Method and system for optimal use of heat produced in a thermal system on board a marine unit, the thermal system comprises one or more heat generating units (11,13, 21, 23) and one or more heat consumer units (12) with a pipeline (23, 24, 26) and valve system associated therewith for circulation of thermal fluid between the various units, the heat generating unit(s) (11, 13, 21, 23) comprising at least one fuel fired heater (11); at least one thermal fluid cooling unit (21) selected from the group of a cooling system on one or more engines (20), exhaust gas economizer (22) associated with the exhaust line(s) from the one or more engines (20) and/or incinerator(s) (13), wherein heat from the cooling systems of the heat generating units (11,13, 21, 23) is recovered and used for delivering heat to heat consumers (12), the thermal system being configured in such way that heat recovered from the heat recovery generators (11,13, 21, 23) is primarily utilized and that heat delivered from the conventional fuel burners (11) is used only, or as a supplement, if the heat from the heat recovery generators (11,13, 21, 23) is not sufficient for the intended purpose.