Pool Leak Detection: How Leisure Centres, Schools and Hotels Find the Water They Pay to Heat

TL;DR: A commercial pool is topped up every day by design. The industry code of practice asks operators to replace 30 litres of water for every bather, and backwashing the filters adds more. That routine top-up hides leaks. Pool leak detection starts with a meter on the make-up water supply, a record of what the pool should use, and a check on the balance tank overnight. A top-up that rises with no more bathers or backwashes means water is leaving somewhere it should not.

Last updated: 2 October 2026

Key takeaways

• The Pool Water Treatment Advisory Group’s code of practice asks operators to replace pool water with fresh water at a rate of 30 litres per bather, with backwash water counting towards it.
• The same code lists a water meter on the make-up water system among the instruments every pool should have, and says ultrasonic flow meters with remote indicators are the easiest to manage.
• Because a pool is topped up every day by design, a leak of thousands of litres a day can hide inside normal top-ups.
• Commercial pools can leak at the tank and its fittings, in the buried pipework to the plant room, at the balance tank or in the plant room itself.
• More than 1,200 pools are over 40 years old and past their expected lifespan, according to a 2026 parliamentary group report.
• Continuous monitoring of make-up flow, balance tank level and plant room floors turns a leak from a surprise on the bill into an alert the next morning.

How do you know if a swimming pool is leaking?

A swimming pool is probably leaking when it needs more make-up water than its bathers, backwashes and evaporation explain. Five signs are common: rising top-ups, a balance tank that refills at night, damp in the plant room or ducts, creeping chemical and heating use, and physical damage around the tank.

1. Top-ups are rising. The make-up water meter shows more water per day than usual, with no increase in bathers and no extra backwashing.
2. The balance tank refills at night. When the pool is closed and no filter is being backwashed, the automatic top-up should be almost idle, covering little more than evaporation. A fill valve that keeps opening through the night is replacing water that is leaving somewhere.
3. Damp where it should be dry. Wet patches in the plant room, in the service ducts around the pool tank or in the rooms below it are often the first physical sign.
4. Chemical and heating use creeps up. Every litre that escapes is replaced with cold, untreated mains water, which then has to be heated and dosed.
5. Physical damage. Cracked or lifting tiles, gaps around inlets, lights and outlets, wet ground or settlement around the pool surround, or air in the pump strainer, which can point to a leak on the suction side.

None of these proves a leak on its own. Together, and measured over time, they tell a duty manager whether to call a specialist or keep watching. The earlier the sign is caught, the smaller the repair tends to be.

Why is a leak in a commercial pool so hard to see?

Because a commercial pool is topped up every day on purpose. Fresh water replaces dilution and backwash water, and an automatic valve keeps the balance tank at its working level, so a leak simply looks like a little more top-up. Nothing drops and nothing overflows. The water just costs more.

The PWTAG code of practice for the management and treatment of swimming pool water is clear that some pollution can only be reduced by dilution with fresh water. It asks pool operators to be prepared to replace pool water with fresh water at a rate of 30 litres per bather, and notes that water replaced when filters are backwashed contributes significantly to that.

Take a pool with 1,000 bathers a day. At 30 litres each, it should be replacing about 30,000 litres of water a day on purpose. A leak of 2,000 or 3,000 litres a day sits inside the normal swing in bathers and backwashes, and without a meter and a record it goes unnoticed.

Deck-level pools make this worse, in a sense. The code says that taking more than 80% of the circulation flow from the pool surface needs a deck-level pool with a balance tank. In these pools the water line in the pool itself never moves: the balance tank absorbs the change and its fill valve replaces whatever is lost. The classic sign of a swimming pool leak, a falling water line, never appears. That is why pool leak detection on a commercial site starts with the meter, not the eye.

Age adds to the risk. A 2026 report by the All-Party Parliamentary Group on Swimming, summarised by Swim England, found more than 1,200 pools over 40 years old and past their expected lifespan, with ageing buildings often needing more maintenance. Older tanks, joints and buried pipework are exactly where leaks develop.

