How the Overflow Channels and the Balance Tank in Swimming Pools Work
Introduction: A deck-level pool is a swimming pool where the water level reaches the edge of the pool and spills over into surrounding overflow channels. Unlike a skimmer pool, where water is drawn directly from the pool through skimmer openings in the walls and the level is slightly below the edge, a deck-level pool maintains a uniform water level at the height of the surrounding flooring using overflow channels and a balance tank. This system provides highly efficient circulation: water flows over from all sides of the pool, collecting floating debris, and accumulates in the overflow channels. From there, gravity directs the water into the balance tank, located between the pool and the pumps, which stores a variable amount of water to balance the pool's level. The water is then drawn from the balance tank by the circulation pump, filtered and disinfected in the machine room, and returned clean to the pool through the inlets—thus completing the cycle. Below is a detailed explanation of how the overflow channels and balance tank function, how to calculate the tank's volume based on the pool type, and maintenance and construction guidelines to ensure optimal system performance.
1. How the Overflow Channels and Balance Tank Work
Water Flow from the Overflow Channels to the Balance Tank: When the pool water level reaches its edge (e.g., due to swimmers entering or waves), the excess water flows uniformly into the perimeter overflow channels. These channels are embedded around the pool edge and collect 100% of the excess water from all directions. The water then continues to flow by gravity through dedicated piping into the balance tank (also called the equalization tank or balancing chamber). The balance tank is usually located near the pool and close to the machine room, sometimes as an underground pit with a separate cover or as an integrated part beneath the machine room. The tank is connected to the channels using the principle of communicating vessels, ensuring that water levels balance passively between the pool, channels, and tank.
Maintaining Water Level and Continuous Supply to the Pool: A key role of the balance tank is to supply water to the pool continuously to maintain its level at the floor height. When swimmers enter the pool or large waves occur, an equivalent amount of water overflows into the tank—preventing excessive rises in the pool's water level. Conversely, when swimmers leave and the water level drops, water flows back from the balance tank into the pool (via the pump) to replenish the loss and maintain a constant height. This balancing process allows the pool to remain "flooded" to the edge consistently, regardless of momentary fluctuations in water volume. Essentially, the balance tank prevents water shortages in the pool during circulation and ensures that despite evaporation, splashes, or even water being drawn back through the filter, the pool remains full. Without a balance tank, the pool water might spill out or drop below the desired level, necessitating frequent manual refilling. Thanks to the tank, the hydraulic system remains a closed-loop.
Preventing Air Intake into the Filtration System: The balance tank serves as the suction point for the circulation pump instead of drawing directly from the channels. The pump extracts water from the bottom of the balance tank (through a lower suction pipe), preventing air from entering the system even if the water level fluctuates. This is critical: if the pump attempted to draw directly from the overflow channels, there is a risk that the water level could drop enough to allow air into the pipes—causing cavitation, which damages the pump. A properly designed balance tank maintains a water volume that prevents air suction, ensuring water is always drawn from the lower level inside the tank. Additionally, level monitoring devices (floats or height sensors) are installed in the tank and connected to valves or pumps to shut off the pump if the level is too low or to close an emergency outlet if the level is too high—keeping fluctuations within a safe range.
Filtering Floating Debris and "Skimming" the Surface Layer: One of the significant advantages of a deck-level pool is the removal of floating debris across the entire water surface. In this design, all light debris (such as leaves, dust, pollen, oils, and hair) that floats on the water naturally flows with the water into the channels. Essentially, a 360-degree skimming effect is created: the entire pool perimeter functions as a continuous skimmer, making the collection of floating debris highly effective. The debris initially accumulates in hair strainers (mesh filters) at the channel piping entrance and later settles or floats within the balance tank itself. The balance tank acts as a collection station for floating debris—where oils, leaves, and other contaminants settle. Some debris sinks to the bottom of the tank, while some remain floating on the water surface inside the tank until removed. Regardless, the water returning from the tank through the filtration system to the pool is significantly cleaner after undergoing mechanical separation of floating debris. This is a primary reason why deck-level pools are considered more hygienic and clean—the surface contaminants are continuously removed, unlike in skimmer pools, where debris may accumulate in areas far from the skimmers.
2. Calculating the Balance Tank Volume and Adapting It to the Pool
Importance of Balance Tank Size: The volume of the balance tank is a critical design parameter. A tank that is too small cannot contain all the excess water during peak use—leading to tank overflow and water loss to the drain, or worse, air intake into the pump. Conversely, an oversized tank unnecessarily increases construction costs and requires storing a larger volume of treated water than needed. Therefore, the tank size must be designed to meet the hydraulic balance requirements of the pool under maximum usage scenarios but should not be excessively large. The required volume primarily depends on the pool surface area (which determines the potential number of swimmers at a given time) and its depth since these factors dictate how much water might overflow under load. The flow rate of the pumps and the length of the overflow perimeter also influence volume calculations—since a more intense circulation system and longer channels allow more water to flow into the tank quickly (thus, in small pools with relatively long overflow channels, increasing the tank size is advisable).
The balance tank volume is typically calculated as a percentage of the pool surface area, based on pool size. The design guidelines recommend:
- Pools under 200 m² require a balance tank with a volume equal to approximately 3% of the pool surface area.
- Pools over 200 m² require a balance tank with a volume equal to approximately 5% of the pool surface area.
For example, a pool measuring 25×12.5 meters (312.5 m² surface area) would require a balance tank of 9.375 cubic meters (5% of the surface area), with additional buffer volume for extra security. This ensures that all excess water collects in the tank rather than spilling into the drainage system.
3. Water Level Control in the Balance Tank
Float switches are installed inside the balance tank to regulate water supply and pump operation based on water height:
- Upper float – Activates freshwater supply to the tank when water level drops too low. It opens a valve to add city water until the level returns to normal.
- Lower float – Protects the pumps: If the water level drops too low, this float automatically shuts off the circulation pumps, preventing them from sucking in air and getting damaged.
4. Maintenance and Construction Recommendations
To ensure optimal operation, periodic maintenance is necessary:
- Regular cleaning of the balance tank to remove accumulated dirt and debris.
- Maintaining disinfection levels by adding chlorine tablets in a floating dispenser.
- Using appropriate coatings like ceramic tiles or epoxy to ease cleaning.
- Providing proper access with a sealed cover to prevent contamination and facilitate maintenance.
A well-maintained balance tank enhances water hygiene, prevents system failures, and ensures efficient pool operation over time.