Heat exchangers are used with indirect cooling systems. The hot engine (fresh) cooling water passes over a stack of pipes containing raw water, which absorbs the heat and carries it away. The engine cooling water flows in the opposite direction to the raw water.
Heat exchangers can be serviced by dismantling them and cleaning the parts. In particular the raw-water flow channels may need cleaning, so long as care is taken not to damage the tubes when rodding.
The header tank level needs to be checked periodically and filled to finger level. The system is flushed by opening the drain tap on the engine block, letting the water/antifreeze mix drain into the bilge.
Hose pipes will crack after a time, and lead to overheating in an indirect system. Replace them after four or five years. Note the temperature sensor wire at the top right.
