When Draining a Swimming Pool Is a Bad Idea

Empty Pool
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While it is possible to make most repairs to your swimming pool underwater, there are circumstances that necessitate draining. However, you should not attempt this unless it is absolutely necessary and you are thoroughly familiar with the steps necessary to do this safely. Depending on the type of pool, draining it can cause serious damage to its structure.

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 difficult 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, which can then 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 groundwater which can make the liner float away from the wall as the level in the pool gets equal to or lower than the groundwater level. Groundwater must be lowered below the bottom of the pool by pumping it out through the well point line installed during construction.

If there's no well point line, 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 groundwater present when the pool was built, this can change over time.

You must also be very careful about rain. Normally most rainwater runs off the surface and does not soak into the soil (except for very sandy soils). However, building a pool disturbs the soil, loosening it, and allows a lot more water to penetrate, 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 and Fiberglass Pools

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

An Extra Tip

Knowledgeable pool owners often ask about the hydrostatic relief valve and why it wouldn't protect the pool in this case.

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, which is designed to equalize the water level in the pool to the groundwater to compensate for a small leak or water loss.

Updated by Dr. John Mullen