How to Close a Pool for Winter

When to Close

Close your pool when nighttime temperatures consistently stay below 65°F. The NSPF Pool & Spa Operator Handbook treats that 65°F threshold as the safest single trigger we've seen, and we agree: it correlates with biological activity slowing enough that algae can't establish under the cover. In most northern states, that lands between late September and mid-October. Closing too early (while water is still warm) lets algae set up shop under the cover, creating a more difficult spring opening.

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

Unbalanced water causes scale, staining, and corrosion over a closed winter. Per the ANSI pool-industry standard, target these values before closing:

• pH: 7.2–7.4 (lower end, reduces scale)
• TA: 80–100 ppm
• Calcium hardness: 200–400 ppm (plaster pools need 200 minimum)
• CYA: No change needed for closing

Do not try to adjust pH and TA on the same day. Fix TA first (use the alkalinity calculator), let it circulate for about 6 hours, then adjust pH (use the pH calculator).

Shock Before Closing

Shock the pool about one week before closing. This kills any existing bacteria and algae before they go dormant under the cover. Target 10 ppm FC for a standard closing (a widely-used practice per Trouble Free Pool community guidelines). The same as a pool opening.

Do not add algaecide while FC is still elevated. High FC oxidizes algaecide quickly, wasting it. Add algaecide when FC has dropped to 5 ppm or below (per ANSI/APSP/ICC-11 and algaecide manufacturer label instructions).

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. Lower water level 4 inches below the skimmer (or per your cover manufacturer)
2. Connect a shop vac or air compressor to the skimmer line
3. Blow air through each line until water stops coming out the returns
4. Immediately plug each return with a winter plug as you finish it
5. Remove the pump strainer basket and blow out the pump housing
6. Remove the drain plug from the pump, filter, and heater
7. Store drain plugs somewhere you'll 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. Wait for FC to drop to 5 ppm or below (per ANSI/APSP/ICC-11 and manufacturer label instructions).
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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