Where do commercial swimming pools leak?

At five points: the pool tank and its joints, the fittings through the tank wall, the buried pipework between the pool and the plant room, the balance tank, and the plant room itself. The last two are often the easiest to fix, yet they are easy to miss because they sit out of sight behind locked doors.

1. The pool tank and its joints. Cracks in the structure, failed tile grout or render, and movement joints that have opened.
2. Fittings through the tank wall. Inlets, outlets and main drains, skimmers, underwater lights and handrail sockets, anywhere a pipe or fitting passes through the structure.
3. Buried circulation pipework. The pipes between the pool and the plant room carry the whole circulation flow. PWTAG says pool water should circulate 24 hours a day, and gives a turnover period of 2.5 to 3 hours for a public pool up to 25 metres long, so the entire pool volume passes through those pipes several times a day.
4. The balance tank. Cracks in the tank itself, and a fill valve that sticks open and sends mains water straight to the overflow and down the drain. That is not strictly a leak, but it costs exactly the same.
5. The plant room. Pump seals, filter vessels, valves and pipe joints, plus backwash valves that do not seat properly and pass water to drain.

How do you test a pool for a leak?

Measure first, then pinpoint. Compare the make-up water with what the pool should be using, run an overnight level test with the automatic fill isolated, and then bring in a specialist to locate the leak with dye and pressure tests. Each step narrows the search and costs more than the one before.

Make-up water review. Read the make-up meter every day for two weeks alongside bather numbers and backwash records. The code already asks for daily records of flow rates, pressure differentials and backwash procedures, so most of the data should exist.

Overnight balance tank test. With the pool closed, the automatic fill isolated and no backwash, record the balance tank level at closing and again at opening. Circulation keeps running as normal. A drop larger than evaporation can explain means water is escaping. Watch the level during the test so the pumps are never starved.

Bucket test for smaller pools. For a skimmer pool or a small pool without a balance tank, stand a bucket on a step, fill it to the pool’s water level and mark both. After 24 hours, both will have lost roughly the same to evaporation. Any extra drop in the pool is water leaving it.

Dye tests. A little coloured dye released close to a suspected crack or fitting in still water is drawn into the gap if water is escaping there.

Pressure tests. A specialist isolates sections of circulation pipework and pressure tests each one to find the run that is losing water. It is the usual next step once the loss is proven and the tank itself looks sound.

How does make-up water metering find leaks early?

It turns the pool into a daily water balance. The meter measures every litre added, bather numbers and backwash logs explain most of it, and what remains is evaporation and loss. When that remainder steps up and stays up, especially overnight, a leak has started, and the chart shows the day it began.

The balance is simple enough to write on one line: make-up water equals dilution, plus backwash, plus evaporation, plus splash and carry-out, plus any leak. The meter gives the total. The records give the planned parts. The rest is what needs explaining.

PWTAG lists the instruments every pool should have to measure how its system performs. They include a flow meter after the circulation pump, before the filters, to measure the circulation rate and the backwash flow rate of each filter, and a water meter on the make-up water system, which gives a measure of dilution. The code adds that ultrasonic flow meters with remote indicators are the easiest to manage.

The night window is the cleanest signal. Between closing and opening there are no bathers and usually no backwash, so any make-up water added then is evaporation plus loss. A night top-up that jumps and does not come back down is the same pattern estates teams use to find leaks in buildings, explained in our guide to night flow analysis.

What should a continuous pool leak detection system include?

A flow meter on the make-up water supply, a level sensor in the balance tank, leak sensors on the plant room and duct floors, and alerts that reach the duty manager. Together they show how much water is added, when it is added, and where any escaping water ends up.

