Tap water con­ta­ins aggres­si­ve salts and lime. The more water a hea­ting or coo­ling sys­tem con­ta­ins, the more mine­rals and salts are brought in while fil­ling. Modern devices like wall moun­ted con­ti­nuous fur­naces, heat pumps and solar hea­ting plants suf­fer more quick­ly from dama­ge by lime pre­ci­pi­ta­ti­on, alre­a­dy at a low hard­ness level of the fil­ling water.