An excessive amount of solids and sludge can insulate the heating surfaces on the waterside of the boiler. The metal overheats and, as a result, the pressure vessel fails. High solids concentrations can also lead to carryover of the boiler water into the steam which can cause water hammer and damage piping and equipment. Low water conditions can also occur.
When boiler water turns to steam, solids are left behind. Unless the condensate return is 100%, the solids content tends to build up when the boiler takes on makeup water. On hot water systems, no makeup water is generally used. The solids' concentration remains the same and no blowdown is needed.