Why does spa drain out when we turn on our pool filter - is attached and flows into but something wacky because it goes reverse?

It is not common for a spa to drain when the pool filter is turned on. In a typical pool and spa setup, the pool filter should not affect the water level in the spa. If you notice that your spa is draining when you turn on the pool filter, there may be an underlying issue with your pool or spa plumbing system.

Here are a few reasons why a spa might drain when the pool filter is turned on:

Incorrect Plumbing: If the plumbing between the pool and the spa is not properly installed or designed, it can cause the water to flow in the wrong direction. This could result in the spa draining when the pool filter is turned on.

Faulty Valves: Check the valves that control the flow of water between the pool and the spa. If a valve is faulty or not positioned correctly, it can cause water to flow in the wrong direction, resulting in the spa draining.

Clogged Pipes: Clogged pipes can restrict the flow of water and create a pressure imbalance between the pool and the spa. This can cause the water to flow out of the spa when the pool filter is turned on.

Air in the System: Air trapped in the plumbing system can cause inconsistent water flow and affect the water levels in the pool and the spa. This can sometimes lead to the spa draining when the pool filter is turned on.

Improper Filter Size: If the pool filter is too large for the size of your pool and spa, it can create excessive suction, causing the water to be drawn out of the spa when the filter is turned on.

If you are unsure about the cause of the issue, it is best to consult with a qualified pool and spa professional to diagnose and fix the problem. They can inspect the plumbing, check the valves, clean any clogged pipes, release trapped air, and ensure that the filter is properly sized for your system.