The white lid at the bottom of the pool is known as a maindrain, here you’ll find a detailed explanation of the hydro-static valve.
Pool Hydrostatic Valve Information Page
Hydro-static Valve
What is a Hydro-static Valve?
A hydrostatic valve is a pressure relief one-way valve or “plug” installed usually at the deepest point of a swimming pool. The valve usually comes in two sizes: 40mm & 50mm A valve removal tool is normally needed to remove it.
Without a Hydrostatic valve styled relief from hydrostatic pressure, a swimming pool may be susceptible to the pressure and “pop” or lift out of the ground, in concrete pools it is less likely to lift but can be prone to cracking concrete by the added hydrostatic pressure.
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Where is the Hydro-valve located?
Usually it is at the deepest point of the pool floor, the valve is located under the lid which sits at the bottom of the pool covering the main-drain.
What does a Hydro-static Valve do?
Also known as a Hydro-valve it prevents ground-water pressure from raising the pool or causing movement. The hydrostatic valve prevents this by allowing ground-water from your surrounding yard to flow into the pool and relieve the pressure on the pool structure.
Why do swimming pools need a hydro-valve?
Swimming pools need a hydro-valve (or hydrostatic relief valve) to protect against groundwater pressure building up beneath the pool. When the water table rises, pressure under the pool can cause cracks, movement, or even lift the pool out of the ground. The hydro-valve prevents this by allowing excess groundwater to safely enter the pool, relieving pressure. If you notice slightly dirty water entering the pool or a small overflow, it’s a good sign the valve is working properly.
Should a hydrostatic valve be removed when a swimming pool is empty?
Yes, it is necessary to remove the hydrostatic valve when the swimming pool is empty. This allows for inspection and ensures the valve is functioning correctly. If the existing valve is blocked, damaged, or faulty, it can lead to dangerous hydrostatic pressure buildup when the pool is refilled. Replacing or servicing the valve during renovation or maintenance is a crucial step in protecting the pool shell from movement or cracking caused by groundwater pressure. A new hydrostatic valve should always be installed prior to refilling the pool to ensure proper pressure relief and long-term structural safety.
What are the chances of a swimming pool moving?
It can vary depending on location, pool structure & other factors, there is no way to determine exact chances or percentage of pools raising. Scenarios may vary, and can be dependant of having a one way valve relief system that is or isn’t working. For example: If your pool is located at higher altitude on top of a small hill, the chances of movement by ground water pressure may be a lot smaller than the exact same pool if it were installed at the bottom of a valley. This is because ground water tends to move deep into the ground and may be more abundant at the bottom of a hill than at the top. A similar rule may apply if your pool is located in front of a beach, the chances of movement may be higher than a pool situated in a suburb much further away from the shore.
Can a swimming pool lift out of the ground?
Can a Swimming Pool Lift Out of the Ground? Yes, a swimming pool can lift or “pop” out of the ground due to excessive hydrostatic pressure, usually caused by a high water table or heavy rainfall. This pressure builds up beneath the pool and, if not managed, can push the pool shell upward—especially during extended periods of wet weather. How Does a Hydrostatic Valve Prevent Pool Movement? To prevent structural damage, a hydrostatic valve (also called a hydro-valve) is typically installed at the deepest point of the pool. This valve allows ground-water to safely flow into the pool when external pressure builds up, reducing the risk of movement, cracks, or lifting. This protection is crucial during swimming pool renovations when the pool may need to be emptied. What If My Pool Doesn’t Have a Hydrostatic Valve? Not all concrete pools have a main drain or hydrostatic valve—some may never have had one, or it may have been removed or sealed off during previous work. That’s why part of our pool inspection process includes evaluating each swimming pool’s structure, history, and signs of stress such as cracking or uneven movement. We tailor every pool renovation based on these findings, ensuring the best long-term outcome for your pools safety and structural stability.
Want to learn about more swimming pool terminology?
When you’re done learning about Hydro-static valves, check out the swimming pool dictionary.
How to remove a hydrostatic valve?
Local Pool Renovations encourages you to hire the help of a pool professional. However, if you have emptied your pool, you may want to remove the hydro-valve as soon as possible, most times the valve is hard to remove by hand. You can use a hydro valve removal tool, usually available for hire at your Local Pool Shop, all you need to do is figure out if you need a 40mm or 50mm removal tool, otherwise you will end up hiring both.
Hydro-valve Removal tool
Other factors to consider about the chances of a swimming pool moving.
