On 05/02/2018 17:12, harry wrote:

You can put in a "clack"valve.
The flap will remain closed against convection losses but open on over pressure
You might need to put in an air vent immediately below the clack valve.
Or drill a small hole in the clack.
The clack will fall shut in the vertical position


https://encrypted-tbn0.gstatic.com/i...9hmjSqr6EWEf7z



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!.