How to Detect Leaks in Pools Without Draining Them

If your pool is losing water, in many cases you can narrow down where the leak is without draining it, though not always. The method depends on the suspected area: the hydraulic circuit (skimmer, inlet, return, sweep, floor drain), the pool shell itself or the buried pipes. Each technique answers a different question and has limits worth knowing before you hire anyone. In this guide we explain what is done in each case, what each test can and cannot tell you, and when it makes sense to call a professional. If you are looking for the service, you can see our page on pool leak detection.

Signs that your pool may have a leak

Before choosing a method, it is worth confirming that there is a real loss of water. These are the most common signs:

• Water level drops without explanation: the water goes down more than evaporation, use or rain can justify.
• The level drops only with the pump running: this points to a circuit, a valve or an equipment connection.
• The level drops with the pump off: this points to the shell or to a pipe that stays full of water.
• Constant dampness around the pool or in the technical room: this may indicate a buried pipe or a leaking wall penetration.
• Puddles or drips at the equipment: filter, pump or valve connections with a worn seal.
• Water noises in the pipes when the equipment is off and no consumption is expected.

None of these signs shows on its own where the leak is. They help you decide which test to do first.

Why you do not need to drain the pool

Draining the pool to look for a leak is usually the wrong step. You lose all the water without yet knowing where the problem is, and the shell is exposed to sudden changes. With water in the pool, most tests work better, because the leak becomes visible when the circuit is pressurised or when water escapes through the faulty point.

Common damage caused by draining

Draining a pool can cause several problems:

• Existing cracks: the sudden change in pressure and moisture can make cracks already present in the shell worse.
• Joints and lining: exposed to sun and drying out, the grout between the tiles and the lining can deteriorate.
• Liners and coatings: with vinyl or resin linings, draining can lift or damage the material.
• Ground pressure: with a high water table, an empty shell can be pushed upwards and deform.

For all these reasons, leak detection without draining is usually the preferable option, both in terms of water cost and risk to the structure.

Where leaks usually appear depending on the type of pool

The type of construction changes the weak points, so it is worth taking it into account before inspecting.

Concrete pools

In concrete shells, cracks in the floor and walls are common, often due to shrinkage or ground movement. Wall penetrations, that is, the places where pipes pass through the structure when the seal ages, are also typical trouble spots.

Mosaic tile pools

In mosaic tile pools, water usually gets in through the grout joints before the tile itself. A broken piece is easy to see; a joint that has lost adhesion is not. That is why the inspection should be done by a diver, who can observe and touch the joints up close.

Liner or fibreglass pools

In liner pools, leaks are usually small punctures, joints around the perimeter or pipe entries, and they can be slow. In fibreglass pools, the areas where nozzles and lights are connected are more likely to leak. In both cases, check the elements that pass through the lining.

Overflow pools

These have an additional overflow and balancing circuit. Leaks often concentrate at the overflow valve, the edge gutter and the buried return pipes, so the circuit test must include those lines.

Which method to use in each case

The table summarises which technique makes sense depending on the question you want to answer. None replaces the others: they are usually combined, and the final confirmation comes from direct inspection.

Circuit pressure test

The test consists of isolating each circuit of the pool and checking whether the pressure holds. The skimmer, the inlet (return nozzles), the sweep or floor cleaner line, the floor drain and, in overflow pools, the overflow circuit are isolated separately. When the pressure drops in a specific section, that section becomes the suspect and the test is repeated only on it.

It is important to be clear about what this test does not do. It indicates in which section there is a loss, but not at which point in the pipe or which part. If all circuits pass the test, the leak may be in the shell or in a line that was not isolated, and then direct inspection is needed.

Shell inspection with dye and professional diver

When the leak seems to be in the structure, a professional diver checks the shell from inside. Their job is to look at and touch each suspect area, note what they find and decide whether an additional test is needed. The points they usually check are:

• Cracks and fissures in the floor, in the corners and in the areas under the greatest load on the shell.
• Mosaic tile and grout joints, to tell a broken piece from a joint that has lost adhesion.
• Return nozzles, lights and floor drain, where aged seals or detached parts often appear.
• Wall penetrations, the points where pipes pass through the shell.

How dye is used

Dye is a colourant applied very close to the suspect area with the pump off and the water calm. If there is an opening at that point through which water leaves the pool, the colourant is drawn towards it; if there is none, it disperses slowly. With the filter running, the currents from the nozzles and the suction move the colourant and the reading is no longer reliable. That is why dye is always interpreted together with the pressure test and with what the diver sees, never on its own.

