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Balancing tank and overflow swimming pool

By April 7,2020 May 12th, 2021 No Comments

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The balancing tank and the overflow swimming pool system always work together. The first one is located on the hydraulic circuit of any in-ground or semi in-ground pool, between the pool and the pump. Some people call it also a compensation tank.

The purpose of the balacing tank in an overflow swimming pool

In a classic skimmers pool, the water is sucked directly into the pool by the pump and is then returned to the pool after filtration and possibly treatment and heating.
In an overflow pool the water circulation is slightly different. The balancing tank is equipped with a bottom drain to allow the pump to suck up the water and send it into the filtration circuit and then to the pool. That excess of water send into the pool overflows from the pool into a gutter and then flows by gravity into the balancing tank.
infinity swimming pool with sea view
The role of the balancing tank is also to compensate variations in the water level in the pool. The modification of the water level in the balancing tank can be due to normal evaporation, number of swimmers, rain; without it, the water could run out and get lost in the nature, which would necessitate daily adding water to compensate for the losses.
Therefore and thanks to the balancing tank the hydraulic circuit of the pool is also a closed circuit.

The different types of balancing tanks

There are mainly two kinds of balancing tanks for overflow swimming pools. The choice depends on whether the pool is built by a professional or not.

Balancing tank built

For a large infinity pool, the balancing tank is usually built in concrete in or near the pump room. For a smaller water body, the gutter can be used as a balancing tank; in this case it is called a gutter/balancing tank. Both solutions require that a waterproofing lining is applied onto the bottom and the walls of the balancing tank.  The construction of a balancing tank is often the solution chosen by swimming pool specialists or builders.

The prefabricated balancing tank

Many suppliers of tanks to be buried (and not especially at a pool equipment manufacturer) can provide with such cisterns. There can be prefabricated concrete or moulded high density polyethylene tanks.
If the balancing tank does not have to be buried, depending on the configuration of the premises, even cheaper plastic type solutions can be found.

How to calculate the volume of the balancing tank?

You have probably already read that the volume of the balancing tank should be between 5 and 10% of the volume of the pool. This is the theory, but several factors come into play.
  • The volume of the pool,
  • the nature of the waterfall or perimeter overflow,
  • the length of the overflow,
  • the expected number of swimmers; this is a question of whether your pool is for strictly private use or whether you expect to have a large number of people in it,
  • the flow rate of the filtration pump,
  • the presence of an auxiliary pump which improves the overflow effect by increasing the thickness of the water layer.
If the balancing tank of a swimming pool is too small, the water will be washed away (and therefore the chemicals with it!); this can happen when the pool will be used by more swimmers or when it rains, for example. By oversizing it, you will spend money for nothing. Don’t take any risk for your pool and talk to us, as overflow pool specialists for over 25 years.

The accessories necessary for the operation of the balancing tank

The balancing tank alone cannot do its job. The design of the filter unit must also include :
  • A suction hose between the balancing tank and the pump; that’s obvious but it doesn’t cost anything to say it.
  • An overflow to prevent the tank from overflowing in the pump room when there is a large number of swimmers in the pool or when it rains. It is usually connected to the main sewer.
  • A non-return valve to prevent the pool from draining by this balancing tank overflow when the pump is stopped. However, if the pump stops when this non-return valve is slightly open, the pool water will return to the balancing tank by gravity and when the water level reaches the tank overflow pipe, the water will flow into the drain. In this way a pool can be siphoned off more or less completely, depending on the level of the overflow.
  • A water level control system is crucial; it acts in both cases: lack of water or excess of water in the balancing tank.
  • A water inlet equipped with a solenoid valve controlled by the regulation system. The water is topped up automatically according to water losses due to evaporation, splashes from swimmers, etc…

What is the price of a balancing tank

For a prefabricated high density polyethylene balancing tank of 3000 L you need to plan around 1000 €; the same prefabricated concrete tank will cost you only 500€, but you’ll have to pay the transportation.  A concrete tank built in the pump room at the same time as the swimming pool will have to be included in the total estimate; the price range is between 2,000 and 2,500 € for the same volume. This price must include the waterproofing of the bottom and walls of the tank. The use of the gutter as balancing tank is a slightly cheaper solution if you agree to leave it open.

For a cistern that is not buried and therefore not subject to the pressures of the ground, you will find it cheaper!
But beware of cheap plastic tanks that can only be buried with various precautions: a blocks wall to be built around the tank or a stabilised backfilling (sand/cement). At the exit this option could be more expensive.
Note: only in some cases it is possible to use the gutter as a balancing tank; if you wish to consult us on this point, we will be happy to help you.


The balancing tank for the overflow swimming pool must always be accessible; it must therefore have an opening (manhole type) for possible cleaning. A basket or a protective net judiciously placed at the exit of the gutter will block the biggest dirt, toys, … and will avoid you a plunge in it. It is easier to clean a simple gutter than to go down into the balancing tank of an overflow pool to clean it.
Some swimming pool owners install the non-return valve in the buffer tank; we do not recommend it at all. It makes cleaning the no-return valve much more difficult.
When installing the balancing tank, make sure that the overflow, the water suction hose to the pump and the hose for the balancing tank leveller devices are properly installed.
These and many other details are overlooked by professionals who are new to infinity pool construction.

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