Indirect Heat Exchanger means equipment in which heat from the combustion of fuel is transferred by conduction through a heat-conducting material to a substance being heated. An indirect heat-exchanger eliminates problems caused by the build-up of limescale and ensures that the panel cannot freeze in winter. An indirect heat exchangers are submerged in the water and in this way cold water circulated to a closed loop system is connected to mass produced fan coil units or other terminals. Indirect heat exchanger designed to transfer heat from combustion gas to the air stream added to the combustion zone.