How to close a pool for winter: water chemistry and step-by-step
Closing a pool well means two things. The water should be clean and balanced, so it doesn't stain or turn into a swamp by spring. And the plumbing and equipment need protecting from freezing. Here's the whole process, with the chemistry explained. Your pool builder's, cover maker's and equipment makers' instructions take priority wherever they differ, especially on how far to lower the water.
When to close
Algae grows slowly in cold water, so many owners wait until the water stays cool, often below about 65 °F, before closing. Closing while the water is still warm gives algae weeks of warm water under the cover. In climates that rarely freeze, many pools stay open all year with the pump running fewer hours. If you close in a mild climate, freeze protection matters less than keeping the water clean.
Step 1: Balance the water for cold
About a week before closing, test everything and adjust:
- pH around 7.4–7.6.
- Total alkalinity in your usual range (often 80–120 ppm).
- Calcium hardness in range for your surface. For plaster, don't let it run low.
- Stabilizer (CYA) in your normal range, so the closing shock isn't wasted.
Cold water is more aggressive toward plaster and grout than warm water with the same readings. Using the standard Langelier equation, the same water's saturation index drops by roughly 0.4 between 80 °F and 40 °F. So water that's exactly balanced in summer becomes noticeably corrosive in winter. Aim for a saturation index at or slightly above zero at your expected winter water temperature. Enter that temperature in the LSI calculator to check.
Step 2: Clean and shock
- Skim, brush the walls and floor, and vacuum. Leaves left in the pool feed algae and stain over the winter.
- Clean the filter or backwash.
- Shock to the shock level for your CYA, about 40% of the CYA reading under a widely used guideline. The shock calculator gives the dose. Do this a few days before you close, so chlorine can fall back toward a normal level before the cover goes on. Very high chlorine sitting under a cover can bleach or weaken some covers and liners.
- Make sure the water is clear and holds chlorine overnight before you close. Closing with algae already growing almost guarantees a green pool in spring.
Some owners also add a winter algaecide or a slow-release chlorine product under the cover. Follow the product label, and never put dry chlorine products where they can sit undissolved against a vinyl liner.
Step 3: Lower the water level
How far to lower the water depends on the pool type and cover, so check the manufacturer's instructions:
- In-ground pools with plumbing that will be blown out are commonly lowered to just below the skimmer opening, or below the tile line, so freezing water can't crack the skimmer or tile.
- Pools with a mesh safety cover often need the water a set distance below the cover, as the cover maker specifies.
- Above-ground pools are often lowered only a little, just below the skimmer and return, or not at all if the skimmer and return are plugged. Draining much more can let a vinyl liner shrink or pull out of its track.
Never drain a pool completely unless a professional advises it. In-ground pools can lift out of the ground when groundwater is high, and liners can be damaged when exposed.
Step 4: Protect the plumbing and equipment
- Blow out the lines. In freezing climates, use a shop vacuum or blower to push water out of the skimmer and return lines, then seal them with winter plugs. Many skimmers take a foam or plastic "ice compensator" that absorbs expansion.
- Pool antifreeze (propylene glycol made for pools) is sometimes added to lines after blowing them out, as extra protection. Never use automotive antifreeze.
- Drain the equipment. Open the drain plugs on the pump, filter, heater and any chemical feeder, and store the plugs somewhere you'll find them in spring (many people drop them in the pump basket).
- Filter: clean cartridges and store them dry. Set a sand filter's valve to "winterize" or "closed" if it has one, or follow the maker's instructions.
- Salt cell: remove, inspect, clean if scaled, and store it indoors.
- Heater and heat pump: follow the manufacturer's winterizing steps, which often include draining the heat exchanger.
- Electricity: turn off breakers for the pump, heater and timers so nothing starts dry over the winter.
If you won't blow out the lines, for example in a mild climate, the alternative is to keep the pump running whenever it freezes. Many timers and automation systems have a freeze-protection setting for this.
Step 5: Cover it
- Safety covers (mesh or solid) anchor into the deck and are designed to support weight. Mesh lets rain through, so the water level rises over the winter.
- Winter tarps held with water bags keep out debris but aren't safety covers. Pump rainwater and snowmelt off the top during the winter.
- Above-ground pools often use an air pillow under the cover, in the middle of the pool, so ice pushes against the pillow instead of the walls.
During the winter
- Check the cover after storms and remove standing water from solid covers.
- On a mesh-covered pool, check the water level now and then, and lower it if rain has raised it above the recommended level.
- Store leftover chemicals safely: cool, dry, acids and chlorine apart, out of children's reach. Liquid chlorine loses strength over the winter, so buy it fresh in spring. See pool chemical safety.
Opening in spring, briefly
Remove and clean the cover, reconnect the equipment, refill to the normal level, and start the pump. Then test everything. CYA and calcium usually stay put over the winter, but rain and snowmelt dilute them. Shock if the water isn't clear, then follow the order in our pool chemical levels guide.
Frequently asked questions
Can I just leave the pump running all winter instead?
In mild climates, many people do, running the pump fewer hours and keeping chlorine up. In places with long hard freezes, a power outage during a freeze can crack pipes and equipment, so a full winterization is safer.
Should I add chlorine tablets before closing?
Some owners put a floating dispenser with tablets under the cover. It keeps a little chlorine in the water, but it also adds stabilizer and lowers pH, and a stuck floater can bleach a liner. If you use one, keep it away from the walls and follow its instructions.
Why is my pool green when I open it?
Usually because it was closed too warm, with too little chlorine, or with algae already present, or because debris got under the cover. Shock and hold the shock level until it clears. See how to shock a pool.