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 — different words for the same vital component.
What is a balancing tank
A balancing tank — also called surge tank or catch basin — 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.
Firstly, concrete provides a durable and long-lasting structure that can withstand the constant exposure to water and chemicals.
Additionally, a concrete balancing tank allows for precise customization and can be designed to accommodate specific pool dimensions and water flow requirements.
Finally, the watertight seal applied to the bottom and walls ensures that the buffer tank effectively prevents water leakage, providing a reliable solution for pool designers and builders.
Prefabricated tank
Many suppliers of tanks to be buried (and not especially pool equipment manufacturers) can provide such cisterns. There can be prefabricated concrete or molded high-density polyethylene tanks. Suppose the site configuration does not allow the burying of the balance tank. In that case, it is possible to find even more economical solutions in plastic.
Morana's advice
Making the surge tank the right size
Suppose 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 forget the security features
Not having safety features in a pool surge tank can lead to various issues. For example, it may not be able to handle sudden surges in water volume.
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 kind: You can choose a perimeter with the surface water flowing over the perimeter or one edge (generally a length) lower than the deck level to give an infinity illusion,
- 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 check 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.
- 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 1 gal (3.79 l) US, you need to plan around $1,500. The same prefabricated concrete tank will cost you only $1,000. A concrete tank built in the pump room simultaneously as the swimming pool will have to be included in the total estimate; the price range is between $2,500 and $3,500 for the same volume. It includes the cost of the waterproofing of the bottom and walls of the tank.
The overflow channel as a balance tank is a slightly cheaper solution if you agree to leave it open.
The cheapest option is an unburied catch basin.
Beware of cheap plastic tanks that can only be buried with various precautions: a blocks wall to be built around the tank or a stabilized backfilling (sand/cement). At the exit, this option could be more expensive.
Conclusion
The balancing tank for the overflow swimming pool must always be accessible; it must therefore have an opening (maintenance hole type) for possible cleaning. A basket or a protective net judiciously placed at the channel’s exit will block the biggest dirt, toys, … and will avoid you from plunging into 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 check valve in the balance tank; we do not recommend it at all. It makes cleaning the check valve much more difficult.
When installing the catch basin, make sure that the overflow, the water suction hose to the pump, and the hose for the balancing tank leveler devices are correctly installed.
Explore our detailed hydraulic drawings for infinity and zero-edge pools
Our clients often ask us these technical questions about the balancing tank; here are our answers.
Frequently Asked Questions — Infinity Pool Balancing Tank
What is a balancing tank in an infinity pool?
A balancing tank — also called a surge tank or catch basin — is a secondary reservoir built alongside the pool to collect all water that spills over the infinity edge. It feeds directly into the filtration and circulation system, maintaining a continuous and stable hydraulic circuit. Without it, an overflow pool simply cannot function.
Do all infinity pools and zero-edge pools need a balancing tank?
Yes, without exception. The balancing tank absorbs the constant fluctuations caused by rain, evaporation, bather load and thermal expansion. It ensures the water level at the overflow edge remains perfectly constant — which is what creates the seamless mirror effect characteristic of a well-designed infinity pool.
How do you calculate the right balancing tank capacity?
The standard starting point is 5 to 10% of the total pool volume. This figure is then refined based on overflow edge length, expected bather numbers, pump flow rate and local climate. There is no universal formula — every project requires precise hydraulic calculation to avoid overflow failure or pump cavitation.