The principle
Water sitting still under a cover for five months does two things: it slowly loses sanitiser, and it slowly acts on whatever surface it is touching. Balanced water does the second harmlessly. Unbalanced water etches plaster, stains, or scales the tile line — and it does it uniformly and permanently, because nothing is circulating to even it out.
So the goal of winterising chemistry is not to sanitise for five months. It is to leave the water chemically neutral toward the pool, with enough chlorine to cover the first weeks, and nothing else.
What genuinely helps
| Product | Worth it? | Notes |
|---|---|---|
| Sodium bicarbonate / acid | Yes | Whatever it takes to land TA and pH in band, a week ahead |
| Chlorine, to the top of range | Yes | On closing day, not a week before |
| Long-acting polymer algaecide | Sometimes | Useful in mild climates and long seasons under cover |
| Metal sequestrant | If you have metals | Test first; otherwise it does nothing |
| Enzyme products | Rarely | Sold hard, help mainly with an oily waterline |
| Floating chlorine dispenser | No | Parks in one place under a cover and bleaches it |
What the kits add
A boxed winterising kit is priced as a bundle and dosed for a pool nobody measured. It typically contains algaecide, a shock, a stain preventer and a clarifier — some of which your pool needs, some of which it does not, and none of which are sized to your actual volume.
If a pool is genuinely balanced and properly chlorinated on closing day, most of the box is redundant. Buy the parts you need individually, dosed to the volume you measured.
The order on the day
- Confirm the balance you set a week ago still holds.
- Add algaecide if you are using one, with the pump running.
- Circulate for a few hours.
- Add chlorine last, to the top of the working range.
- Then, and only then, lower the water and start the physical closing.
Never mix
Algaecide and shock added together can bind or gas off. Hours apart, pump running, and never a chlorine product into an acid or vice versa.
What it looks like in spring
A pool closed with balanced water and a tight cover opens cloudy and comes back in three or four days. One closed unbalanced, or with a slack cover, opens green and takes a week of holding chlorine to recover. The difference in effort is enormous; the difference in chemical cost at closing is a few dollars.