This non-return valve is often installed to help keep water from getting into your spa's blower. (There should also be a loop of pipe above the water line). It is installed on the air-line between the air jets and the blower. This valve has a 1/4-pound spring and slip fits inside of 1.5" inch pipe fittings.

This one-way valve is ideal for air pipes, but we don't advise that you use it for water circuits.  Its design will slow down the flow rate and tends to catch foreign objects like leaves or hair, which will subsequently prevent the valve from closing properly and can be very difficult to remove.