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How to winterise a swimming pool

Updated: 21 October 2026

The pump is still running and you are about to pour a winter chemical because the forecast has turned. How to winterise a swimming pool in Britain is a sequence, not a single glug. Clean and shock first, dose while the pump can still spread the product, then protect the pipes from frost. The bottle does not do the frost step. Which bottle to buy is on the [pool winteriser comparison](/hub/pool-care/pool-winteriser). Peter F. used UK trade sheets and the amazon.co.uk listings dated 19 October 2026. This is not a lab procedure. A qualifying purchase can earn a commission at no extra cost to you.

The pump is still running and you are about to pour a winter chemical because the forecast has turned. How to winterise a swimming pool in Britain is a sequence, not a single glug.

Our top picks

Kleen Pool Kleen Pool algaecide, 1 L

Best overall

Kleen Pool Kleen Pool algaecide, 1 L
7.5/10about £17.95
View offer
Blue Horizons Blue Horizons winter time algaecide, 5 L

Rank 2

Blue Horizons Blue Horizons winter time algaecide, 5 L
8.6/10about £29.20
View offer
Quimicamp Quimicamp CATAGEN PS, 2 L

Alternative

Quimicamp Quimicamp CATAGEN PS, 2 L
4.1/10about £12.70
View offer
On this page
  1. 1.When to close
  2. 2.Clean, balance, shock
  3. 3.Dose, then drop the level
  4. 4.Cover, ice, and a later top-up

When to close

Do not close while the water stays warm. HTH says do not winterise above 15°C. PoolRound uses a consistent 12–15°C with leaf fall. Bayrol keeps the filter on through an autumn stage around 15–20°C and treats below 15°C as winter. Use the figure on your bottle if it prints one. Get the debris out before the cover goes on. A single cold night is not a run of days under the temperature your label uses.

Clean, balance, shock

Stay out of the water while you dose, and do not mix chemicals in one bucket. The UK sheets say clean, set the pH, shock, then the winter product. Water Technics describes a shock to about 10 mg/l first. Use the pH band on your bottle, not an average of Lo-Chlor, Blue Horizons, and HTH. Which bottle matches the pool is the pool winteriser buying guide.
Kleen Pool Kleen Pool algaecide, 1 L

Testsieger

Kleen Pool Kleen Pool algaecide, 1 L
7.5/10about £17.95
View offer

Dose, then drop the level

With a main drain, Water Technics says lower to about 150–200 mm below the skimmer, then dose while the pump still draws from the drain. Without a main drain, dose before you drop the level. Once the pump is drained for frost, it cannot circulate a chemical you add afterwards. Do not empty a vinyl, concrete, or fibreglass shell. DP Pool Services warns of liner shrink and groundwater pressure. CATAGEN says treatment without filtration. Lo-Chlor and HTH want the pump running. Follow the bottle in your hand, then drain the plant. A heater you cannot drain is the point where a pool engineer should take over.

Cover, ice, and a later top-up

The chemical slows algae. It does not stop ice, and it does not stop a cover blowing off. Use a winter debris cover, tied down, and store a solar cover unless the maker says otherwise. Partly filled bottles are ice relief, not a second dose. Top up after about three months, and after heavy rain. Do not pour the top-up and a shock into one bucket. A rigid above-ground pool then changes the structure. An inflatable is usually stored empty. That split is how to winterise an above ground pool.

Common questions

When do I start winterising?
Start when the water has settled around the temperature your label uses, often under about 15°C, and when leaves are falling so you can clean before the cover goes on. HTH says do not winterise above 15°C. Other UK notes use a run of days in the 12–15°C range, or an autumn stage while the filter is still on. A single frost overnight, with the water still warm the next afternoon, is early.
Do I empty the pool?
Do not empty a vinyl, concrete, or fibreglass pool that is built to stay full. DP Pool Services cites liner shrink and groundwater as the risks. Lower the level only as far as the main drain and the cover require, often about 150–200 mm below the skimmer when Water Technics' drain method applies. An inflatable is the usual exception and is stored empty. If the cover maker says keep the normal level, do that.
Do I shock before or after the winteriser?
Shock before the winteriser, on the UK sheets used here. Water Technics describes a shock to about 10 mg/l first, then the winter product on clean water. Pouring the winteriser into dirty, unshocked water is the step these guides tell you to avoid. Do not mix the shock and the winteriser in one bucket. Keep swimmers out while you dose.
How long do I run the pump?
Use the time on your label. Lo-Chlor's sheet asks for about 6–8 hours of circulation. Blue Horizons sheets say run until the product is distributed. HTH's closing note includes about 2 hours. CATAGEN says treatment without filtration. That is that listing, not the instruction for HTH or Lo-Chlor.
What pH should I aim for?
The band on your bottle, not an average of every brand. Lo-Chlor's sheet uses about 7.5–7.8. A Blue Horizons Ultimate sheet uses about 7.6–7.8. HTH's preparation advice uses about 7.0–7.4. Those ranges overlap and they are not the same target. Set the pH before the shock and the winteriser. If the listing prints no pH, use the bottle, not another brand's figure.
When do I add more?
After about three months, and after heavy rain, on the Water Technics guide and on the CATAGEN bullet. Rain dilutes what you put in at close-down. The top-up is another dose from the same product's instructions, not a guess of half a cap. If the cover has been off, clean again before you add more.
When is this a job for an engineer?
Call a pool engineer for a heater you cannot drain, buried pipes, a holiday let or a club pool, or an automatic cover you are not sure how to set. HSG179 is operator guidance for people who run pools, not a garden dose chart, and there is no separate Act that sets a domestic winter dose. This page does not check each Amazon name against the HSE biocides list. If you are unsure the plant is isolated from frost, stop.

Related pages

Copper-free or copper winteriser

Pool winteriser

How to winterise an above ground pool safely in the UK

Best pool winteriser UK: which listing fits your pool

Swimming pool winteriser: the checks before you buy

Sources

Public UK sources used for this page in October 2026. Shelf prices and star lines are the amazon.co.uk figures for these nine listings, dated 19 October 2026.

  1. HSE: Biocidal product authorisation (retrieved 21 October 2026)
  2. HSE: Managing health and safety in swimming pools (HSG179) (retrieved 21 October 2026)
  3. HSE: Swimming pools (retrieved 21 October 2026)
  4. PWTAG: Microbiological testing TN53 (retrieved 21 October 2026)
  5. Lo-Chlor: Winteriser algaecide tech sheet (retrieved 21 October 2026)
  6. Blue Horizons Ultimate winteriser sheet (retrieved 21 October 2026)
  7. Water Technics: Swimming pool winterising guide (retrieved 21 October 2026)
  8. HTH: Super Winterprotect 3 L (retrieved 21 October 2026)