How to Drain a Pool: The Complete Step-by-Step Guide

When Should You Drain a Pool?

The first question is whether you actually need to drain the pool at all. Full drainage is not a maintenance task – it is a last resort for specific situations where no other intervention will work.

The main reasons to drain a pool are a TDS (total dissolved solids) level so high that chemistry can no longer be balanced through normal treatment, persistent cyanuric acid (CYA) buildup above 100 ppm that has suppressed chlorine effectiveness beyond recovery, an algae infestation so severe that standard shock treatment has not resolved it after multiple attempts, structural repairs or resurfacing that require the pool to be empty, and acid washing to remove deep staining or calcium scale from the pool surface.

If you are considering draining because the water looks cloudy or green, try a thorough shock and clarifier treatment first. A proper chemical reset handles the majority of water quality problems without the time, expense, and risk involved in full drainage. Draining as a shortcut for poor water chemistry is usually unnecessary.

Important Preparations Before You Start

Check for Hydrostatic Pressure Risk

This step is critical for inground pools and cannot be skipped. An inground pool shell is balanced by the weight of the water inside it. When the pool is drained, groundwater pressure from the surrounding soil pushes inward and upward from beneath. If groundwater levels are high, this pressure can physically lift a fibreglass or vinyl liner pool out of the ground, or crack a concrete shell.

This is not a rare worst-case scenario. It happens, and the repair costs are significant. The risk is highest after heavy rain, in areas with high water tables, and in low-lying or clay-heavy soils.

Before draining an inground pool, check that the weather forecast is dry for the duration of the job, assess whether your area has a high water table, and confirm that your pool has hydrostatic relief valves (small plugs in the floor that allow groundwater to enter the shell at equalised pressure rather than lifting it). If you are unsure about any of these factors, consult a pool professional before proceeding.

Above-ground pools do not have the same risk profile and can be drained more straightforwardly.

Turn Off All Equipment

Switch off the pump, heater, chlorinator, automatic timers, and any other automated systems before draining begins. A pump that continues to run as water levels drop below the skimmer line will run dry, causing immediate and often irreparable damage to the pump impeller and seals.

If your system has automatic timers that would normally switch the pump on during the draining process, disable them manually.

Reduce Chlorine Before Discharging

Highly chlorinated water is harmful to soil biology, vegetation, and aquatic environments. Before draining, stop adding chlorine and allow the pool to sit in sunlight for several days without a cover. UV exposure degrades free chlorine relatively quickly. Test the water before draining; a free chlorine level below 0.1 ppm is generally considered safe for controlled discharge to a sewer connection.

Do not drain water with chlorine levels above 1 ppm into any natural environment.

Know Where the Water Will Go

This is not optional. Pool water must be discharged to an appropriate point. In Portugal, the correct option is a municipal sewer connection or a properly designed cesspool. Discharging onto a lawn destroys grass and harms soil biology. Discharging near trees, garden beds, or any planted area will cause damage. Draining toward a street or into a drainage ditch that connects to a watercourse is harmful to local ecosystems and may carry legal consequences.

Plan your discharge route and run your hose to the correct endpoint before starting the pump. A hose that is too short, incorrectly positioned, or poorly directed is one of the most common sources of avoidable problems during pool drainage.

How to Drain a Pool: Methods

Submersible Pump

This is the recommended method for most full drainage situations. A submersible pump is placed directly in the pool water and pushes water out through a connected discharge hose at a controlled flow rate. It is faster than gravity drainage, more controllable than using the filter system’s waste setting, and does not depend on the pool’s built-in drain valve.

Position the pump at the deepest point of the pool for maximum efficiency. Connect the discharge hose and run it its full length to the drainage endpoint before switching on. Check for kinks in the hose – a single kink can reduce flow rate significantly.

As the water level drops, monitor the pump and reposition it as needed to keep the intake submerged. Switch the pump off before it loses suction – the sound of the motor tone changing and the flow rate dropping are the first indicators that water is too low.

The last few centimetres of water will sit below the pump intake. Remove this with a wet vacuum, mop, or sponge once the pump can no longer reach it.

Filter System on Waste Setting

For pools with a sand or DE filter fitted with a multiport valve, the waste setting bypasses the filter and directs water straight out through the backwash line. This is more practical for partial drains than for full drainage, since the flow rate is limited by the pump’s capacity and the process requires active monitoring to avoid running the pump dry.

Switch the pump off before rotating the multiport valve to waste, then restart the pump. Route the backwash hose to your discharge point. Monitor water levels closely and switch off before the skimmer line is exposed.

Cartridge filter systems without a multiport valve do not have a waste setting. Use a submersible pump or the built-in drain valve instead.

Built-in Drain Valve

Most above-ground pools include a drain valve at the base of the pool wall. Connect a garden hose adaptor to the valve and run the hose to the discharge point. Remove the inner rubber plug to open the flow, and gravity does the rest.

This method requires no electricity and no additional equipment, but it is slow. For small above-ground pools it is practical enough, particularly overnight. For larger pools, a submersible pump is significantly more efficient.

Tilt the pool slightly toward the drain valve once the water level drops low enough to remove the remaining volume. The last few litres will collect in the low corner and can be removed with a sponge.

Siphon

A siphon requires a long hose, a discharge endpoint lower than the pool floor, and a few minutes to set up. Submerge the hose completely to fill it with water, seal both ends with your thumbs, place one end at the pool floor and the other at the lower discharge point, and release both ends simultaneously. Gravity pulls water continuously through the hose.

