Overflow Pool Balancing Tank: Design & Function

The balancing tank is an essential part of every infinity pool system. Positioned between the pool and the pump, it forms the hidden reservoir on the hydraulic circuit of an in-ground or semi-in-ground pool. Depending on the country or technical context, it may also be called a surge tank, balance tank or catch basin.

What is a balancing tank

A balancing tank is the hidden reservoir that collects the water overflowing from an infinity or zero-edge pool. From this tank, the pump draws water back into the filtration system before returning it to the pool. It is the key element that makes the infinity pool circulation possible.

What does a balancing tank do in an overflow pool?

The surge tank maintains a constant water level despite evaporation, rainfall or the number of swimmers. By absorbing these variations, it ensures the overflow remains smooth and continuous. This balance must be designed from the start, as explained in our Infinity Pool Design & Construction Guide

The different kinds of balancing tanks

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

Built-in balancing tank

For a large overflow pool, the balancing tank, also named surge tank, is generally built in concrete in the pump room.
For a smaller pool, the overflow gutter can be used as a balancing tank.

These 2 solutions require a watertight seal to be applied to the bottom and walls of the tank. Using a concrete surge tank for a large pool offers several advantages.

Concrete has three advantages. It withstands constant exposure to water and chemicals. It can be built to the pool’s exact dimensions and flow rates. And once the floor and walls are sealed, it is fully watertight.

Prefabricated tank

Many suppliers of buried tanks, not only pool equipment manufacturers, can provide these cisterns. They are prefabricated concrete or moulded high-density polyethylene. If the site does not allow the balance tank to be buried, a plastic tank above ground is a cheaper option.

How to calculate the balancing tank capacity?

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 overflow type: a perimeter overflow has a much longer spill edge than a single infinity edge, so it needs a larger balancing tank.
• the overflow length,
• the expected number of swimmers; this is a question of whether your pool is for strictly private use or whether you expect to have numerous people in it,
• the flow rate of the pump and the filter diameter,
• the presence of an auxiliary pump, which improves the overflow effect by increasing the thickness of the water layer.

The key components required for the operation of the balance tank

The balancing tank alone cannot do its job. The design of the filtration system must also include :

• An overflow to prevent the tank from overflowing in the plant room when there are many swimmers in the pool or when it rains. It is usually connected to the main sewer.
• A non-return valve, which stops the pool from draining back into the balancing tank when the pump is off. If this valve sticks slightly open, pool water flows back into the balancing tank by gravity, rises to its overflow pipe and runs to waste. Depending on how high the overflow pipe sits in the balancing tank, the pool can lose some or most of its water.
•  A water level control system is crucial; it acts in both cases: lack of water or excess of water in the balancing tank.
• The regulation system controls a water inlet with a solenoid valve. The water is topped up automatically according to water losses due to evaporation, splashes from swimmers, etc…
• A suction hose between the balancing tank and the pump; that’s obvious, but it doesn’t cost anything to say it.

What is the price of a balancing tank

For a prefabricated high-density polyethylene balancing tank of 4 m³ you need to plan around €1,300. The same prefabricated concrete tank will cost you only €900. A concrete balancing tank can also be built in the plant room at the same time as the pool. Expect €2,200 to €3,000 for the same volume, waterproofing of the floor and walls included.

The overflow channel as a balance tank is a slightly cheaper solution if you agree to leave it open. Beware of cheap plastic tanks: their walls are too thin to withstand the pressure of the surrounding soil. Burying them means building a blockwork wall around the tank or using a stabilised sand-and-cement backfill. In the end, this option can work out more expensive.

Conclusion

The balancing tank must stay accessible, with a maintenance opening for cleaning. A basket or a net at the gutter outlet stops the larger debris, and toys, before they reach the tank. Cleaning the gutter is simpler than climbing into the tank.

We do not recommend placing the check valve in the balance tank. Cleaning it becomes much harder. When the tank is installed, the overflow, the suction line to the pump and the level-control lines have to be in the right place.