Energy certification of 2016 buildings New FAQ on water treatment in heating systems published on the website of the Ministry of Economic Development the 1 August 2016 to clarify that it is water treatment is always mandatory in the heating circuit of the heating systems dedicated to winter air conditioning. If the plant also produces sanitary water (combined boiler), water treatment is mandatory in both circuits (heating and sanitary) ,. The treatment is instead recommended in the case of heating systems used only for the production of domestic hot water (water heaters).