The AQUAIOT Clamp-on Ultrasonic Flow Meter fits the outside of live pipework from DN8 to DN100, using transit-time measurement, with no cutting and no shutdown. It is built for monitoring, water balance and leak detection rather than billing, which is exactly the job on a make-up supply. It gives the pool the meter PWTAG asks for without draining anything or stopping circulation.

The AQUAIOT Radar reads the balance tank level from above, without contact, to within 2 mm on still surfaces. Our radar level monitoring service sets high and low thresholds and rate-of-change alarms, with persistence rules that filter out short spikes, so a tank that keeps falling overnight raises an alarm.

A leak sensor on the plant room floor, in the ducts around the pool tank and under filter vessels catches water where it lands. Our water leak detection service sends real-time alerts by SMS, email and app, with escalation rules so the alarm reaches someone who can act.

What does a pool leak really cost?

More than the water. Every litre that escapes is replaced with cold mains water that has to be heated to pool temperature and dosed again. Escaping water can also wash out the ground under slabs and pipework, and on an ageing pool a small leak rarely stays small for long.

Heat is the expensive part. PWTAG sets pool temperatures from 26 to 28°C for competitive and fitness swimming, 28 to 30°C for leisure water, and 32 to 36°C for hydrotherapy and rehabilitation pools, ideally 34.5°C. Replacement water arrives at mains temperature and has to be brought all the way up.

Energy is also dearer than it was. The parliamentary group report found energy prices still nearly double their pre-pandemic levels. Swim England had already warned in its 2021 Decade of Decline report that almost 2,000 pools could be lost by the end of the decade as those built in the 1960s and 1970s reach the end of their lifespan. For an operator trying to keep an older pool open, heated water leaking into the ground is money it cannot spare.

Do spa pools and hydrotherapy pools need leak monitoring too?

Yes, and their small volumes make losses show faster. PWTAG gives commercial spa pools a turnover period of just 6 minutes and says their filters should normally be backwashed daily, while hydrotherapy pools run at up to 36°C. A spa pool leak or a hydrotherapy pool leak costs heat quickly.

Spa pools sit outside the PWTAG code. They are covered by HSE guidance HSG282 on controlling legionella and other infectious agents in spa-pool systems. Pool leak detection works the same way there: meter the make-up water, watch the level overnight, and keep the plant area dry.

Hydrotherapy pools matter in NHS, care and special school estates, where the pool supports treatment and closure affects patients. A rising top-up on a pool held at 34.5°C is worth investigating the same week, not at the next planned shutdown.

Frequently asked questions

How much water loss is normal for a swimming pool?

There is no single figure, because evaporation, bather numbers, water features and backwashing differ from pool to pool. The practical answer is the pool’s own baseline: meter the make-up water for a few weeks, record bathers and backwashes, and treat any sustained rise above that baseline as a possible leak.

Can you find a pool leak without draining the pool?

Yes. Make-up water metering, overnight level tests, dye tests and pressure tests on isolated pipework all work with the pool full. Draining is usually a last resort, once the leak has been narrowed down to the tank itself.

What is a balance tank in a swimming pool?

A balance tank collects the water that overflows from a deck-level pool before it goes to the filters, and holds the extra volume displaced by bathers. An automatic fill valve tops it up with mains water, which is why a leak in a deck-level pool shows as extra top-up rather than a falling water line.

Where can a swimming pool leak?

In the tank and its joints, at fittings through the tank wall such as inlets, outlets and lights, in the buried pipework to the plant room, in the balance tank and its fill valve, and in the plant room itself, at pump seals, filters and backwash valves.

Where to start

Start with two numbers: yesterday’s make-up water reading and yesterday’s bather count. If there is no make-up meter, fitting one is the first step, and the PWTAG code already asks every pool to have one. After that, a week of night readings usually shows whether the pool has a leak worth chasing.

For pool leak detection across a leisure estate, see our page on leisure centre leak detection. To plan metering and alerts for your pool, tell us about the site and a UK engineer will come back within one working day.

By GP, Digital and IoT lead at AQUAIOT.