Doctor Drivel wrote:

... a collector above an unpressurized tank with the ends of the supply
and return pipes underwater and a small hole above the water line. This
would need less pumping energy than a system with the return pipe above
the water line.

The air hole in the pipe needs to be exposed when the pump is operating...
If the water level is too high then no drain down.

simpler to have the vent hole above the top of the tank. Add a sleeve
to prevent splash possibility.

It has to be "in" the tank. The pump will squirt water everywhere. Best to
have say 15mm into the tank, then immediately up to 28mm (1")...

Or put a T 6' above the tank with a small tube that goes back to the tank.

Nick