Guides · Updated Oct 31, 2026

How to winterize a pool

Winterizing protects your pool from freeze damage and keeps the water in good enough shape that spring opening is quick. The steps differ depending on your climate: in regions with hard freezes you need to "close" the pool fully, while in mild climates many owners keep running the pump at reduced hours instead. This guide covers a full closing for freezing climates and notes where mild-climate pools differ.

When to close

Wait until water temperature stays below about 65°F (18°C). Algae grows slowly in cold water, so closing after the water cools means less chance of opening to a green pool. Close too early, while the water is still warm, and the chlorine you add will be used up long before spring.

1. Balance the water a week ahead

About a week before closing, test and balance:

• Total alkalinity: 80–120 ppm
• pH: 7.4–7.6
• Calcium hardness: 200–400 ppm for plaster
• Cyanuric acid: within your normal range

Balanced water protects plaster and metal surfaces during the months the pool sits still. Cold water is more aggressive toward plaster, so don't leave pH or calcium low.

2. Clean thoroughly

Skim, brush walls and floor, and vacuum. Empty skimmer and pump baskets. Every leaf left in the pool feeds algae and stains over winter. Clean or backwash the filter so it goes into storage clean.

3. Shock and let chlorine drop

Raise free chlorine to shock level a few days before closing to destroy organics. Let it fall back toward normal levels before adding any winter algaecide, since high chlorine can break down some algaecides. Follow the product label for dosing.

4. Lower the water level

For pools with a solid or mesh safety cover in freezing climates, lower the water below the skimmer opening and typically below the tile line or return fittings — check your cover and skimmer manufacturer's guidance. With some covers (and many above-ground pools using skimmer covers or plugs), you lower less or not at all. Never fully drain an in-ground pool; groundwater pressure can lift or crack the shell.

5. Blow out and plug the lines

Water left in pipes can freeze, expand and crack plumbing. This is the most important step in a freezing climate:

1. Use a shop vac or blower to push water out of each return line and skimmer line until you see bubbles at the outlet.
2. Plug return fittings with threaded winter plugs while air is still flowing.
3. Insert a skimmer plug or a freeze-expansion device (such as a Gizzmo) in each skimmer.
4. Blow out the main drain line until bubbles appear, then close the valve to hold an air lock.

Some pros add pool-grade antifreeze (propylene glycol, never automotive antifreeze) to lines as extra insurance.

6. Drain and store the equipment

• Pump: remove drain plugs and store them in the pump basket so they don't get lost.
• Filter: open the drain, remove cartridges for cleaning and storage indoors, and set multiport valves to the winterize or closed position.
• Heater: drain it fully following the manufacturer's instructions; trapped water can crack a heat exchanger.
• Chlorinator and salt cell: empty tablets from feeders, and remove and clean salt cells.

Turn off power at the breaker, and if equipment is outdoors, cover motors loosely so they can breathe.

7. Remove accessories

Take out ladders, handrails, diving board hardware if recommended, and any automatic cleaners. Store them dry.

8. Install the cover

A well-fitted safety cover keeps out debris and adds protection for children and pets. Check straps and anchors and tension the cover evenly. For water-bag style covers, fill bags about two-thirds full so they don't burst when frozen. Through the winter, pump standing water off solid covers and clear heavy snow loads from above-ground pool covers.

Mild-climate pools

Where freezes are rare and short, many pools stay open. Reduce pump run time as the water cools, keep chlorine and pH in range, and test every week or two. Most modern variable-speed pumps and automation systems include freeze protection that runs the pump when air temperature drops near freezing — confirm it's enabled and the sensor works.

Common winterizing mistakes

• Closing while water is still warm.
• Leaving water in plumbing or the heater.
• Draining an in-ground pool completely.
• Skipping the cleaning step.
• Using automotive antifreeze.

Document the closing

Write down the final readings, what was added, which plugs were used and any equipment issues to fix in spring. Pool companies using Swimming Pool On Demand attach photos and readings to a closing report the homeowner receives automatically — and you'll have everything you need for a smooth spring opening.

Pool pros: log every reading and send photo reports with Swimming Pool On Demand.