With the cover on and the power off, a hot tub should lose almost nothing overnight. Retailers typically put normal evaporation at 25 to 50 mm a week with the cover on. A drop you can measure in one night is a leak, and the pump unions, pump shaft seal and heater fittings inside the cabinet are where most start.
The waterline was on the mark when you put the cover on last night. This morning it is a finger’s width lower, and it was lower yesterday too. Four adults climbing out dripping and a cover that no longer sits flat can explain a slow loss over a week. They cannot explain a tub that drops overnight with nobody in it. Here is how to prove which one you have, and where to look. When a tub is sunk into decking with the pipework out of reach, our pool and spa leak detection service finds the spot without lifting the whole deck.
Splash-out, evaporation or a leak
Warm water in cool air evaporates, and the wider the gap between water and air temperature, the faster it goes. Running the jets churns the surface and speeds it up again. Bathers move the level too, and four people leaving the tub take a surprising amount out on skin and towels. Retailers typically put normal loss, cover on, at 25 to 50 mm a week, and a worn cover or heavy use pushes that higher.
What evaporation does not do is empty a tub overnight with the cover on. So test it rather than guess.
- Fill to the normal line. Then switch the tub off at the wall isolator, the rotary switch outside, not the topside panel.
- Put the cover on and mark the waterline with a strip of tape on the shell.
- Come back after 24 hours. A drop you can barely see is evaporation. A drop you can measure is a leak.
- Refill and run it for 24 hours, then compare. More loss running than resting points to the pressure side: the pump, heater and the pipework out to the jets. The same loss either way points to the shell, a fitting below the waterline or a suction line.
- Let it keep falling with the power off. The level stops at the height of the leak.
Where hot tubs leak, in order of likelihood
Almost everything that fails on a hot tub sits inside the cabinet, and most of it is a seal or a joint rather than a hole in the shell.
- The pump. Two large plastic union nuts join the pipework to the wet end, and they loosen with vibration and the warm-cool cycle. The O-rings inside wear as well. Behind the unions is the shaft seal, which shows as water under the pump body, rust or scale on the motor, and often a noisier motor.
- The heater. Unions at each end of the metal heater tube, plus gasketed fittings for the sensor and flow switch. Rust streaks down the tube or a puddle beneath it are the tell. A heater tube that has ever frozen will crack.
- The jets. Each one is sealed to the shell with a gasket that hot, treated water slowly breaks down, with a glued joint behind it.
- Manifolds and flexible pipe in the foam. The pipework feeding the jets is buried in insulating foam behind the cabinet panels, where a joint can weep for months before anything shows outside.
- Everything else. The light niche seal, the drain valve gasket, the filter housing and the diverter valves. A true shell crack is the least common finding.
A tub that seems to leak from underneath is nearly always one of these, running down through the foam.
The cabinet-off checks you can do yourself
Switch off at the wall isolator first. Then take off the cabinet panel on the equipment side and look before you touch anything. Look for puddles, white scale trails, rust streaks, and foam that is dark or spongy.
Press dry kitchen roll against each pump union, the heater fittings and the drain valve. A drip a minute that you cannot see will darken the paper. Then stand clear, switch back on and let the pump run for a few minutes, because some leaks only show under pressure. Switch off again before you reach back in.
If the bay is dry and the level still falls, rest the tub with the cover on until it settles, then dye test on that line: a few drops of food colouring in still water beside each jet, the light and the footwell fittings. Water leaving through a gap draws the colour in.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Water under the pump, rust on the motor | Union O-rings or the shaft seal | Kitchen roll on each union, power off; watch the pump run from outside the bay |
| Rust streaks on the heater tube | Heater fittings or a cracked tube | Look only. Heater work is for an engineer |
| Level stops falling at jet height | Jet gasket or the glued joint behind it | Food colouring beside each jet on that line, pump off |
| Level falls to the footwell | Light niche, suction fitting or drain | Food colouring at the light and footwell fittings |
| Cabinet base damp, foam wet, bay dry | Flexible pipe or manifold joint inside the foam | Have it traced rather than digging out foam on a guess |
| Water only after a winter empty | Freeze cracks in heater, filter housing, pump or manifold | Torch inspection before refilling, then a part fill and watch |
Should you try a leak sealer?
