How to prepare your pool for winter

How to prepare your pool for winter

Sebastian, self-taught DIY-er, Exeter

Guide written by:

Sebastian, self-taught DIY-er, Exeter

Depending on where you live, your swimming pool may need full or partial overwintering during the cold months. From cleaning and treating the water to covering the pool and installing a frost protection box, gizmo or pool float, read on for our top tips on preparing your pool for winter.  

Important features

  • Overwintering period
  • Active or passive overwintering
  • Steps for full wintering
  • Steps for partial wintering
  • Mistakes to avoid
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How do you overwinter a pool?

Winterising a swimming pool is a yearly process designed to protect your pool and filtration system over the cold months when your pool is not in use. One approach is to completely close down the pool, clean it thoroughly, reduce the water level slightly, cover the pool with a winter cover and shut down the filtration system. Alternatively, if the goal is to partially close the pool, the filtration system should simply be used less frequently and the pool will need to be meticulously cleaned once time comes to open it up full-time in the spring.

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When should you start to prepare your pool for winter?

The process of winterising a swimming pool should be put into motion once the pool water consistently drops to below 12°C. That said, waiting as long as you can to start the process can help to keep your pool water cleaner for longer.

There is no set period for winterising a pool given that the temperature of the water will depend on where you live, the outdoor temperature, the amount of sun exposure and whether or not your pool is covered or heated. The only real indicator will be the water temperature but you'll generally want to start preparing your pool from the end of September to the end of October.

Passive or active winterising

Passive overwintering

Passive overwintering

Also referred to as full winterising, this approach involves completely shutting down your pool pump and temporarily stopping water treatments. This means that your swimming pool will be completely out of use over the winter. Passive winterising is the most common process in the UK.

Active overwintering

Active overwintering

Active overwintering, also known as partial wintering, consists in keeping your filtration system on but only for around two hours a day. You will also need to check the pH level of the water with a pool water tester. This approach involves treating your water and cleaning your pool on a regular basis. Active overwintering is usually only possible in warmer countries.

Active vs. passive overwintering: advantages and disadvantages



Active overwintering (partial wintering)

- Easier to implement

- Pool is quicker to set up in spring

- Water quality is maintained

- Pool equipment in constant use (avoids need to clean, flush, etc.)

- Requires regular care

- Consumes electricity and wears equipment down

- Requires the use of different water treatments

- Requires the purchase of a frost protection box

Passive overwintering (full wintering)

- Minimal maintenance required over winter

- Does not consume electricity

- Takes longer to set up pool in spring

- Takes longer to re-use pool (adding more water, water treatments)

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Passive overwintering a swimming pool in 8 steps

1. Treat the water and adjust the pH level

Treat the water and adjust the pH level

Add a shock treatment to your pool and check and adjust its pH level as required: the pool water should be clear and the pH level neutral (not too acidic or alkaline).

2. Clean the pool

Clean the pool

The pool liner should be thoroughly cleaned, including the base, walls and along the water line. We would recommend using a pool vacuum, pool brushes or a pool robot. Remember to clean your pool ladder, skimmers, filters and pumps.

3. Reduce the water level

By reducing the total volume of pool water by about 10%, you'll be able to add some much needed freshness to the pool when it comes time to open it back up. You can simply drain a bit of pool water through the bottom drain.  

4. Shut down the filtration system

The pool filter should be turned off, drained, cleaned, dried and stored in a warm, dry place.

5. Add a winterising treatment to the water

Add a winterising treatment to the water

A winterising treatment is designed to limit limescale formation or buildup on the pool liner. This type of treatment should be added directly added to the pool water. The amount you add depends on the volume of your pool.

6. Block off any holes

The swimming pool jets, drain plug and sockets should be blocked up after you've removed any stagnant water.

7. Install gizmos and winterising floats

Install gizmos and winterising floats

Gizmos (or gizzmos) are placed in the skimmer basket to absorb pressure created by the frost that could otherwise damage the skimmer casing. Winterising floats perform a similar role and work to absorb the pressure caused by the pool water which increases in volume as it freezes. They are designed to be placed diagonally and partially submerged in the water.

8. Cover the pool

Cover the pool

The pool needs to be covered with a winter cover to protect the water from external pollutants (dead leaves, dust, etc.).

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Actively overwintering a swimming pool in 6 steps

1. Add a winterising product to the pool water

Add a winterising product to the pool water

Use a special overwintering treatment once the temperature of the pool water has dropped too low to swim. Apply the treatment as many times as indicated by the manufacturer's instructions.

2. Check the water quality and equipment

It's important to check the pH level of the pool water on a regular basis and add pH treatments as required. You'll also need to check the skimmer basket, the surface of the water (for leaves, etc.), and so on.

3. Install a frost protection box

The pool filtration system should be programmed to run about two hours a day but it can also be fitted with frost protection system to trigger automatic filtering when the pool water reaches a certain temperature.

4. Preventing the pool from freezing 

Running the skimmers and the pump creates movement in the water which should prevent freezing.

5. Cover the pool

Use a winter cover to cover up the pool and protect the water from impurities (dead leaves, dust, etc.).

6. Monitor pool equipment

Check the condition of the pool regularly by looking at the general state of the pool liner, the jets, the water surface and the pump. When the time comes to open up your pool, you will need to clean the pool from top to bottom.

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6 mistakes to avoid when overwintering your pool

1. Emptying the pool completely

Emptying the pool completely

This is pointless and potentially damaging to the swimming pool as the structure relies on the constant pressure of the water; this helps the liner to stay in contact with the pool walls.

2. Winterising the pool too early

You are better off waiting for the water to drop to about 12°C as the water quality will stay fairly stable until this point (any lower and bacteria will start to set in).

3. Not cleaning the pool before winterising

No matter which approach you choose, your pool water and equipment must be cleaned before winter sets in.

4. Not using a winterising treatment

When your pool is not in use or is being filtered less often, it requires specific treatments. Fail to treat your water correctly and you run the risk of having to use intensive or expensive treatments when spring rolls around. At worst, you may even have to replace your pool water.

5. Using a bubble cover or no cover at all. 

Over the winter, you need to protect your pool with a winter cover. Fail to cover your pool at all and you're just asking for cloudy and dirty water in the spring!

6. Failing to prevent your pool from freezing

Water expands in volume when it freezes and this has very expensive consequences for all your swimming pool equipment. Be sure to prepare for the risk of freezing according to whatever overwintering method you choose.

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Guide written by:

Sebastian, self-taught DIY-er, Exeter

Sebastian, self-taught DIY-er, Exeter

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