Stand a bucket of pool water on the top step, mark both levels and wait a day. If the pool drops further than the bucket, it is leaking. An uncovered outdoor pool typically loses a few millimetres a day to evaporation. Repeat with the pump on and off: more loss with the pump running points to the return pipework.
The hose is out again. You filled the pool at the weekend, the weather has been nothing special, and the tile line is already showing. You can settle the first question yourself, with a bucket, in 24 hours. The second question, where the water is going, has a free test too. This guide walks through both, then explains what swimming pool leak detection adds when the tests say leak and you cannot see one.
How much water a pool should lose
Every uncovered pool loses water without a leak, and it is more than most owners expect. The pool trade typically quotes around 6 mm a day of evaporation for an uncovered outdoor pool in warm, breezy weather, and less for an indoor one. Heated water evaporates faster than cold, and an onshore wind across a pool near the north Cornish coast lifts more off the surface than a still day inland.
Then there is the water that leaves on people. Swimmers carry some out on skin and hair, children take splash over the edge, and backwashing the filter sends a good volume down the waste line. Across a busy week an outdoor pool can lose a couple of centimetres without anything being wrong.
So a few millimetres a day in a hot spell is normal. Needing the hose every other day, or losing several centimetres a week in cool weather with the cover on, is not. The bucket test tells the two apart.
The bucket test, step by step
A bucket of pool water sitting in the pool evaporates at the same rate as the pool itself, because it is at the same temperature in the same air. If the pool level falls further than the bucket level, the extra water has gone somewhere other than the sky. You need a plain bucket, some tape or a wax pencil, and a brick to stop the bucket floating off.
- Fill the pool to its normal level. Switch off any automatic top-up valve, or it will hide the result.
- Fill the bucket from the pool. Weight it and stand it on the top step so the water inside is roughly level with the pool water outside.
- Mark both levels. One mark inside the bucket, one on the outside at pool level.
- Leave it for 24 hours. No swimming, no backwashing. Light rain falls into both equally and does not spoil the result; heavy rain does, so pick a dry day.
- Compare. The same drop in both means evaporation. A pool that has fallen further than the bucket is leaking, and the difference is roughly how much.
While you wait, walk the plant room and find the outlet of the waste line. A drip from a union or a trickle from the backwash pipe is a leak you can see, and it saves you the rest of the search.
Pump on, pump off: which half of the pool
A positive bucket test tells you there is a leak. It does not tell you where. The next free test narrows it to one side of the system, using the same tape mark.
Pool pipework has two halves. The suction side runs from the skimmer and main drain to the pump. The pressure side runs from the pump through the filter and heater and back to the pool through the return jets. When the pump runs, the pressure side is pushing water outwards, so a split there loses more. A crack on the suction side is under vacuum while the pump runs and pulls air in instead; when the pump stops, the weight of the pool pushes water out through it.
- Run the pump for 24 hours and measure the drop from your mark.
- Refill to the mark, leave the pump off for 24 hours and measure again, starting at the same time of day.
- More loss with the pump on: the return lines, the jets, or the plant after the pump.
- More loss with the pump off: the suction lines or the skimmer.
- The same loss either way: the shell, a fitting, or the light niche. The pump makes no difference to those.
Bubbles in the pump basket while it runs are a second clue that a suction line is drawing air.
Where pools leak
Pools fail at a short list of joints and fittings, and the level itself often tells you which one. Let the pool find its own level over a day or two with the pump off, and note where it settles.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Level drops to the skimmer mouth, then holds | The skimmer throat or its joint with the pool wall | Dye test around the skimmer with the pump off |
| Level stops at a return jet or the light | That fitting, or the light niche seal | Dye test at the fitting rim, pump off |
| Level keeps falling below all the fittings | The main drain, a floor fitting, or the shell or liner | Specialist survey with dye and listening equipment |
| Loses more with the pump running | A return line or the plant after the pump | Plant room walk-through, then pressure test each return |
| Soggy or sunken ground, a green patch of lawn | Buried pipework between plant room and pool | Pressure test each line, then pinpoint with tracer gas |
| Puddle under the pump or filter | Shaft seal, union, drain plug or clamp band | Look with the pump running; replace the seal or gasket |
Do not forget the plant room. A multiport valve with a worn spider gasket lets water cross to the waste port while the handle sits on Filter, and the pool loses water for every hour the pump runs without a drop touching the floor. Dry the end of the waste pipe, run the pump and watch it for five minutes. A puddle under a heat pump is often condensation, and a chlorine test strip settles it: condensation has none, pool water does.
Holiday-let pools in Cornwall
A pool at a holiday let has a particular problem: nobody measures. Guests top it up, the changeover team tops it up, and the auto-fill valve covers the rest. A leak that started in May can run until October, and every litre that leaves has been filtered, heated and dosed. What replaces it is cold water with no chlorine in it, so the heater and the dosing work harder all season.
If you manage a let with a pool anywhere from Padstow to the Roseland, add two lines to the changeover routine. Note the waterline against a fixed mark before guests arrive and again when they leave, and log it beside the water test. Take the property’s water meter reading at the same points. If the meter moves with the auto-fill switched off and the house quiet, the overnight meter test will show whether the house itself has a leak, which is a separate question from the pool.
When the tests say leak and you cannot see one
The bucket and pump tests get most owners to “pressure side, plant room dry” or “shell side, level stops below the light”. They rarely get you to the exact spot, particularly with a buried pipe or a hairline crack in a shell. That last step is instrument work, and digging on a guess under a patio is the expensive way to do it.
A specialist survey runs in the order the tests suggested. The plant room is checked first, because it is above ground and the cheapest place to find a fault. Then each pipe run is isolated, filled and pressurised: a gauge that holds clears that line without a spade going in. A line that drops is filled with a tracer gas that rises through soil and paving to a detector at the surface, so the break is marked from above. Dye and acoustic equipment cover the shell, the fittings and the light niche. Our guide to what happens during a leak detection survey describes the process for a house; a pool follows the same logic with the plant room in place of the airing cupboard.
The result is one marked spot and a written report. Repair is then a small excavation over the break, or on many runs the pipe can be relined from its ends without lifting the patio. We provide leak detection across Cornwall for private pools, holiday lets and park pools, and a pool survey is usually booked within the same week.
Frequently asked questions
How much water should a pool lose in a day?
An uncovered outdoor pool typically loses a few millimetres a day to evaporation, more in warm or windy weather and more again if the water is heated. Splash, swimmers climbing out and backwashing add to that. A steady loss well beyond that, or a real drop with the cover on, points to a leak.
Can I do the bucket test with the pump running?
Yes, and you should do it both ways, refilling to the same mark between them. More loss with the pump running points to the return lines or the plant after the pump. More loss with it off points to the suction lines or the skimmer. The same loss either way points to the shell or a fitting.
Does rain ruin the bucket test?
Light rain does not, because it falls into the bucket and the pool equally and the comparison still holds. Heavy rain masks the result, because the pool has a far larger surface than the bucket and collects run-off from the surround. If a wet day is forecast, wait for a dry one.
My pool only loses water in summer. Is that still a leak?
Possibly. Evaporation rises sharply in warm weather, so summer loss can be innocent. The pump also runs for longer hours in summer, so a leak in a return line loses more water in July than in April. The bucket test settles it in a day, and the pump comparison tells you which explanation fits.
Bucket says leak?
Tell us what the bucket and pump tests showed and we will take it from there. We isolate and test each line, mark the spot, and give you a written report so the repair is one hole rather than a dug-up patio.
