Many of today’s pools are built combined with a spa. The spa water level is at a slightly higher elevation in respect to the pool’s water level. Pool water circulates when water is drawn from the pool towards the suction side of the pool pump. This water travels through plumbing lines from the pool to the pump, due to negative pressure “vacuum” created by the suction side of the pump. Next, the positive pressure side of the pump pushes this water through other plumbing lines which now returns it back to the pool.
In most pool spa combinations, branching off of the same plumbing line returning water back to the pool would be another line. This line is also returning water back, but to the spa instead. This water keeps the spa full, which then spills over into the pool when the spa is too full. A waterfall is created due to the slight elevation difference between the two bodies of water.
Since the spa is slightly higher than the pool, and since both have this return line in common, the following may occur. When the pool pump is off, the spa water can now back-siphon through the pump and back into the pool due to gravity. The spa level will drop, but the pool level will rise, until both reach that equalized level and then the process will stop. There is no water loss due to all of this, although it may seem like it. That is unless you do have an actual leak somewhere in your pool or spa.
To prevent back-siphoning when the pump is off, installing a one way valve on the spa spillover line eliminates the problem. Another name for this valve is a check valve. Water can be to be pumped to the spa, but when the pump is off, this valve stops or “checks” the water from flowing backwards to the pool due to gravity The spa water level should now stay right at the base of the spillover when the pump is off.
Chances are you already have a check valve installed on the spa spillover line, but it may no longer work correctly and simply needs to be replaced.