How to Close a Pool for Winter

When to Close

Use measured pool-water temperature and your local freeze forecast, not nighttime air temperature as a substitute. Cooler water slows growth but does not guarantee that algae cannot establish. Keep sanitation and filtration operating until a complete winterization can be done before freezing.

In southern states where freezing is rare, some pools stay open year-round. If you do close, the timing is less critical. Focus on getting the chemistry right rather than a specific calendar date.

Balance Chemistry First

Choose closing chemistry for the actual surface and expected coldest pool-water temperature, using measured pH, TA, CYA, calcium hardness and dissolved solids. Cooling lowers calcium-carbonate saturation; automatically lowering pH to 7.2–7.4 can make plaster water aggressive. Have a qualified service assess cold-water saturation and the finish maker’s limits before setting targets. LSI is not a metal-corrosion index.

Cold-water saturation and winterization reference. Coordinate pH and TA corrections using fresh readings; follow the label’s circulation and waiting periods.

Shock Before Closing

Resolve active algae before covering, using the CYA-aware treatment and its completion tests. Otherwise maintain routine chlorine for measured CYA and follow the closing-product instructions.

Follow the exact winter chemical and cover labels for chlorine compatibility and waiting periods.

Blow Out the Lines (Freeze Protection)

This is the step that prevents the costly freeze damage: cracked pipes, burst fittings, and split pump housings. Water expands 9% when it freezes. Any water left in plumbing lines will crack something.

1. Set water level only after checking the cover, finish and pool construction requirements. A dilution percentage describes water exchange, not permission to drain. Before substantially lowering water, have a qualified pool professional assess construction, liner/support needs and groundwater, and choose a suitable exchange plan. Groundwater can lift a shell; unsupported liners or walls can be damaged. Account for the replacement water’s chemistry.
2. Shut the pump off at the breaker, then remove the drain plugs from the pump, filter and heater and let all three drain completely. They are drained separately, on their own plugs — not by blowing air through them
3. Move air through the plumbing, not through your equipment. Air is blown line by line from the equipment pad with the filter and heater taken out of the path. Forcing air through a filter or heat exchanger can damage grids, laterals and coils. Exactly which valves to set, and where to connect, depends on your pad — follow your equipment manual, or have this step done by a pool professional. Do not improvise a valve position
4. Blow one line at a time until water stops and you see only a light spray or mist at the far end
5. Plug that return with a winter plug while air is still flowing, then move to the next line
6. For the main drain, blow until bubbles appear at the drain, then close its valve while the air is still on. That traps an air lock in the line, which is what keeps water out of it all winter. Do not plug a main drain
7. Leave every drain plug out for the winter, and store them where you will find them in spring

If you have lines you cannot fully blow out (bends, traps), add pool-safe propylene glycol antifreeze. Never automotive antifreeze.

Common Winterizing Mistakes

1. Draining an in-ground pool completely. Do not drain an in-ground pool for winter. If your water table is high, an empty pool can literally float out of the ground. A repair that costs thousands.
2. Leaving equipment outside in freezing temperatures. Store the pump, heater header, and filter cartridges indoors. Any water inside equipment components will freeze and crack.
3. Adding algaecide before FC drops. High FC oxidizes algaecide immediately, wasting it. Use the product label’s permitted chlorine range and waiting period.
4. Not securing the cover properly. A cover blown off by winter wind defeats the entire purpose. Use water bags or proper anchor hardware. Not bricks or heavy objects that can puncture the cover or fall in.

Frequently Asked Questions

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