On Mon, 5 Feb 2018 18:00:59 +0000, GB wrote:

I can't actually find a clack valve. Would a spring loaded non-return
valve do much the same. It gets in the way of any convection currents,
but allows water through if there's an over-pressure. Plus, it's cheap!.

Don't forget any inertial over presure surge when a pump starts up.
This may push water past the valve that can't return. Subsequent
starts shoving a bit more and bit more etc eventually leading to each
start giving a burst of pump over. If there is enough pressure with
the system running to open the valve this won't happen.

--
Cheers
Dave.