How to winterise a swimming pool: a practical guide for installers and service technicians

Winterising a swimming pool is more than simply carrying out a few maintenance tasks at the end of the season. Proper preparation of the pool system, water and equipment reduces the risk of faults, makes spring start-up easier and helps prevent costly repairs. For installers and service technicians, it can also mean fewer call-outs and problems after the first frosts.

In this guide, we explain how to prepare a swimming pool for winter step by step: from closing the season, cleaning the pool and adjusting water parameters to protecting the pump, filter, pipework, skimmer and cover. We also look at when an outdoor pool should be winterised, which components can remain outside and why in-ground, framed above-ground and inflatable-ring pools may require different approaches. The aim is to provide a clear winterisation procedure from a professional servicing perspective, rather than focusing solely on the end user.

When should you start winterising a swimming pool?

The end of the swimming season does not necessarily mean that the pool should be closed as soon as the first cooler weekend arrives. Water temperature is a useful indicator. Once it has remained below 15°C for several consecutive days, winterisation can usually begin. Cooler water is less favourable to algae growth, whereas closing a pool too early while the water is still relatively warm can increase the risk of water-quality problems developing beneath the cover.

In a temperate European climate, this will often be around late September or early October, although the actual timing should depend on water temperature and the weather forecast. If the first frosts are expected, water should not remain trapped in parts of the system that are exposed to freezing temperatures. Water expands when it freezes, and ice inside a pipe, pump or filter can cause serious damage.

When a service company has several pools to close for the season, it makes more sense to plan the schedule around water temperature rather than a fixed calendar date. A pool that is closed too early may require more water treatment, while one left unprotected for too long is more vulnerable to an unexpected frost.

The right timing is therefore a balance: not so early that water quality becomes more difficult to maintain, but not so late that there is insufficient time to protect the pool before freezing conditions arrive.

How should you prepare a swimming pool for winter – where should you start?

Before draining any part of the system or removing equipment, check the overall technical condition of the installation. The inspection should include the pool shell, walls, skimmer, inlets, valves and fittings, pump, filter and all visible pipe connections. If there are cracks, leaks or signs of corrosion, it is better to deal with them before winter rather than leave the problem until the following season.

The next step is a thorough clean. Remove deposits from the floor, waterline and walls, and clear leaves and other debris from the water. If a robotic pool cleaner or pool vacuum is used, the end of the season is a good time for one final cleaning cycle. Clean the pool thoroughly before lowering the water level, as some areas may become more difficult to reach once part of the water has been drained.

The skimmer baskets and pump pre-filter should also be emptied at this stage. Effective cleaning reduces the amount of debris left beneath the cover over winter and makes spring start-up considerably easier. Attention should not be limited to the pool itself: before closing the system, the service technician should also inspect baskets, covers, seals and any areas where water could remain trapped after the season.

How should pool water be prepared for winter?

Water parameters should be checked before the system is shut down. The pH, alkalinity and disinfectant level are particularly important. There is no single value that is suitable for every pool, as the correct range depends on the water-treatment system, the pool finish and the manufacturer's recommendations. The pH should therefore be adjusted in line with the documentation for the chemicals and equipment in use.

Chlorine should not simply be added in excess without checking the concentration. If the system uses chlorine, salt electrolysis or automatic dosing, both chlorine and pH levels should be checked before the equipment is switched off. Overly aggressive water chemistry can increase the risk of discolouration and material degradation, while incorrect parameters may also contribute to corrosion. On the other hand, insufficient disinfection can encourage algae growth and bacterial activity. Correctly balanced water also helps reduce scale formation while the pool is out of use.

Some systems use a dedicated winterising product to limit deposits and microbial growth. Any such product should be dosed in accordance with the manufacturer's instructions. Winterising chemicals do not replace proper water balancing or basic disinfection. If a customer asks how their pool should be winterised, it is worth making clear that water treatment is only one part of the process and cannot replace correct protection of the hydraulic system. Proper winter preparation helps maintain water quality and can make reopening the pool in the new season much easier.

Does the water need to be drained from the pool?

There is no single answer for every type of pool construction. With in-ground pools, completely emptying the pool for winter is generally not standard practice. The water provides stabilising weight, and removing it can be problematic where groundwater levels are high or with certain types of pool construction and finish. A completely empty in-ground pool may be affected by groundwater pressure, while liner and fibreglass pools can also be vulnerable to damage.

With passive winterisation, however, it is often necessary to lower the water level to protect the skimmer and parts of the hydraulic system. There is no universal level that applies to every pool. Some manufacturers require the water to be lowered below the skimmer, while other systems may require it to be below both the inlets and skimmer. Arbitrary values such as 10 or 20 cm below these components should not be used without first checking the relevant documentation.

If part of the pool water needs to be drained, ground conditions and the method of installation should also be taken into account. A pool designed to be installed in the ground may require a different approach from a lightweight seasonal above-ground pool. Ground conditions, drainage and the construction of the pool shell all play a role in safe winterisation.

How should the pipework, skimmer and inlets be protected?

Water left inside the hydraulic system is one of the greatest winter risks. If it freezes, expansion can damage pipes, fittings, valves and equipment housings. A system that is being shut down for passive winterisation should therefore be drained in accordance with its design, with additional protection provided wherever water could remain trapped.

Professional pool servicing often involves blowing out the pipework with compressed air and sealing the lines with suitable winterising plugs. The skimmer can also be protected with a compensating element that helps absorb some of the forces generated by freezing water. Bosta offers the Gizmo wintering plug , which is fitted into the skimmer to provide a compression point in freezing conditions.