Sealer products are sold for minor plumbing weeps and small shell leaks, and the makers’ own instructions tell you the scope: take the filter cartridges out first, circulate for a couple of days, drain, and expect the repair to last a year or two rather than for good. Sealer cannot cure a pump shaft seal, does nothing for a loose union, and a cracked part needs replacing. It also tells you nothing about where the leak is. Our article on why leak sealer often works for a month and then stops covers the same trade-off on heating systems.
Holiday-let tubs and the changeover problem
A hot tub at a Cornish holiday let leaks for a season because nobody measures. The guest tops it up, the cleaner tops it up at changeover, and the tub is drained and refilled every week anyway. Two things make that worse than the water bill. A low waterline starves the pump, and the heater depends on flow across its element, so most controllers throw a flow error and refuse to heat, the Friday night message from a cottage in Newquay that nobody wants. Guidance for hot tubs in business use expects the water to be tested and recorded during a let, and every top-up is untreated mains water going into dosed water, so a tub that needs filling daily is a tub whose morning reading no longer describes what guests are sitting in by evening.
Add three lines to the changeover sheet: the waterline against a fixed mark on arrival and departure, a look inside the equipment bay and around the cabinet base, and a note in the welcome pack asking guests to tell you about a low level rather than top it up. If the property is metered, take the reading at changeover too. Our guide to a water meter that moves when nothing is on covers what to do when the house, rather than the tub, turns out to be the problem.
When to call a specialist
A hot tub engineer with the panels off can usually find and fix a union, seal or jet gasket on a free-standing tub. Call for leak detection when the bay is dry and the tub still drops, when the foam is wet with no obvious source, or when the tub or swim spa is sunk into decking with the plant boxed in elsewhere, because then the pipe runs are buried and guesswork gets expensive.
The method is the one used on pools. Each pipe run is isolated and pressure tested on its own, so a line that holds is cleared without a board coming up. A line that drops is filled with a tracer gas that rises through decking or paving to a detector above the break. On a heated spa, escaping warm water shows as a plume under the boards on a thermal camera. Dye and listening equipment cover the shell and fittings. You get one marked spot and a written report that tells the engineer which part has failed, and the decking comes up in one place. We provide leak detection across Cornwall for tubs, swim spas and pools at private homes, lets and parks.
Frequently asked questions
How much water should a hot tub lose in a week?
With the cover on, retailers typically put normal evaporation at roughly 25 to 50 mm a week. Heavy use, a high set temperature and a worn cover that no longer seals push that up. If the level drops faster than that with the cover on and the tub switched off, treat it as a leak.
Why does my hot tub only lose water when it is running?
Because the leak is on the pressure side. When the pump runs it pushes water through the heater and out to the jets, so a loose union, a worn shaft seal or a heater fitting loses water under pressure and barely weeps at rest. Check the pump unions and heater fittings first.
Does a leaking hot tub damage the pump or heater?
It can, once the level drops far enough. A low waterline starves the pump, and the heater relies on flow across its element. Most controllers throw a flow or dry heater error and refuse to heat, and an element that runs without water can burn out. Top up, switch off, then find the leak.
Can you find a leak in a swim spa without lifting the decking?
Usually, yes. Each pipe run is isolated and pressure tested, so the lines that hold are cleared without a board coming up. A failing buried line is filled with tracer gas, which rises through the decking above the break, and a thermal camera shows warm water escaping from a heated spa. The decking comes up in one place.
Tub dropping overnight?
If the cabinet checks have not found it, or the tub is sunk into decking, we will trace the leak and give you a written report that tells the engineer exactly which part to replace.