Fibreglass & Vinyl pools
Vinyl & fibreglass pools have a greater chance of lifting or shifting because these structures weigh less and therefore it would take less ground water pressure to cause movement. Fibreglass pools in particular have been known to have the floor “popping up” once empty.
Concrete Pools
Concrete pools are less likely to shift or raise, because it is a steel reinforced concrete structure that weighs a considerable amount more than any other pool build. The important thing to remember is that all pools have a tolerance and all pools can and might move, empty or not.
How long can a swimming pool stay empty?
There is no general rule that can answer this question, so many factors come into play that it makes it difficult to answer, and the only problem with having an empty pool is that it increases the likelihood of movement that may cause problems, refer to the above what are the chances of a swimming pool moving.
How often do I need to change the Hydro-static Valve?
It is recommended that hydrostatic valves be changed on average every 6/7 years, as the rubber seals may deteriorate over time due to corrosion from pool chemicals.
Do all swimming pools have a hydro-valve?
Not all pools have a hydrostatic valve. However most swimming pools do usually have some kind of pressure relief system in place. Older pools may have a similar version of the valve known as a hydro-plug, which also acts as a pressure relief system. A hydro-plug usually looks like a plastic white rim around the size of a dinner plate located at the bottom of the pool, Inside the rim it usually has the same render as the rest of the pool interior. The majority of swimming pools do have a plastic/pvc hydro-valve and main-drain. Some pools have a main-drain but find that inside they have a copper fitting instead of a hydro valve, this copper usually oxidezes very quickly, is almost impossible to remove & renders the fitting useless. It’s best to remove this fitting without causing damage or install a new main-drain and pressure relief system.
Hydro plug
Applicable to some older swimming pools.
Some older concrete swimming pools were built without a main drain and hydrostatic valve system. Instead, they rely on a hydro-plug, which serves a similar purpose in managing groundwater pressure beneath the pool. Inside the hydro-plug is a rendered surface with a pop-up section. In the event of excessive groundwater pressure, this plug is designed to lift slightly, allowing water to enter the pool and relieve the pressure under the shell—helping prevent damage such as cracks or lifting. Why is there a small pipe or plug in the floor of my pool? Next to the hydro-plug on the pool floor, there is often a second pipe. This is typically a secondary suction point connected to the skimmer box. It serves two purposes: 1. It helps filter debris from the pool floor. 2. It provides an alternate water intake if the skimmer box becomes blocked or full, reducing strain on the pool pump. While hydro-plugs are functional, they are considered outdated. During pool renovations, we often recommend upgrading to a modern main drain with a hydrostatic valve system for improved reliability and protection.
Can a hydro-plug be changed?
Yes, a hydro-plug can be removed and replaced. In fact, we recommend removing the hydro-plug and installing a proper main drain with a hydrostatic valve system. This upgrade provides a more reliable and effective way to manage groundwater pressure, helping to protect the pool structure from movement, cracking, or other damage. Replacing the hydro-plug with a modern hydrostatic valve system also brings your swimming pool up to current safety standards, improving its long-term performance and durability. The best time to make this change is during a pool renovation, when the pool is already empty and there is easy access to the pool floor. This allows for a smooth and efficient installation with minimal disruption. For those interested in learning more, we have helpful videos on our YouTube and Instagram channels that explain the process in detail and show real examples of hydro-plug replacements and upgrades. These resources can guide you through what to expect and how this important upgrade can protect your pool for years to come.
This photo shows the Hydro plug and a secondary suction connected to the skimmer box, the surface is a deteriorated marblesheen render.
Should swimming pool water be changed?
When changing the hydrostatic valve it is also a good opportunity to replace the pool water, Pool water is treated with strong chemicals, and has biological remains such as oils, dead skin and so forth… Swimming Pool water can no longer be effectively treated after an estimated 6-7 years, this can be longer with regular top ups and if in an area where rainfall is common.
Groundwater Pressure and What’s Really Happening Below Your Pool
Groundwater pressure is caused by water building up in the soil beneath and around a swimming pool. When the ground becomes saturated — from heavy rain, poor drainage, or a naturally high-water table — that water has nowhere to go. The pressure then pushes upward against the underside of the pool shell.
When a pool is full, the weight of the water inside the pool helps counteract this pressure. When a pool is emptied, that balancing weight is removed, allowing groundwater pressure to act directly on the structure. This is when movement, cracking, or lifting becomes far more likely.
