Scale is caused by the accumulation of hardness salts in the boiler water which, if not removed can cause localised overheating, reduced heat transfer efficiency, and in severe situations tube failure.

Hardness salts can either be removed before they enter the boiler system or they can be treated inside the boiler itself with suitable antiscalants/scale inhibitors. Even when hardness is removed from the make-up water it is important to use an appropriate scale control programme within the boiler water to ensure that any hardness slippage is prevented from causing troublesome deposits.