On a car, the heater is fed with hot water from the primary side of the
cooling system, ie. "before" the thermostat. This makes sense as the water
going through the heater matrix is at engine temperature regardless of the
weather outside etc.

Now, I'm being told by my boat engine supplier to connect the inlet hose to
my new calorifier by drilling and tapping a plate on the engine heat
exchanger. This point is clearly on the secondary side of the fresh water
cooling circuit ie. "after" the thermostat. This doesn't seem correct to me
as the water may only get luke warm, unless I'm misunderstanding something
about normal practice for marine diesel installations.

Any help appreciated.

Thanks in advance, Jonathan