Active or passive winterisation: the choice that shapes your winter

The debate has always existed in the trade, and it is usually settled by ideology rather than analysis. Here are the real criteria.

I. What each method involves

Passive winterisation shuts the pool down completely. You lower the water below the return inlets, drain the pipework and plant room, fit floats and gizzmos, dose the water with a winterising product and cover it. The pool does not run from November to March.

Active winterisation — sometimes called dynamic — keeps the filtration running, generally two to four hours a day, with light treatment. The water level stays normal. Frost is handled by a thermostat-controlled timer, or by the heat pump's frost mode.

II. The criteria that decide

Water temperature. Passive winterisation is set up when the water falls durably below 12 °C, generally in late October or during November around the Bay. Doing it too early, with water at 15 °C, guarantees green water in spring: algae are still growing, with no filtration to hold them back.

Frost risk. The Bay sees a few nights a year below −3 °C, very rarely below −7 °C. Active winterisation with proper automatic frost protection handles that without difficulty. The vulnerable point is not the pool itself — a 50 m³ body of water does not freeze through — but the plant room pipework and the skimmers.

Power cuts. This is the decisive argument in a second home, and the most underrated. Active winterisation depends on a permanent supply. If the breaker trips in January and nobody notices, filtration stops, frost protection with it, and the water stagnates for two months. Without anyone to notice, active winterisation becomes improvised passive winterisation — the worst of both.

Intended use. A pool recommissioned in April justifies active winterisation: restarting takes a day. A pool that will see nobody before July has no reason to run all winter.

Equipment. A submerged cover or slatted cover changes the picture: fewer leaves, less photosynthesis, less evaporation. An open pool under pines accumulates needles and degrades much faster under either regime.

III. The passive protocol, in order

1. Full clean: walls, waterline, floor, skimmer and pump baskets.
2. Shock treatment and continuous filtration for twenty-four to forty-eight hours, until the water is perfectly clear.
3. Clean or replace the filter medium, full backwash.
4. Balance the water: pH between 7.0 and 7.4, alkalinity and hardness within range. Unbalanced water standing for five months will either scale or corrode.
5. Lower the water below the return inlets, about ten centimetres below the skimmers.
6. Fully drain the plant room: pump, filter, heater, electrolyser, pipework. Drain plugs removed and stored in the pump basket.
7. Fit gizzmos in the skimmers and winterising floats diagonally.
8. Distribute winterising product over the surface.
9. Fit and secure the cover. Around the Bay, securing is not optional: a badly fixed cover becomes a sail.
10. Isolate the plant room supply.

IV. What happens if you do nothing

A pool left as it is, with no winterisation and no supervision, through a winter on the Bay:

The water fills with pine needles, which acidify as they break down. pH falls, the water turns corrosive to metal parts and permanently stains the liner. Algae colonise in spring as soon as temperatures rise. Scale forms at the waterline. A pipe caught by frost may split.

Recommissioning then no longer means starting a pump; it means draining, treating, sometimes replacing the liner. You go from two hours of work to a budget in the thousands — for the sake of not spending a few hours in November.

V. Our position

For a second home on the Bay, we recommend active winterisation provided three conditions hold: a reliable power supply monitored remotely, working automatic frost protection, and a physical visit at least monthly to confirm everything is still running.

If any one of the three is missing, passive winterisation is safer. It takes more work in November and March, but it depends on nothing for five months. In an empty house, autonomy beats performance.