Check valves, bypasses, dosing systems and disinfection equipment should also be inspected. A pool installation cannot be treated as one simple pipe run: every closed section in which water remains is a potential failure point. Correctly selected plugs and compensating components provide additional protection, but the most important measure is still to remove water from areas where it must not remain. Careful preparation reduces the risk of damage and costly repairs.

How should the pool pump, filter and filtration system be prepared for winter?

The pump and filter require separate attention. Once the power supply has been isolated, the pump housing, pre-filter and connected pipework should be drained, and drain plugs removed where this is specified by the equipment design. Pumps and pipework should not remain filled with water where sub-zero temperatures are possible.

Many pool pumps available from Bosta include features that make winter preparation easier. For example, Hayward Powerline pool pumps are equipped with drain plugs designed to simplify winterising. Hayward Max-Flo models also include drainage solutions that support seasonal shutdown and servicing.

The filter should also be drained in accordance with the manufacturer's instructions. A sand filter may require backwashing before shutdown, while cartridge filter elements should be cleaned and inspected. The Norsup ICF cartridge filter available from Bosta includes a bottom drain plug designed to support efficient winterisation, cleaning and rinsing.

If the pump or another component can be removed, it is best stored in a dry, protected location. The motor should not be tightly wrapped in impermeable plastic, as condensation trapped underneath can contribute to corrosion. If equipment is to remain outdoors, use a suitable cover designed for that particular product and ensure that adequate ventilation is maintained.

How should a heat pump and other pool equipment be prepared for winter?

A pool heat pump needs to be isolated from the water circuit, the heat exchanger drained and the unit protected in accordance with the manufacturer's instructions. Simply switching off the electrical supply is not enough. Water left inside the heat exchanger can freeze and cause damage that could otherwise have been prevented during proper autumn servicing.

If the unit remains outdoors, it should be protected with a suitable cover rather than a random sheet of impermeable tarpaulin. The cover should reduce exposure to rain and debris while still allowing moisture to escape. Bosta recommends suitable ventilated winter covers for pool heat pumps, and selected Flotide inverter heat pumps are supplied with a winter cover.

Chlorinators, automatic dosing systems, UV units and other pool equipment require similar attention. Sensors and probes should be stored in accordance with the manufacturer's requirements, as some probes must not be allowed to dry out. Electrical and measuring equipment should always be protected as specified in its documentation. Labelling pool equipment, automation components and sensors during removal can make recommissioning in spring quicker and reduce the risk of mistakes.

How should a swimming pool be covered for winter?

Once the water and hydraulic system have been prepared, the pool can be protected with a suitable cover. A properly selected cover reduces the amount of leaves, dust and other debris entering the pool, while some winter covers also restrict the amount of light reaching the water. Depending on the design of the cover and conditions in the pool, this can help prevent or significantly reduce algae growth.

Bosta offers a range of swimming pool covers. The Energy Guard winter cover is designed to help reduce chemical use by limiting algae growth in the pool.

The cover should be suitable for the type of pool and installed in accordance with the manufacturer's instructions. Where fixing systems, winter pillows or floats are used, their purpose may include helping the cover cope with loads from snow and ice. Improvised ropes stretched across the pool should not be used unless the cover system has specifically been designed for this method of installation.

The condition of the cover should also be checked during winter. Excessive snow, ice or standing water can add significant load. It is therefore worth planning several inspections throughout the winter rather than assuming the pool requires no attention once it has been closed.

What should be checked throughout the winter?

Winterising a swimming pool does not end when the cover is installed. The pool should remain part of the service schedule throughout the winter. Every few weeks, check the water level, the condition of the cover, visible structural components and the area around the technical equipment. If water appears in the plant room, the cover becomes distorted or a leak develops, dealing with the problem during winter may prevent more extensive damage.

Depending on the winterisation method, water quality may also need to be checked periodically. If the pool remains partially operational, filtration and disinfection should continue according to a separate winter schedule. With a passive winter shutdown, access to the water is more limited, which makes correct preparation at the beginning of the process particularly important.

Correct winterisation makes it easier to recommission the pool when the swimming season returns. During spring opening, winter protection should be removed, drain plugs refitted, the pipework checked for leaks and the pump and filtration system restarted. Water parameters should then be tested and adjusted again before the pool is returned to normal operation.

Preparing a swimming pool for winter – key points

A swimming pool should be treated as a complete technical system rather than simply a pool shell. It includes the hydraulic installation, water-treatment system, heating equipment and protective components. Every pool should therefore have a clearly defined seasonal shutdown procedure. A properly prepared pool is easier to recommission in spring, and where regular servicing is carried out, it is good practice to document the work completed each year in a consistent way.

To reduce the risk of mistakes, it is worth completing a short service record once the winterisation work has been finished. This can include the water level, water-treatment parameters, circuits that have been drained and any equipment that has been removed. The same record can then be used during spring commissioning, without having to determine which winter protection measures were used several months earlier.

• Correct pool winterisation reduces the risk of frost damage to the hydraulic system and equipment.
• Winterisation is best started once the water temperature remains consistently below 15°C.
• Before closing the pool, clean it thoroughly and check the technical condition of the pool shell and installation.
• The water should have the correct pH, while chlorine and other treatment parameters should be adjusted to suit the water-treatment system in use.
• Not every pool should be completely drained. The required water level and drainage procedure depend on the pool construction and the manufacturer's instructions.
• The pump, filter, pipework, skimmer and inlets must be protected against water freezing inside them.
• Components that can be removed should be cleaned and stored in a dry, protected location where appropriate.
• The pool should be fitted with a winter cover suitable for its construction, and the condition of the cover should be checked throughout the winter.
• Correct preparation for winter helps prevent damage, simplifies spring commissioning and reduces the risk of costly repairs.