How to Winterise Your Pool: Complete 2026 UK Guide
As the swimming season draws to a close, taking proper steps to close your pool protects your investment from severe frost damage, pipe bursts, structural lifting, and costly spring clean-ups.
In the UK, winter weather brings prolonged freezing temperatures and high rainfall. Leaving a pool unmanaged over winter can result in cracked heat exchangers, ruined liners, and green, stained water by April.
This guide details the step-by-step winterisation process recommended by independent pool surveyors.
Quick Summary: UK Winterisation Checklist
Water Level: Lower water to just below the skimmer mouth. Never drain a pool completely.
Water Chemistry: Target pH 7.2 to 7.6, Total Alkalinity 80 to 120 ppm, and shock chlorinate 48 hours before adding algaecide.
Note: If you do not have a way to measure these levels, use a reliable [multi-parameter pool test kit] or a digital photometer to check your pH, alkalinity, and free chlorine accurately before dosing any winter chemicals.
Plant Equipment: Drain heat pumps, filter pumps, and sand filters completely. Set multiport valves to WINTER.
Skimmer Protection: Fit a Gizzmo expansion plug in every skimmer to absorb ice pressure.
Electrical Isolation: Switch off main pool circuit breakers to prevent pumps running dry on automatic timers.
Covering: Secure a fitted woven winter debris cover across the pool.
The Golden Rule: Never Fully Drain a UK Swimming Pool
The single most expensive mistake a pool owner can make in the UK is emptying the pool for winter.
Groundwater Hydrostatic Pressure: UK winter soil becomes saturated with rainwater. Without the downward weight of water inside the pool, ground pressure can lift concrete shells out of the earth or cause one-piece fibreglass pools to bulge and pop.
Liner Shrinkage: Vinyl liners rely on water weight to maintain their shape. Draining a liner pool causes the vinyl to shrink, dry out, wrinkle, and tear off its tracking.
For UK installations, lower the water level to roughly 100mm to 150mm below the skimmer mouth, leaving the remaining water in place to protect the structure all winter.
Step-by-Step Pool Winterisation Guide
Step 1: Balance the Water Chemistry
Correct water chemistry prevents staining, scale formation, and liner bleaching during the dormant months. Test your water and adjust to the following targets:
pH Level: 7.2 to 7.6
Total Alkalinity: 80 to 120 ppm
Calcium Hardness: 180 to 250 ppm
Once balanced, perform a shock chlorination (superchlorination) to raise free chlorine levels to between 5 and 10 ppm. Allow the pool to circulate for 24 to 48 hours so the chlorine can destroy organic contaminants before adding winter algaecide.
Step 2: Clean the Pool Thoroughly
Vacuum the floor, brush the walls, and skim all floating debris off the surface. Organic material left in the pool will decompose over winter, consuming your sanitiser and feeding algae growth beneath the cover.
Step 3: Add Winter Chemical Treatments
After chlorine levels settle back down to roughly 3 to 5 ppm, add a dedicated [long-life winter algaecide]. Copper-free algaecides are ideal for liner pools to prevent copper staining.
For additional protection throughout winter, place a [floating winter chemical dispenser] in the pool water.
Step 4: Protect Plant Equipment and Pipework from Freezing
Trapped water expands by roughly 9 percent when it freezes. This expansion exerts immense force, easily cracking plastic pump housings, PVC pipework, and metal heating components.
To protect your equipment:
Isolate Electrical Power: Turn off the main electrical breakers at the consumer unit. This prevents digital timers from running pumps dry after pipework has been drained.
Drain the Heat Pump or Heater: Remove the drain plugs at the base of your air-source heat pump or gas boiler. Titanium heat exchangers trap water inside; if left undrained, freezing water will destroy the heating element.
Drain Filter Pumps and Sand Filters: Remove the drain plugs on the pump basket housing and at the bottom of the sand filter tank to let trapped water escape.
Set Multiport Valves to WINTER: Move your sand filter multiport valve handle to the "WINTER" or "CLOSED" position. This lifts the internal rubber spider gasket off its seat, preventing frost from seizing the valve.
Fit Skimmer Expansion Plugs (Gizzmos): Thread a hollow expansion tube, known as a [Gizzmo skimmer plug], into the bottom thread of every skimmer basket. If ice forms inside the skimmer, the hollow tube collapses under pressure, absorbing the force and preventing the plastic skimmer housing from cracking.
Step 5: Lower the Water Level Correctly
Lower the water level to just below the skimmer intake using your submerged pump or waste line.
Special Note for Vinyl Liner Pools: Do not drain the water more than 150mm below the skimmer line. Lowering water too far removes the weight keeping the liner stretched against the walls.
Step 6: Secure a Fitted Winter Debris Cover
Fit a heavy-duty, UV-stabilised winter debris cover over the pool. Ensure spring straps and perimeter anchors are tensioned tightly. A quality winter cover allows rainwater to filter through while blocking leaves, dirt, and sunlight, stopping algae from photosynthesising.
Frequently Asked Questions
When is the best time to winterise a pool in the UK?
Winterise your pool in autumn when water temperatures fall consistently below 15 degrees Celsius, usually between late October and early November. Closing the pool while water is still warm allows algae to multiply rapidly under the cover.
Do I need to drain my heat pump for winter?
Yes. Draining your heat pump is essential. The titanium heat exchanger inside an air-source heat pump holds water even when main pipes are drained. Failing to remove the heat pump drain plug before the first hard frost will result in severe internal freeze damage.
What is a Gizzmo and why is it needed?
A Gizzmo is a hollow plastic tube screwed into the bottom of a skimmer mouth. During winter, water left in the skimmer can freeze. The Gizzmo absorbs ice expansion by collapsing inward, preventing the skimmer box from splitting open.
Should I run my pool pump on a freeze stat during winter?
You can either drain the system completely or run an automated freeze-protection system that circulates water when air temperatures drop near freezing. If you choose to run a freeze-stat system, the pipework must remain full, and water must flow continuously during cold snaps.
Ensure Your Pool Is Sound Before Closing
Winterising a pool that has an undetected leak, liner failure, or structural fault can cause severe underlying damage while the pool sits covered for six months.
At Swimming Pool Surveys, we provide independent, comprehensive pool surveys across the UK. We do not sell repairs, maintenance contracts, or equipment, giving you an entirely unbiased assessment of your pool's structural condition, pipework integrity, and plant equipment.
Get in touch with our surveying team for complete peace of mind before closing your pool for the season.
Telephone: 01234 783569
Website: Contact Swimming Pool Surveys online to book a survey.