The underwater camera is used to record what is found and to compare areas, which helps to explain the diagnosis with images. When underwater inspection is needed, a professional diver carries it out. The result is explained to the client and, if the contracted service includes a written report, it is recorded in that report.

Camera inspection of the circuit pipes

An inspection camera introduced through an inspection point, a skimmer or another access point lets you see inside a section: scale build-up, breaks, displaced joints or roots that have entered through a joint. It is useful when the pressure test points to a specific line and you want to know whether the cause is a break, an open joint or a deposit narrowing the passage.

The camera has limits. It needs a suitable access point, the pipe must be passable, and the image is lost in tight bends, very long runs or cloudy water. It also cannot see what lies outside the pipe, such as the ground around it. The other methods are used for that.

Buried pipes: geophone and tracer gas

When the circuit is buried and the pressure test points to a section, the problem becomes one of locating it in the ground. There are two techniques, and they are applied to pipes, never to the shell.

Geophone and acoustic listening

The geophone is a sensor that is placed on the ground or on an accessible point of the pipe to listen to the sound of water escaping under pressure. The route is walked, readings are compared and the point where the sound is strongest is sought. It works best when the circuit is pressurised and the route is known.

Its main limitation is that it is not reliable inside the pool. Water dampens the sound, and the water mass itself, together with the equipment, creates background noise that makes it hard to separate a real leak from other sounds. For this reason the geophone is used on buried lines, not to look for cracks in the walls or floor. Pump noise, traffic or very dry ground can also make the reading difficult.

Tracer gas in drained sections

The tracer gas is applied to circuit pipes that can be drained. The section is pressurised with the tracer and a detector moves across the surface above the route; if there is a leak, the gas escapes into the ground and the detector records it. It is a technique designed for buried pipes and it cannot locate leaks in the pool shell, nor in sections that cannot be isolated and drained.

That is why the first step is to isolate and drain the section, which depends on the valves and the design of the installation. If the section cannot be drained, tracer gas is not the right option and the technician should propose another method.

Thermography: when it helps and when it does not

Thermography measures temperature differences on surfaces. In a pool it has little use for two reasons: water has a large thermal mass and changes temperature slowly, and sun, wind and evaporation alter the image on the surface. It can add something when the water is clearly warmer or cooler than the ground and what you are looking for is moisture in the soil, in a buried pipe or in a technical room. Even so, the thermal image only suggests areas to check; it does not replace the pressure test or the inspection.

Selector valve and filter equipment

In many cases the loss only appears when the filter is running. The selector valve, which directs the water towards filtration, backwash, drain or recirculation, is a frequent point of wear: its seals age with each change of position and with use. If the pool only loses water with the pump on and the pressure test points to that circuit, check the valve and the filter drain before considering any other intervention.

The bucket test as a first step

Before any professional test, you can do the bucket test. Fill a bucket with water to the same level as the pool, place it on a step and compare over 24 hours how much the level in the bucket drops and how much the level in the pool drops. If the pool loses clearly more than the bucket, there is a loss that deserves investigation. If they lose the same amount, the drop is explained by evaporation.

The test does not show where the leak is, but it prevents you from opening the ground for a loss that is actually evaporation.

When to call a professional

There is no need to call a professional for every drop in level. It is advisable to do so when one of these criteria applies:

• The loss repeats after topping up and is not explained by evaporation, use or rain.
• The pressure test points to a circuit, but it is not clear which section or which part is failing.
• There is persistent dampness, puddles or mould around the pool or the technical room.
• Cracks, loose joints or detached pieces are visible in the shell.

A good diagnosis follows an order: first the tests that require no works, such as the bucket test and the circuit pressure test; then direct inspection with a diver or camera; and only at the end any intervention. If the technician proposes breaking up the floor without having narrowed down the area first, it is reasonable to ask them to explain why.

What happens after the leak is located

Once the point has been identified, the diagnosis should state what has been found and which type of repair fits. For some circuit leaks a repair without works can be considered, for example rehabilitating pipes with resin or sealing nozzles and lights. For large cracks in the shell or crushed pipes, an opening may be needed to see the real condition before deciding.

Next steps

If you suspect a leak, start by confirming the loss with the bucket test, and note when the level drops and whether it happens with the pump on or off. That information organises the technician's work from the start. If you would prefer us to check it directly, you can request a no-obligation assessment.