Why you don't want to drain your pool.

Aboveground Pools
After draining, the liner can shrink which may then tear when refilling. The older the liner, the less it will stretch when refilling. Do not drain the pool in cold weather as this also reduces the stretching capability of the liner. After draining, complete your repairs and begin refilling as quickly as possible. As the pool is refilling, you may need to shift the liner around to make sure it is aligned properly. You will need to do this with only an inch or so of water in it because the weight of the water will quickly prevent you from being able to shift the liner.

Inground Vinyl Liner Pools
This type of pool is the most dangerous to drain and should only be done by a professional. Older pools may not have been built structurally to hold back the weight of the dirt against it when the pool is drained. This can cause the walls to collapse. These pools were backfilled with dirt as the water level came up, equalizing the pressure as it fills. Modern vinyl pools have been designed and built to hold the weight of the dirt without water in the pool.

The next problem you must deal with is ground water which can make the liner float away from the wall as the level in the pool gets equal to or lower than the ground water level. Ground water must be lowered below the bottom of the pool by pumping it out through the well point line installed during construction. No well point line? Then you will need to install at least two (one on each side of the deep end) to pump out the water. Even if there was not any ground water present when the pool was built, this can change over time.

You must also be very careful about rain. Normally most rain water runs off the surface and does not soak into the soil (except for very sandy soils). However, when the pool was built, it disturbed the soil, loosening it, and this can allow a lot more water to penatrate, filling the bowl that was excavated and causing the liner to float. We have even seen this happen to a pool that was full. That is why you may find your liner floating and/or wrinkles in it after a heavy rainstorm.

Inground Concrete Pools
Here, you are dealing with the same groundwater problems as for a vinyl pool. Most concrete pools are built structurally to withstand the weight of the dirt against them when drained. However, if the ground water is high enough, it can push the entire pool out of the ground. The concrete pool shell acts like a ship and floats up in the ground water.

Inground Fiberglass Pools
This type of pool is very susceptable to floating. It is a lot lighter than concrete and would float much quicker when drained.

Extra Tip
When telling pool owners about the above concerns, we are often asked about the hydrostatic relief valve and why this wouldn't protect the pool. A hydrostatic relief valve only allows as much water to flow through as the force of gravity permits. You are draining the pool much faster than water can flow through the hydrostatic valve. It is designed to equalize the water level in the pool to the ground water to compensate for a small leak or water loss.