How to Close a Pool for Winter
Plan for cold water and freezing. Set balance targets for your finish and cold water. Finish any algae treatment and maintain chlorine for your CYA reading. Follow the equipment manuals to protect the equipment and plumbing before the first freeze.
Estimate a dose for algaeIf algae needs treatment, use gallons, measured CYA and fresh FC.
Supplies for your closing plan
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- Winter pool cover: Check price on Amazon →
- Cal-hypo shock (only if algae treatment is needed before closing): Check price on Amazon →
- Winterizing algaecide (if your closing plan calls for it): Check price on Amazon →
- Antifreeze labeled for pool plumbing: Check price on Amazon →
Before You Cover the Pool:
- Choose closing targets for your pool finish and the coldest water temperature you expect. Use measured pH, TA, CYA, calcium hardness and dissolved solids. As water cools, calcium-carbonate saturation falls; automatically lowering pH to 7.2–7.4 can leave plaster water aggressive. Before you choose targets, have a qualified service assess cold-water saturation and the finish maker’s limits. LSI describes calcium-carbonate saturation. Have metal-corrosion concerns assessed separately.
- Complete algae treatment, maintain FC for measured CYA and follow closing-product labels
- If indicated by the labels, add algaecide or stain/scale treatment while the pump still runs, and circulate for each label’s mixing period before equipment is drained
- Have plumbing winterized with the approved procedure and plugs
- Drain and protect equipment from freezing according to its manual
- Install winter cover
Choosing Your Closing Date
Measure the pool water itself and check the local freeze forecast to decide when to close. Algae can still grow in cooler water, though growth slows. Keep sanitation and filtration running until you can finish winterizing before a freeze.
Keeping the pool open through winter means continuing to test, chlorinate and filter. When a freeze is forecast, protect equipment using the freeze-protection procedure in its manual, even if your winters are usually warm.
Assess Water Balance for Winter
Choose closing targets for your pool finish and the coldest water temperature you expect. Use measured pH, TA, CYA, calcium hardness and dissolved solids. As water cools, calcium-carbonate saturation falls; automatically lowering pH to 7.2–7.4 can leave plaster water aggressive. Before you choose targets, have a qualified service assess cold-water saturation and the finish maker’s limits. LSI describes calcium-carbonate saturation. Have metal-corrosion concerns assessed separately.
Cold-water saturation and winterization reference. Use fresh pH and TA readings for each correction, with the label’s mixing and waiting periods between additions.
Treat Algae Before Covering
Treat any algae to completion at the target for your CYA reading before putting on the cover. Without algae, continue routine chlorine dosing.
The closing-product and cover instructions determine permitted chlorine levels and waiting periods.
Estimate Any Needed Chemical Additions
Use your pool’s gallons and current tests to calculate any chemical additions. Treat algae if it’s present, and have cold-water balance assessed before setting closing targets.
Protect Plumbing and Equipment from Freezing
Water trapped in plumbing or equipment can damage it when it freezes. Get the winterization procedure for each component, including the correct connections and valve positions at your equipment pad.
Use the equipment’s approved winterization procedure.
Compressed air can damage equipment or eject plugs. Use only the approved air source, connection and pressure limit for your system. Keep the filter and heater out of the air path and wear eye protection. If the manuals do not establish a safe procedure, have a pool professional do the work.
- Choose the closing water level from the cover, finish and pool construction requirements. A calculated dilution is a water-exchange estimate. Before substantially lowering the water, have a qualified pool professional assess groundwater, pool construction and the support needed by liners or walls. Groundwater can lift a shell; unsupported liners or walls can be damaged. Use an exchange plan suited to your pool and account for the replacement water’s chemistry.
- Isolate electrical power before work. Drain the pump, filter and heater using their own manual-specified drain points. Do not try to drain these components by forcing air through them.
- Keep filters and heaters out of the air path. Follow your equipment’s instructions for connections and valve positions. Forcing air through a filter or heat exchanger can damage internal parts. If the correct air path is uncertain, stop and use a pool professional.
- Use the approved procedure to clear each line; follow the specified completion check.
- Install the specified winter plugs in the sequence the procedure requires.
- Have a pool professional establish the main-drain winterization method for your plumbing. Do not attempt to plug a submerged main drain or improvise an air-lock procedure.
- Leave or reinstall drain plugs as each component’s manual directs; store removed plugs together.
Where a manual permits pool-plumbing antifreeze, follow the approved product’s label. Do not use it as a substitute for an assessed winterization plan. Never use automotive antifreeze.
Closing Mistakes That Can Damage Your Pool
- Emptying an in-ground pool without assessment. Groundwater can lift an empty shell. Have water-level changes assessed before lowering water, including the support needed by liners or walls.
- Leaving water trapped in equipment before a freeze. Drain and protect each component as its manual directs; trapped water can cause freeze damage. Check which assemblies should stay in place and which are designed to be removed.
- Ignoring closing-product compatibility. Use any algaecide only within its label’s permitted chlorine range and waiting period.
- Using improvised cover weights or anchors. Use the cover’s specified anchors or weights. Loose bricks or improvised heavy objects can puncture the cover or fall into the pool.
Frequently Asked Questions
When should I close my pool for winter?
Measure the pool water itself and check the local freeze forecast to decide when to close. Algae can still grow in cooler water, though growth slows. Keep sanitation and filtration running until you can finish winterizing before a freeze.
Do I need to drain my pool for winter?
Keep water in the pool for winter. Before lowering the level, have its construction and groundwater assessed, including the water support your liner or walls need. Groundwater can lift an empty shell. Use the pool and cover’s required closing level and winterize the plumbing separately.
What chemicals do I add when closing a pool?
Maintain chlorine for your CYA reading and finish any algae treatment before covering. Choose extra closing products only if they suit your pool, cover and water condition, using the label’s compatibility, dose and waiting instructions.
Can I use antifreeze in my pool plumbing?
Use only antifreeze explicitly approved for pool plumbing, and only where the equipment instructions allow it. Follow its concentration and application directions. Never use automotive antifreeze; it can be toxic to people and animals.
Save the Opening Checklist for Spring
Keep the opening checklist handy for spring. You’ll need to restart equipment, test the water and decide between routine chlorine dosing and algae treatment.
How to Open a Pool →