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"Why two stages?"
If you drain too much water from a solid pool (gunite, plaster, fiberglass) and the ground water level is high, the pool can float up in its hole -- tearing up the pipes and in general, ruining the pool owners day.
Draining too much water from vinyl lined pool can result in the liner floating off the bottom of the pool. Not as bad as the previous example, but still one of those things that just doesn't need to happen.
"Why two stages?"
If you drain too much water from a solid pool (gunite, plaster, fiberglass) and the ground water level is high, the pool can float up in its hole -- tearing up the pipes and in general, ruining the pool owners day.
Draining too much water from vinyl lined pool can result in the liner floating off the bottom of the pool. Not as bad as the previous example, but still one of those things that just doesn't need to happen.