. Calcium carbonate has a small but real solubility, so a thorough boil leaves a residual alkalinity floor of roughly 50 ppm as calcium carbonate. To go below that you must dilute with reverse-osmosis or distilled water or add acid.

Why does calcium matter for boiling out alkalinity?

The precipitate is calcium carbonate, so each unit of alkalinity removed consumes calcium in a fixed ratio of about 0.4 ppm of calcium per ppm of alkalinity. If the water is high in bicarbonate but low in calcium, it runs out of calcium and the alkalinity stops dropping until you add some.

What is permanent versus temporary hardness?

Temporary hardness is the part tied to bicarbonate that boiling can remove as carbonate scale. Permanent hardness comes from sulphates and chlorides and is unaffected by boiling. This tool models only the temporary, carbonate fraction.

When should I boil my brewing water?

Boiling is a cheap, acid-free way to cut high alkalinity in hard, chalky tap water, which suits brewing pale, hoppy beers that need lower mash pH. Boil, let the chalk settle, then rack the clear water off the sediment before brewing. For very high alkalinity, combine boiling with dilution or acid.

Brew Water Boiling Hardness Reducer

Pre-boiling is the oldest way to soften hard, chalky brewing water without chemicals. As the water heats, calcium bicarbonate breaks down and drops out as harmless chalk, lowering the alkalinity that would otherwise push mash pH too high. This calculator estimates how much alkalinity a boil actually removes.

How it works

Above roughly 80 °C, dissolved carbon dioxide escapes and the bicarbonate equilibrium shifts, precipitating calcium carbonate:

Ca(HCO₃)₂ → CaCO₃ (precipitate) + CO₂ + H₂O

Two limits decide how far the alkalinity can fall:

The solubility floor. Calcium carbonate is slightly soluble, so a thorough boil cannot drive alkalinity below about 50 ppm (as CaCO₃). That residual stays in solution no matter how long you boil.
Available calcium. The precipitate is calcium carbonate, so removing alkalinity consumes calcium in a fixed ratio — about 0.40 ppm of calcium per ppm of alkalinity removed. Water that is rich in bicarbonate but poor in calcium runs out of calcium first and stops dropping.

The tool applies whichever limit binds first and reports the final alkalinity, the amount removed, and the calcium consumed.

Worked example

Hard tap water at 250 ppm alkalinity and 120 ppm calcium has plenty of calcium, so boiling drives the alkalinity down to the 50 ppm floor — a 200 ppm reduction — consuming about 80 ppm of calcium along the way. By contrast, water at 250 ppm alkalinity but only 30 ppm calcium is calcium-limited: it can only shed about 75 ppm of alkalinity before the calcium runs out, leaving roughly 175 ppm.

Tips and notes

Boil, then let the chalky sediment settle for a few hours, and carefully rack or decant the clear water off the bottom so the precipitate stays behind. If the result is calcium-limited, adding a pinch of chalk or calcium chloride before boiling lets more alkalinity precipitate. To get below the 50 ppm floor, blend in reverse-osmosis or distilled water, or finish with acid. Permanent hardness from sulphates and chlorides is unaffected by boiling and is not modelled here. All calculation runs locally in your browser.