Double check valve

[1] It is different from the two-way check valves (sometimes erroneously referred to as double check valves) used in air brake systems on heavy trucks which select from the highest pressure source.

[2] This employs two operating principles: firstly one check valve will still act, even if the other is jammed wide open.

Secondly the closure of one valve reduces the pressure differential across the other, allowing a more reliable seal and avoiding even minor leakage.

Small valves may be so compact as to be barely noticeable particularly when they are integrated into the bodies of existing taps (faucets).

It is commonly used on lawn irrigation, fire sprinkler[2] and combi-boiler systems.