There are several circumstances in which replacing some or, less commonly, all of the water in a swimming pool may become necessary. As we discussed in Part 1, these can include certain repairs and renovations as well as water-chemistry conditions that can’t be adequately corrected without replacing some of the water.
Before reaching for the pump, however, it’s important to understand that draining an inground swimming pool isn’t simply a matter of removing the water and filling it back up again. Water inside the pool provides substantial weight and structural support. Once that water is removed, groundwater and soil outside the pool can exert forces that the pool doesn’t ordinarily experience.
The risks and proper procedures also vary considerably depending on whether you have a concrete, fiberglass or vinyl-liner pool. For that reason, completely draining an inground pool is one of those maintenance projects where knowing the type of pool you own really matters.
Draining a Concrete Pool Requires Caution
Concrete pools may sometimes need to be completely drained for resurfacing, major repairs or other substantial work, but that doesn’t mean an empty concrete pool is automatically safe.
Groundwater beneath and around an inground pool can create hydrostatic pressure against the shell. When the pool is full, the tremendous weight of the water helps counteract that pressure. Remove the water, and the forces acting on the structure change dramatically.
Some concrete pools incorporate hydrostatic relief valves or plugs intended to help relieve groundwater pressure beneath the pool. Their location, design and operation can vary, however, and homeowners shouldn’t assume that every pool has the same system or that simply opening a particular fitting makes draining safe.
Groundwater conditions are particularly important. Recent heavy rainfall, poor drainage, a high water table and local soil conditions can all increase the risk associated with an empty pool.
If your concrete pool needs to be completely drained for resurfacing or major repair, find out how its hydrostatic-pressure protection works and what the groundwater conditions are before removing the water. If you’re uncertain, have an experienced pool professional handle or supervise the draining process.
An empty concrete pool also shouldn’t be left that way unnecessarily. In addition to groundwater concerns, an exposed pool finish can be affected by environmental conditions while the pool is empty. If draining is part of a repair or renovation, coordinate the draining, work and refilling so the pool isn’t sitting empty longer than the project requires.
Most importantly, don’t assume that because concrete feels substantial, the pool can’t move or suffer structural damage. The amount of water normally contained in an inground pool is an important part of the equation.
Be Especially Careful About Draining a Fiberglass Pool
Completely draining a fiberglass pool should not be treated as a routine do-it-yourself maintenance procedure. Fiberglass pool shells depend on the surrounding installation and the water inside the pool, and removing that water can expose the shell to forces from groundwater and the surrounding soil.
If groundwater is present around an empty or substantially lowered fiberglass pool, it can place significant pressure on the shell. Depending on the installation and conditions, damage can include movement, distortion or other structural problems.
Manufacturers may have specific requirements concerning how far the water can safely be lowered and what procedures must be followed if a fiberglass pool needs to be emptied. Some specifically advise homeowners not to drain their pools without the involvement of an experienced pool professional.
Conditions can also change quickly. Recent or approaching rainfall, drainage around the pool, groundwater levels and the original installation can all affect the risk. A pool that has remained perfectly stable while full shouldn’t be assumed to remain that way once thousands of gallons of water have been removed.
If a fiberglass pool needs to be completely drained for repair or another unusual circumstance, consult the pool manufacturer’s instructions and have an experienced professional evaluate the situation before proceeding.
Avoid Completely Draining a Vinyl-Liner Pool Unless Necessary
Vinyl-liner pools present another set of concerns. The water in the pool helps hold the liner against the walls and floor. When too much water is removed, the liner can shift, wrinkle or pull away from its proper position.
An older liner may be especially vulnerable. Once an aged vinyl liner has been completely drained and allowed to move or contract, getting it to fit properly again may be difficult or impossible.
There are circumstances in which a vinyl-liner pool needs to be drained as part of a liner replacement or significant repair, but those procedures should be planned around the work being performed. Don’t completely empty an otherwise functioning vinyl-liner pool simply because you want to clean it or replace the water.
If only a partial water replacement is needed to correct a water-chemistry problem, determine how much water actually needs to be replaced rather than automatically draining the pool to the bottom.
There Isn’t One Safe Draining Method for Every Pool
Perhaps the most important lesson is that the material your pool is made from is only one part of the decision. The age and condition of the pool, groundwater level, recent weather, surrounding drainage, soil conditions, pool construction and reason for draining can all affect what should be done.
That’s why general internet instructions can only take you so far. Before completely draining an inground pool, consult the documentation for your pool and equipment and make sure you understand the groundwater and hydrostatic-pressure issues that apply to your installation.
If you’re uncertain, this isn’t a bad project to hand over to an experienced pool professional. The cost of professional assistance is considerably easier to absorb than discovering that your empty swimming pool has shifted, cracked, distorted or lifted.
In our final article in this series, we’ll look at some additional practical considerations to keep in mind when water actually needs to be removed from an inground pool.
Leave a Reply