KH (carbonate / temporary hardness)

The portion of water hardness made up of bicarbonates — the component that forms scale when heated. KH is what most "softening" filters actually remove (via ion exchange).

Why it matters — High KH means rapid scale buildup inside boilers. Carbonate hardness is what causes the white crust on heating elements and the limescale blockages in solenoids.

Good to know

• Specialty Coffee Association recommends 40–75 ppm KH for brewing water.
• Reverse osmosis removes nearly all KH; ion-exchange filters reduce it specifically; carbon filters don't touch it.

Related: Water Filtration