Overflow and infinity pools: balance tanks, channels and why they lose water differently

How an overflow pool differs from a skimmer pool

In a skimmer pool the water sits a little below the coping, and skimmers in the wall draw off the surface water. Dubai Municipality's pool plan guidelines describe the purpose of skimmers as removing dust, debris, floating leaves and thin oil layers from the surface.

In an overflow pool the water spills over an edge into a channel (also called a gutter), and runs from there to a balance tank, sometimes called a surge tank. The pump draws from the tank, sends the water through the filter and treatment, and returns it to the pool. In a deck level or perimeter overflow pool the edge runs all the way round. In an infinity or vanishing edge pool, water falls over one or more weirs into a catch channel below.

The edge has to be accurate. The US Model Aquatic Health Code (MAHC), a public pool code published by the CDC, requires perimeter gutters to be level within plus or minus 1.6 mm around the pool, to be able to remove water at 125 percent of the design circulation flow, and to allow inspection, cleaning and repair. Villa pools are not built to that code, but it shows how precise an overflow edge needs to be to work evenly.

What the balance tank does

When people get in, they push water over the edge. The tank has to hold that water, plus the water flowing through the channels and pipes at any moment, without overflowing or running dry.

• Capacity. The MAHC requires perimeter overflow systems to have a net surge capacity of at least one gallon per square foot of pool surface (40.7 litres per square metre).
• Marked levels. It requires the minimum, maximum and normal operating levels to be marked on the tank.
• Overflow to waste. Surge tanks must have an overflow pipe that sends excess water to waste through an air gap or similar protection.
• Automatic make up. Automatic make up water equipment must be provided to keep skimming continuous.

AQUA Magazine's builder guide to vanishing edges adds the practical side. A surge chamber always needs some water in it, because if it runs nearly dry the pumps can lose prime. The builder interviewed also says the commercial rule of thumb for surge capacity is often not enough for a busy family pool, where there are more swimmers per square metre.

Why they lose water differently

The pool always looks full. The pumps refill the pool to the edge, so the water line in the pool tells you very little. The builder in the AQUA article describes service visits where the pool looked full every week while the level that actually mattered, in the surge chamber, was the one falling.

Evaporation over the edge. Water flowing over a weir is spread thin and exposed to sun and wind. The same builder says early infinity pool owners often thought their pools were leaking because of how much water evaporated when water ran over the edge for long periods. His firm now runs the edge for a limited time each day, partly to reduce evaporation and partly because constant flow leaves calcium deposits on the outer face of the wall.

The auto fill hides a leak. An automatic top up keeps the tank at its set level, so a leak can run for weeks without anyone seeing the level drop. The AQUA article notes that some builders avoid auto fills for exactly this reason.

Water goes to waste. If the pumps are switched off after heavy use, the water in transit drains back to the tank. If the tank is already high, it can overflow to waste. Heavy rain does the same.

There are more places to leak. Channels, gravity pipes, the tank itself, the weir wall and its outer face all hold water. The outer face of an infinity wall needs a finish that survives constant water and sun.

How a specialist tests an overflow pool

Expect the specialist to work with the tank rather than a bucket on a step.

1. Switch off or isolate the auto fill, and record the tank level against a mark.
2. Measure the loss with the circulation running and with it off. Losses that only appear when water is flowing point towards the edge, channels, pipes or evaporation over the weir.
3. With the pumps off, check whether the pool itself holds its level. If it drops, the same approach used for skimmer pools applies: see where the level stops and inspect the fittings at that height. LeakTronics, a maker of leak detection equipment, lists the skimmer, plumbing, light niches and return fittings as the most common leak points in general.
4. Test the tank on its own, and pressure test the individual lines.

Questions worth asking about your own pool

• Where is the balance tank, and are its normal, minimum and maximum levels marked?
• How is the auto fill set, and is there a water meter on the top up line?
• How many hours a day does the edge run, and does it need to?
• Who checks the tank level on service visits, and is it recorded?

If the answers are unclear, a specialist who works with overflow pools can set up the levels, check the auto fill and give you a baseline for normal water use, which makes the next leak far easier to spot.