Simply put, a check valve allows flow in one direction and automatically prevents backflow (reverse flow) when fluid or air in the line reverses direction. They are one of the few self-automated valves that do not require assistance to open and close. The design of the check valve is such that it will operate in any orientation. When used in conjunction with any of our pond aeration kits, it is best that they are installed in-line roughly six inches before each diffuser. This will increase startup airflow to the diffusers and help avoid heat build-up to the air pump.