This DN100 (4") flap valve, consists of a plastic body and stainless steel inners. This provides protection against ground water from entering the storage tank via the overflow.
This devive works by a flap being forced shut if water pressure (minimal required) presses the "wrong way" in the valve. Being DN100, this is suitable for all domestic and smaller commercial rainwater harvesting applications.
It can be positioned anywhere after the the overflow from the storage tank and good practice suggests that it should be installed in such a way to allow future maintenance (rodding, blockage removal).
Delivery to UK mainland is included.