How to close (winterize) a pool

When to close

Leslie’s, the pool retailer, says to wait until the water is consistently below 65°F. Warmer water burns through sanitizer, and a cover over warm water is a good way to open a green pool in spring. In places that never freeze, many owners don’t close fully at all: they keep the pump running for fewer hours and keep chlorine up through the cool season. The rest of this guide is the full winterizing for climates where water in pipes can freeze.

A week before: test, balance, clean

Leslie’s gives these levels to aim for before closing. They sit inside the CDC’s and the Model Aquatic Health Code’s ranges, and they leave the water slightly protected against both corrosion and scale while it sits cold.

Worked doses for a typical pool: how much baking soda to raise alkalinity in a 10,000 gallon pool, how much muriatic acid to lower pH in a 10,000 gallon pool and how much calcium chloride to add to a pool. The reasons for the order are in how to balance pool water.

Then clean: brush the walls and floor, skim, vacuum, empty the skimmer and pump baskets, and backwash or clean the filter. Debris left under a cover feeds algae all winter.

Shock, and wait before the algaecide

Most closing kits include a shock and a winter algaecide. Follow your kit’s label for amounts and order; many say to shock first and add the algaecide only once chlorine has come back into the normal range, because high chlorine can break some algaecides down. Shocking at dusk a day or two before you cover gives it time to work and come back down. For the dose, see how much shock for a 10,000 gallon pool or, from a jug, how much liquid chlorine to shock a 10,000 gallon pool. Run the pump after adding closing chemicals — Leslie’s suggests about 4 to 12 hours, depending on pump size and pool volume.

Inground pools: step by step

1. Remove accessories: ladders, handrails, skimmer baskets, return eyeball fittings, the automatic cleaner and any floater.
2. Lower the water, to protect the skimmer and tile from ice. Leslie’s gives 4–6 in below the skimmer opening for vinyl pools and 4–6 in below the tile line for plaster pools. If you will blow out the lines without a skimmer blow-out plug, it goes lower, to below the skimmer and returns (about 18 in below the tile). Never drain a vinyl or fiberglass pool further than its maker allows.
3. Shut off power to the pump, heater and any timer or automation, so nothing starts while the lines are open.
4. Blow out the lines with a shop vacuum or compressor, return by return and skimmer by skimmer, and plug each one as the air comes through clear. Put a winterizing plug in each return and a skimmer winterizer (an expansion bottle or guard) in each skimmer.
5. Add pool antifreeze if your climate calls for it: non-toxic pool antifreeze only, never automotive. Leslie’s suggests about 1 gallon per 10 ft of pipe.
6. Drain the equipment. Hayward’s filter manual says to drain the tank completely and leave the drain cap off for the winter, set the multiport valve between two positions so it can drain, and winterize the pump by its own instructions. Pull the drain plugs from the pump, heater and chlorinator and keep them together (Leslie’s suggests the pump basket) so you can find them in spring.
7. Cover it. Center the winter or safety cover without tenting it. Water bags or blocks hold a solid cover; a safety cover anchors to the deck. Keep a cover pump handy for rain and snowmelt.

Above-ground pools

• Water level. Above-ground pools are usually lowered only a little, enough to keep water out of the skimmer and return. Leslie’s notes that if you drop the water more than 12 in, or live where it freezes hard, you should allow an extra 2 ft on each cover measurement.
• Hoses and equipment. Disconnect and drain the hoses, pump and filter, and bring the pump and filter indoors if you can. Plug the skimmer and return openings in the wall.
• Air pillow. Float an air pillow under the cover to take the push of ice off the wall.
• Cover. Run the cable through the cover’s grommets and tighten it until snug; Leslie’s warns against hanging weights from the grommets.
• Soft-sided and frame pools. Some makers’ manuals say to drain and store these pools for winter rather than winterize them in place. Check yours before you buy a cover.

Not sure how much water you are dealing with? How many gallons in a 24 ft round pool shows why the water depth, not the wall height, sets the chemical doses.

Over the winter

Pump water and snow off the cover so it doesn’t sag into the pool, check the water level and cover once a month, and test chlorine and pH on a mild day if you can get a sample. Leslie’s suggests checking the chemistry about monthly. If stabilizer ended the season high — common after a summer on tablets — spring, when you would replace water anyway, is a good time to fix it: see how much water to drain to lower CYA.

Common mistakes

• Closing too early. Warm water under a cover uses up chlorine and grows algae.
• Adding algaecide with the shock. Follow the kit’s order.
• Automotive antifreeze. It is toxic; use pool antifreeze only.
• Losing the drain plugs. Keep every plug in one place, and remember Hayward’s warning: winterizing plugs left in suction outlets are an entrapment hazard when the pool reopens.
• Draining too far. A vinyl liner can shrink or shift, and an inground shell can lift in wet ground. Lower only as far as your pool’s maker allows.

In spring, the same steps run in reverse — see how to open a pool.