End-of-Summer Pool Maintenance in Portugal: A Complete Guide

Mid-August on the Margem Sul: the pool has been earning its keep since May, the kids have been in and out of it daily, and the sun cream, dust and long hot afternoons have all left their mark. What you do with your pool between now and October largely decides whether you open a sparkling pool next spring — or spend a small fortune rescuing a green one.

If you've moved here from the UK or northern Europe, forget most of what you know about "closing" a pool. On the Margem Sul, from Costa da Caparica to Setúbal, winters are mild and frost is rare. Pools here almost never freeze, which means the drain-it-down, board-it-up approach common back home is usually the wrong one. The local logic is different: less about frost protection, more about keeping water chemistry stable through a wet, mild winter.

Here's what actually needs doing at the end of a Portuguese summer, in the right order.

Start with the water chemistry

After three months of heavy use, high UV exposure and topping up with mains water, your pool chemistry is almost certainly out of balance. Before anything else, test and correct:

• Free chlorine — heavy bather loads consume it fast; bring it back to 1–3 ppm.
• pH — aim for 7.2–7.6. Sun cream, sweat and fresh top-up water push it around all summer.
• Cyanuric acid (stabiliser) — this is the one that catches people out. Months of stabilised chlorine tablets under intense Portuguese sun often leave levels far too high, which "locks" your chlorine and makes it ineffective. If it's very high, partial water replacement is the only real fix.
• Total alkalinity and calcium hardness — worth checking now rather than discovering scale or corrosion problems in spring.

A shock treatment (superchlorination) at the end of the season kills off the invisible organic load that a summer of swimmers leaves behind. Do it in the evening, run the filtration overnight, and don't let anyone swim until levels drop back to normal.

Deep clean while the weather is still on your side

September is the ideal month for a proper clean, before autumn leaves and the first Atlantic rains arrive:

• Brush walls and the waterline thoroughly — the greasy line of sun cream residue is a biofilm waiting to happen.
• Vacuum the floor carefully, including corners and steps.
• Clean skimmer and pump baskets, and backwash the sand filter (or clean the cartridge).
• Degrease the waterline with a purpose-made product, not household cleaner, which can foam and upset the chemistry.
• Don't forget the surroundings: a pressure washing session on the pool deck and terrace now stops winter algae getting a foothold on the paving.

If your filter sand hasn't been changed in five or six years, end of season is the sensible moment to do it — worn sand is one of the most common reasons pools turn cloudy no matter how much chlorine goes in.

Active vs passive winterising: what makes sense here

This is where the Margem Sul climate changes the calculation entirely.

Active winterising means keeping the pool running through winter at reduced hours — typically a few hours of filtration a day, adjusted downwards as the water cools. You keep the water treated, keep an eye on it every week or two, and the pool stays presentable year-round. In our climate, where January afternoons can hit 16–18°C, this is what most local professionals recommend, especially for villas in Charneca da Caparica or Quinta do Conde where the pool is visible from the house all year.

Passive winterising means shocking the water, adding a winterising product, lowering the water level slightly, protecting the pipework and covering the pool completely until spring. It saves electricity but only works well with a good-quality cover and correct preparation.

What you should almost never do here is fully drain the pool. Empty shells can crack or even lift out of the ground when winter groundwater rises — a genuinely expensive mistake, and one we see foreign owners make because draining feels "logical" if you grew up with freezing winters.

The mistakes that cost people money

A few errors come up again and again at spring openings:

• Switching everything off in October and walking away. Untreated water in a mild climate doesn't freeze — it grows algae happily all winter.
• Covering a dirty pool. Leaves and organic matter sealed under a cover for five months become a swamp.
• Ignoring the equipment. Pump seals, O-rings and chlorinator cells should be checked before winter, not discovered broken in June.
• Cheap bubble covers left on all winter — they degrade in UV and can shed plastic fragments into the water.
• Forgetting the salt system. If you have a salt chlorinator, cold water reduces its output dramatically; many need to be switched to winter mode or supplemented manually.

When it's worth calling a professional

If you're comfortable testing water and your pool is straightforward, end-of-season care is doable yourself — the products are readily available at any bricolage store or pool shop in the region. But it's worth bringing in a professional pool maintenance service if:

• your stabiliser levels are high and a partial drain-and-refill needs doing safely;
• you have a salt system, heat pump or automatic cover that needs proper winter configuration;
• you're only in Portugal part of the year and need someone checking the pool while you're away — very common among expat owners around Azeitão and Sesimbra;
• the pool is already green or cloudy and shock treatments aren't holding.

A couple of professional visits in autumn typically costs far less than a full spring recovery of a neglected pool, in both chemicals and labour.

Wrap it up before October

The end-of-summer checklist, in short: balance the chemistry, shock the water, deep clean, service the equipment, then choose active or passive winterising — with active being the natural fit for our mild Atlantic climate. Do it in September, while the water is still warm enough to work with and before the autumn rains start diluting your efforts.

If you'd rather hand the whole thing over, Arranja Já looks after pools across the Margem Sul all year round — send us a message on WhatsApp for a free, no-obligation quote.