The steps to winterize the pool are simple:
- Drop the water level until it's below the skimmer, and also below the return lines.
- Blow air out of the skimmer and the return lines. To keep the figure simpler, we don't show those lines.
- Blow air out of the main drain line.
You know the skimmer line and the return lines will be safe. Because you dropped the water level below the skimmer, you know that the water has no way to return to these pipes over winter.
But no one ever dives to plug the main drain line. How do you stop the water from bouncing back (blue area) in the main drain line until it reaches the water level in the pool, after blowing the main drain line?