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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.

Hydro-Static Valve

How_to_remove_a_Hydro_static_Valve_what_is Hydro-Static Valve Information

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.  

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?

A build-up of ground water can apply surprising amounts of hydraulic pressure on structures such as swimming pools.

Dirty water in the pool or a slight overflow can be the result of the excess pressure, which is a good sign the valve is working.

Does a hydro-static valve need to be removed when a pool is empty?

Yes, it is necessary to remove the hydrostatic valve when the pool is empty in case it is not working and a new valve must be installed prior to re-filling.

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 move or crack, lift or shift?

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.

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.

Inside of the Hydro plug is a rendered surface within the rim, this is the part that would pop up to allow excess water pressure into the pool.

On the swimming pool floor next to the hydro-plug is another "pipe" this is typically a second suction point for your skimmer box & serves two purposes:

  1. As a secondary suction to filter debris off the pool floor.
  2. In case the skimmer box is extremely full and is struggling, this may help relieve the pressure that is being applied to the pool pump by allowing an additional flow of water.

Can a hydro-plug be changed?

Yes, we recommend that the Hydro-plug be removed & have a new main-drain installed with the Hydro-static valve system.

This would essentially be more a reliable system and also modernize your swimming pool.

The best time to do this would be during a swimming pool renovation.

Hydro_plug_valve_suction Hydro-Static Valve Information

This photo shows the Hydro plug and a secondary suction connected to the skimmer box, the surface is a deteriorated marblesheen render.

Changing Swimming pool water

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.


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.