Real-life case: the cover lifted in February, the swamp in April
- The situation: winterising wrapped up too early (water still at 16 °C) + a cover partially unhooked by a February storm.
- The result in April: opaque brown-green water, a carpet of leaves, a pond smell — the worst possible wake-up.
- Rescue: massive mechanical cleaning, then the brown-water protocol (sequestrant BEFORE chlorine), boosted shock, flocculation.
- One week to swimming — and the next winterising done by the book, at the right moment.
The winterising had been done “properly” — but on an October weekend that came too early, with the water still at 16 °C, because it was the only free slot. Then winter did its work, and a February storm unhooked two of the cover’s fixings on the walnut-tree side. Nobody checked — who looks at their pool in February? The answer came in April, when the cover came off: a swamp. Opaque brown-green water, a carpet of leaves on the bottom, a pond smell. The kind of sight that has you looking up a drainage company’s number. Wrong reflex: here’s the rescue, without draining.
Autopsy of a botched winterising
Two faults added up. The date: at 16 °C, biology is still running — the water lived on under the cover all autumn, burning through its winterising product in a few weeks. The open cover: from February on, light + walnut leaves + rainwater = a complete ecosystem. Add the tannin from the leaves and a hint of dissolved metals, and April’s opaque brown-green was written in advance.
The rescue, a full week
Days 1-2: mechanics first. Flat-bottomed net for the leaf carpet (hours of it), deposits vacuumed to waste — straight to the drain, not through the filter, which would have clogged in ten minutes. Level topped back up, circuit re-primed, long filtration to stir things. No products yet: treating water full of debris is throwing the treatment away.
Day 3: the test — and the surprise. Strip photo: chlorine nil (no surprise), pH out of kilter, and the brown water turning rust-coloured at the edges. The app steers towards its “brown-water recommissioning” plan: the colour wasn’t just from the leaves — there were dissolved metals (thanks to a whole winter’s rainwater). And here, a crucial order: sequestrant first, chlorine second. Chlorinating iron-laden water directly would have darkened it further and tattooed the liner with permanent rust stains.
Days 3-4: sequestrant dosed, pH brought to the low end of the range, continuous filtration. Day 5: chemistry takes back over — a boosted shock calculated on the volume and the starting point of nil. The water turns grey-green: the algae dying en masse, the normal sequence. Days 6-7: flocculation, overnight settling, slow vacuuming of the floc, re-test. Clear water, numbers in range, swim indicator green. Seven days after the swamp — and roughly the price of a drain saved, never mind the risks to the structure.
The following autumn
The same pool was winterised in November this time, water below 12 °C (the app gave the green light by tracking the temperature), cleaned and balanced before the cover went on, fixings checked after every gale — an app reminder there too. Spring result: water barely hazy, back in service in half a day. The difference between the two springs comes down to two things: the right moment, and a watched cover.
What this case teaches
A botched winterising isn’t visible in October — it’s discovered in April, with interest. The right moment (water lastingly below ~12 °C) counts for more than any product, and brown spring water has its own specific trap: never chlorine before the sequestrant. In both directions, the app holds your hand: it says when to winterise, when to reopen, and rolls out the right protocol for whatever you find under the cover — even when it’s a swamp.
Frequently asked questions
Does a pool turned swamp have to be drained?
Almost never: even badly degraded, water can be recovered in stages (mechanical, chemistry, flocculation). A full drain carries its own risks for the structure and is reserved for extreme cases like thick sludge or several years of abandonment.
Why is winterising too early a mistake?
Above ~12 °C, algae and bacteria are still active: you're sealing under the cover water that goes on living, with no ongoing sanitisation. That's the recipe for green spring water, even with a winterising product.
What should you check on the cover during winter?
Its tension and fixings after every gale, and the absence of water pockets or tears. A gaping cover lets in light and debris — the two fuels of algae.