Should the hydrostatic valve be left open when it is raining?
A "relief valve" can only be accessed when the pool is empty,
and no swimming pool should ever be left empty. The only exception
is when absolutely necessary due to repair or resurfacing, and that
should be completed within the shortest possible time.
Any pool that is empty, and that has a relief valve, should have
the valve removed for the entire time, regardless of the weather
forecast.
A "hydrostatic" relief valve opens and closes when needed as it
attempts to equalize the pool water pressure with the ground water
pressure. Human intervention is not involved because the pool is
still filled. Hydrostatic valves are great in theory, but seldom
work after a few years.