Flow rate is limited by hose diameter. A standard garden hose siphon on a large pool will take an extremely long time. This method is best suited to small above-ground pools or situations where no other option is available.

Draining a Pool Step by Step

Step one: reduce chlorine levels and test the water before discharging. Stop chlorination and allow UV exposure for several days, then test that free chlorine is at or below 0.1 ppm.

Step two: check the weather forecast. Confirm dry conditions for the duration of the drain and at least 24 to 48 hours of refilling time. Do not drain an inground pool if heavy rain is forecast.

Step three: switch off all pool equipment and disable automatic timers.

Step four: set up your submersible pump at the deepest point, run the discharge hose its full length to the sewer or cesspool connection, and check for kinks or restrictions.

Step five: start the pump and monitor. Do not leave the process unattended for extended periods. Check regularly that the hose is discharging correctly and that the pump remains submerged.

Step six: switch off the pump before it runs dry. Remove remaining water with a wet vacuum or mop.

Step seven: once the pool is empty, inspect the shell, walls, and floor before doing anything else. This is the best opportunity to identify cracks, staining, calcium deposits, or areas needing repair.

What to Do With an Empty Pool

An empty pool is the ideal moment for tasks that are impossible or impractical to do with water present.

Acid washing removes persistent calcium scale, mineral staining, and deeply embedded algae from the pool surface. It is aggressive – acid washing should not be done on every drain cycle, only when staining genuinely cannot be resolved by other means, and it removes a fine layer of the pool surface with each application.

Surface inspection and crack repair is far easier on dry walls and floor. Check all fittings, return jets, skimmer housing, and the pool floor carefully. Small cracks sealed when empty are far less expensive than the same cracks diagnosed after refilling.

Tile cleaning and grout work, resurfacing, replacing light fixtures, and installing new fittings are all best done with the pool empty and dry.

Do not leave a vinyl liner pool empty in hot sun for extended periods. Direct UV exposure degrades an empty liner, causing it to shrink, stiffen, and become brittle. If repairs take longer than anticipated, cover the pool or refill it.

For concrete and fibreglass pools, avoid leaving them empty for extended periods as well. Concrete can develop shrinkage cracks as it dries out, and fibreglass shells are at ongoing risk from hydrostatic pressure below.

Refilling and Rebalancing

Refilling a pool is not as simple as turning on the hose and walking away. As fresh water fills the pool, the chemistry starts from zero. Tap water in Portugal varies significantly in mineral content and pH depending on the municipal supply and region; water in Lisbon tends to be harder and more alkaline than in parts of the Algarve or Minho.

Allow the pool to fill completely before beginning any chemical adjustments. Do not attempt to test and balance while the water level is still rising, since chemistry will shift as each new volume of water enters.

Once full, test alkalinity first and adjust to between 80 and 120 ppm. Then adjust pH to between 7.2 and 7.6. Then add chlorine to bring free chlorine to between 1 and 3 ppm. Run the pump for at least 24 hours to circulate the chemistry fully before testing again and fine-tuning.

Do not swim in a newly refilled pool until chemistry has been tested, balanced, and stable for at least 24 hours. Unbalanced water with extremes of pH or chlorine causes skin and eye irritation and can damage swimwear.

Draining Pools in Portugal: Environmental and Legal Context

Portugal does not have uniform national legislation specifically governing domestic pool drainage, but municipal regulations in many areas restrict or prohibit the discharge of chemically treated water into storm drains, public gutters, or watercourses. The safest and most consistently compliant approach is to always discharge pool water to a sewer connection or registered cesspool.

In areas served by ETAR (estação de tratamento de águas residuais) wastewater treatment infrastructure, pool water discharged to the sewer will be treated before re-entering the water cycle. In rural areas with septic systems, large volumes of chlorinated water can disrupt the bacterial balance of the septic tank – this is another reason to reduce chlorine to near-zero before draining.

During summer drought conditions, which affect Portugal regularly, water authorities in some municipalities may issue guidance on pool water reuse. Water from a pool being drained can, if chlorine levels are adequately reduced, be used to irrigate areas of non-edible landscaping. This is worth considering during the drier months as an alternative to allowing treated water to go directly to waste.

A Note on Natural Pools

Natural pools and swimming ponds operate on a fundamentally different basis and rarely need to be drained. The water in a natural pool is part of a living system; it is processed biologically by aquatic plants and beneficial microorganisms in the regeneration zone rather than by chemicals. There is no TDS buildup from chemical accumulation, no cyanuric acid problem, and no scenario in which the water chemistry becomes so imbalanced that the only solution is to empty and refill.

Partial water exchange – replacing a portion of the water volume to address mineral concentration – is occasionally relevant, but full drainage is almost never necessary. For pool owners who have drained their conventional pool multiple times and are starting to question the overall logic of the system, understanding how natural pools work differently is a useful exercise.

Conclusion

Draining a pool is the kind of task that rewards preparation. The actual process is not complicated, but the decisions made before starting – where the water goes, whether the ground conditions allow safe drainage, whether the equipment is correctly turned off, whether chlorine levels are safe for discharge – determine whether the job ends cleanly or expensively.

Done with a plan, drainage becomes a straightforward maintenance window: a chance to inspect the shell, complete any repairs, and start the next season with fresh water and a clean baseline. Done without one, it becomes the thing pool owners talk about with regret.

Prepare properly, drain slowly and monitored, discharge responsibly, and use the empty pool well. The refill and rebalance takes a day. Everything else is just good sequencing.