I just took on a pool built in the late 1980's. It has a (field built) atmospheric vent that is blowing water, draining the pool when the pump runs. Does anybody know what would cause this? How to cure this? Here is an extract I found on the net addressing such vents:
II) SUCTION-LIMITING VENT SYSTEM - A suction-limiting vent system with a tamperresistant atmospheric opening.
Staff interpretation: A suction-limiting vent system is also called an atmospheric
vent. It is a pipe teed to the suction side of the circulation system on one end and
open to the atmosphere on the opposite end. The pipe is normally full of water
equal to the same height as the pool. When a blockage occurs at the main drain,
air is introduced into the suction line thus causing the pump to lose prime and
relieving the suction forces at the main drain (suction outlet).