Hydrostatic valves exist for this exact reason. They provide a controlled release point, so pressure does not build unchecked beneath the pool.
High Water Tables and Why Location Matters
A high-water table simply means groundwater sits closer to the surface than normal. In these areas, pressure beneath a pool can increase quickly, sometimes even without obvious surface water or flooding.
Pools located in low-lying land, valleys, coastal zones, or areas with clay-heavy soils are more exposed to this risk. Even properties that appear dry on the surface can experience groundwater pressure beneath the pool shell after prolonged rainfall.
This is why two identical pools can behave very differently depending on where they are built. Location, soil conditions, drainage, and surrounding terrain all play a role in how much pressure develops beneath a swimming pool.
Maintenance, Inspection, and Replacement Considerations
A hydrostatic valve is a working component, not a set-and-forget fitting. Over time, seals can deteriorate, debris can build up, and internal components can seize or fail.
Whenever a pool is emptied — particularly during renovations — the valve should be inspected. If it shows signs of wear, blockage, corrosion, or stiffness, replacement is strongly recommended. A faulty valve may look intact but fail to open when pressure builds, which defeats its entire purpose.
Replacement is straightforward when the pool is already empty and accessible, and far less costly than repairing structural damage caused by pressure buildup. For this reason, installing a new hydrostatic valve during renovations is considered best practice.
Once the pool is refilled and operating, the valve should be left undisturbed but checked periodically during future maintenance to ensure it remains clear and functional.
A Practical Reality Check
Hydrostatic pressure events are unpredictable. Some pools will never experience an issue, while others can move after a single prolonged wet period. There is no warning sign when pressure reaches a critical point — failure happens suddenly.
The role of a hydrostatic valve is not to eliminate groundwater, but to manage it safely. When it works correctly, it does so quietly in the background. When it fails or is ignored, the consequences can be severe.
What h appens if a hydrostatic valve is blocked?
If a hydrostatic valve is blocked or stuck closed, groundwater pressure has no release point. Pressure can build rapidly beneath the pool shell, increasing the risk of cracking, movement, or the pool lifting. This can occur suddenly, especially during wet weather or when the pool is empty.
Can a hydrostatic valve look fine but not work?
Yes. A hydrostatic valve may appear intact from above but still be seized or blocked internally. Sediment, calcium buildup, corrosion, or deteriorated seals can prevent it from opening when pressure builds. This is why inspection during renovations is important.
When is the highest risk time for hydrostatic pressure damage?
The highest risk period is when a swimming pool is empty during wet conditions. Rain raises groundwater levels while the removal of water inside the pool removes the counterweight that normally holds the shell down.
How do I know if my hydrostatic valve is working?
Signs that a hydrostatic valve is functioning include minor water seepage into the pool floor area, slightly dirty water entering after heavy rain, or small temporary changes in water level. These indicate pressure is being relieved as intended.
Why do hydrostatic valves need replacing over time?
Hydrostatic valves operate in harsh conditions at the lowest point of the pool. Over time, seals can perish, internal parts can seize, and debris can block movement. For this reason, valves are commonly replaced during renovations as preventative protection.
Is replacing a hydrostatic valve necessary during pool renovations?
While not always mandatory, replacing the hydrostatic valve during renovations is considered best practice. The pool is already empty, access is easy, and the cost is minimal compared to repairing structural damage caused by pressure buildup later.
Can groundwater pressure occur even if my yard looks dry?
Yes. Groundwater pressure can exist beneath the surface without visible water. Properties in low-lying areas, coastal zones, clay soils, or former wetlands are especially prone, even when the surface appears dry.
Is a hydrostatic valve a plumbing fitting or a structural safety device?
A hydrostatic valve should be treated as a structural safety component. Its role is to protect the pool shell itself, not just manage water flow.
Disclaimer.
The above information is provided purely for informative purposes, the explanations and information above are from a collection of personal experiences and researched data. LPR do not claim to be engineers and cannot guarantee the accuracy of the information provided. This information is not supplied for anyone within the pool industry to use as a reference or guide. LPR does not recommend having a pool empty, especially for long periods of time. Local Pool Renovations does not encourage anyone to “Do it yourself”. Liability for any resulting consequence of action weighs solely on the pool owner as we encourage pool owners to hire swimming pool professionals. LPR does not accept any legal liability for “problems or damage” that may result from work being performed by you or hired staff outside of LPR employment. Pool owners may be breaching their contractual obligation or voiding their warranty by emptying the pool without first advising their former pool builder or renovation specialist.