The balance tank: the “heart” of an overflow pool

What the tank does

In an overflow pool, the water comes right up to the edge and spills into the channel under the grating. From there it flows down by gravity into the balance tank. The pump draws from the tank, the water is filtered and goes back into the pool through the return inlets. The tank does three jobs:

• It absorbs displacement. When people get in, the volume of their bodies raises the water level and the extra water flows into the channel. The tank holds it instead of letting it go to waste.
• It returns the water when swimmers get out, so the pool fills back up to the edge.
• It protects the pump, which must always have water on the suction side.

How its volume is worked out

The volume is not a fixed figure; it is the sum of the water that has to fit in every situation. The engineer takes into account:

1. Bathers. Each adult displaces roughly as many litres of water as they weigh in kilos, i.e. a few dozen litres. A busy pool at guest accommodation needs a bigger margin than a family pool.
2. The thin layer of water spilling over along the whole length of the overflow edge. The longer the perimeter, the more water is on its way to the tank at any moment.
3. The water in the channel and pipes, which drains down into the tank when the pump stops.
4. Waves and rain, plus a safety reserve so the pump is never left without water.

That is why a single-side overflow, with a shorter overflow edge, usually needs a smaller tank than a perimeter overflow on a pool of the same size.

Level control

The water level in the tank goes up and down all the time, and that is normal. To keep it within safe limits, the tank usually has:

• Level sensors or float switches, read by the control panel.
• Automatic top-up through a solenoid valve, which opens when the level drops due to evaporation or a filter backwash.
• Dry-run protection, which switches off the pump if the water drops too low.
• A safety overflow to the drain for heavy rain.

Location and access

The tank sits lower than the channel, so the water flows down by gravity, and close to the plant room, so the pump's suction line is short. It is usually buried in the ground, with an access lid in the paving. The lid must open easily, because that is where you check the floats and clean. Its material and size are decided at the site survey, depending on the pool and the plot.

Maintenance

Once a year, usually before the season, the tank is cleaned of leaves and sediment that get through the grating. Check that the floats move freely and that the solenoid valve closes properly. A valve that does not close keeps filling the tank, and the water is lost through the safety overflow without you noticing.

When the pool is closed for winter, its water level is lowered below the edge, so the overflow stops and the tank is no longer fed. The automatic top-up is switched off so it does not fill the tank for no reason, and in areas with frost the tank and any exposed pipework are drained.

Questions

See also