Not sure of your engine/heat exchanger configuration, but the hottest
cooling water would where it comes out of the engine and goes into the heat
exchanger, then the return water would go into the heat exchanger, where you
disconnected to divert the water. Let the engine cooling pump do the
circulation through the hot water tank coil.

That is the simplest since there won't be much flow until the engine
thermosat opens up. If you circulate the water from the wrong point, you
will warm the cooling water with the already hot water in the tank.

You might check with your engine dealer or manufacturer.. They may have
already addressed this question and can speak with some authority that you
could fall back on if something goes wrong.

It would be adviseable to have the water heater/storage tank higher than the
engine. This will keep you hot water heat from escaping back to the engine
block by thermal convection.

I puzzled over this question recently because I want to have a way for the
generator to heat the water as well. The valve complications cause me to
consider adding another heating coil in the water heater tank.. Simpler and
but not cheaper. One possibility I have is to remove the electric element
and put the second coil in that hole. Right now I just want to see how well
the engine does at heating the water.. My Volvo still has the OEM 140 deg
thermotat installed. (Volvo says leave it even if I switch to fresh water
cooling because all the clearance/tolerances, etc are for SW cooling at 140
deg.)

Steve
s/v Good Intentions