Skip to content


Home / Blog / When to Drain a Swimming Pool
When to Drain a Swimming Pool

When to Drain a Swimming Pool

It’s every pool owner’s worst nightmare, a green pool chock full of chunky chunks of algae, floating debris, and maybe even a dead animal. It looks hopeless. You may throw up your hands and decide the best course of action is to drain the pool and start over. In fact, draining a pool should be a last resort. 

Most in-ground pools will have to be drained and refilled at some point. But cleaning should not be the reason to do it. Pools will need to be drained and refilled every 5-7 years on average, or if there is a major necessary repair. Otherwise, avoid draining your pool if at all possible. Basically, if you want to know when to drain a swimming pool, the answer is: only when you have no other alternative! But read on for more information. 

Why You Should Not Drain a Pool

Draining a pool is a risky business for several reasons. Your pool is not meant to be empty. Whether you have a vinyl, concrete, or fiberglass pool, it is at its best when it’s full of water. Once the water is drained, you open yourself up to all sorts of damage, so drain a pool only when there is no other option.

The greatest danger you face when draining a pool is hydrostatic pressure. That’s just a fancy term for the pressure of all of the groundwater around and under your pool pushing against your pool’s walls and floors. Water is heavy, and hydrostatic pressure can be significant. If you haven’t adequately planned for the onslaught of hydrostatic pressure, your pool could be severely damaged.

The Danger of Hydrostatic Pressure

One source of damage is the pressure of groundwater underneath the pool pushing up on the pool floor. If you have a concrete pool, the pressure of the water below can cause the floor to shift and crack. In a vinyl or fiberglass pool, the hydrostatic pressure below can lift the pool floor, causing severe damage to the floor and the walls.

One way of dealing with this pressure is with “hydrostats,” short for hydrostatic pressure valves. A hydrostat is a valve installed in the deep end of your pool. The cover is usually sealed tight, and in a cement pool, it may take a chisel to remove it. Under the cover is a pipe that leads down right into the dirt or gravel beneath your pool. Opening up the valve allows water to come up from under the pool and pour into the pool, relieving the pressure. If you have hydrostats in your pool, never drain the pool without opening them.

The other danger from hydrostatic pressure is the weight of the water around your pool. If the water table is higher than the floor of the deep end of your pool, water in the ground around your pool will put pressure on its walls. The amount of pressure depends on how the pool was built.

Backfill and the Bowl Effect

When any pool is installed, you start by excavating a hole larger than the pool. The pool is built (or, in the case of a fiberglass pool, placed) inside that hole. The area around the pool is then filled in with dirt, usually from the excavation. That replaced dirt is called backfill and is generally not as dense as the virgin earth around it. The excavation and backfilling create a bowl effect, where the virgin earth forms a bowl that collects water in the looser backfill. That water is heavy and presses on your pool. When your pool is full of water, the pressure of the pool water balances the groundwater. But once the pool is drained, there is nothing pushing back against the pressure from the groundwater.

If you do need to drain your pool, you will need to remove the groundwater around it. This can be done with a sump pump. It is also very important that you drain your pool when the weather will be dry until it is refilled. Heavy rain will saturate the ground around the pool, and the pressure of all that water creates a high risk of damage to the pool floor and walls.

Other Dangers to a Drained Pool

The other possible danger to a drained pool comes from the exposure of the pool floor and walls to the elements. The material of your pool walls and floor is meant to be submerged. A pool with a vinyl liner can become dried out and even crack. Vinyl and cement are less fragile but can also be damaged by the elements if the pool is not refilled quickly.

Yet another risk is that of insect infestation. Insects may enter through cracks, seeking out cool, moist places. This can cause damage to your existing pool and create an entirely new set of issues!

Is There Any Other Option?

It may seem hopeless when your pool is green and full of chunky algae. But the truth is that there is almost always another option. All it takes is patience, some elbow grease, and a commitment to work on your pool almost daily for up to two weeks. 

It can seem daunting, but it’s still probably better than draining your pool. The cost of the materials you will need to clean your pool is almost certainly less than your water bill after filling a 20,000-gallon pool. Keep that in mind as you make your decision on whether to drain.

Clearing Debris

If you can see the floor of your pool at its shallowest part, there is hope. If there is a lot of large debris, chances are you can remove it manually. Even a foot of fallen leaves on the floor of your pool can probably be dealt with using a rake and a pool vacuum. 

In most cases, you can vacuum your pool to the “waste” setting of your pump. If not, a pool company like The Pool Butler can do the vacuuming for you with their portable equipment. Either way, you don’t need to drain the pool. Just don’t panic!

Declaring Chemical Warfare on Algae

A serious algae bloom can be scary to look at. A dark green pool with floating clumps of thick, gooey algae can be pretty disgusting. But you can declare war on that algae with an arsenal of pool chemicals. 

You will definitely need to shock the pool, likely more than once. You can also use algaecide, which is made specifically to kill algae. Then you will have to add a whole bunch of chemicals to rebalance your pool water. But it can be done. 

If you feel like this kind of chemical warfare is beyond what you can do or beyond what you want to do, The Pool Butler can help. We are pool cleaning experts and stand by, ready to de-green even the most disgusting pool water.

Can Your Filter Handle It?

This is important when deciding whether to drain your pool or just clean the water. Smaller filters, especially cartridge filters, may have a hard time with thick, super-dirty water. In that case, you may have no option but to drain the pool. 

However, if you have a large, effective filter, especially a DE filter, there is a strong chance that your filter can help with your situation. You will need to leave it running 24/7 for at least a few days. For a DE filter, you should turn it off for about two hours a day to let the DE and all that debris slough off the grid. 

This could overpower a small sand or cartridge filter, but a larger filter should be able to handle it. You will likely need to backwash your filter when you are done.

When to Drain A Swimming Pool (Finally)

If you have so much debris in your pool that it can’t be removed, or if your filter can handle the cleanup even with a large dose of chemicals, you will have to drain your pool. However, remember that almost any type of pool cleaning can be done with water in the pool. If you feel like you have to drain the pool, another option is to drain the pool partially and then refill it. That will reduce the mess without the danger created by hydrostatic pressure.

Another time to drain your pool is if you need to make significant repairs. Your pool professional will make the call on whether a repair warrants draining the pool.

Finally, most pools should be drained and refilled about once every 5-7 years or when the total dissolved solids (TDS) reaches over 2500 ppm. When this happens, it will become difficult to balance your pool water. You will have to use more and more chemicals. Over time, the chemicals you use to clean and shock your pool, environmental factors, and the buildup of organic waste like hair, dead skin, and oils will simply become too much for regular maintenance to take care of.

How Often Should Pools Be Drained and Refilled?

While there is no definitive answer, most experts agree that pools should be drained and refilled every three to five years. This will help to keep the water clean and clear and will prevent any build-up of dirt or debris. Additionally, draining and refilling your pool will help to extend its lifespan. If you have any questions about when to drain your pool or if you need help doing so, be sure to contact a professional like The Pool Butler.

DIY or Hiring Professionals?

This one is easy! Always hire professionals to drain your pool. If you think your pool needs to be drained, a professional pool maintenance and repair company like The Pool Butler can confirm your suspicion and help with the process. Improper draining could irreparably damage your pool, so don’t try to do it yourself.

If you’re dealing with an exceptionally dirty pool, or if you think you need to drain it, contact The Pool Butler today. We’ll help figure out what needs to be done to solve your pool problems. And if it really does need to be drained, we can safely drain and refill your pool.

When to Drain a Pool [infographic]

The Pool Butler