How to winterize an above-ground pool

Why this afternoon matters more than any other

A seasonal pool spends more of the year closed than open, and almost every expensive failure happens during the closed part. The pattern is consistent: nothing appears to go wrong in October, and in April the liner is cracked, the pump housing has split, or the water is unrecoverable.

None of those is difficult to prevent. All of them are impossible to fix afterwards. That asymmetry is why closing deserves a careful afternoon rather than a rushed hour.

The sequence, and why the order matters

Most closing failures are sequence errors rather than knowledge failures. The full step list is below; three points deserve emphasis.

Wait for genuinely cold water

Chlorine is consumed faster in warm water and algae grows in it. A pool closed while the water is still warm has months of biological activity under a cover with no filtration and no fresh sanitizer. Wait for consistently cold water rather than the first cold week — a warm spell after you close is worse than closing two weeks later.

Balance before, not after

Water sitting still for five months at the wrong pH is water working slowly against your liner and fittings the entire time. Set alkalinity, then pH into the CDC’s published 7.0–7.8 range, then chlorine, before the cover goes on. Detail in pH and alkalinity.

Drain the equipment completely

This is the ten-minute step that prevents the single most common winter loss. Water trapped in a pump housing, a filter body or a hose expands when it freezes, and the housing cracks. Disconnect everything, drain it, and store it somewhere that does not freeze. There is no chemical or cover that compensates for skipping this.

The steps in full

1. 1

   Wait for cold water

   Consistently cold, not one cold week. A warm spell after closing is worse than closing later.
2. 2

   Deep clean

   Skim, brush every surface including the waterline, vacuum the floor. Organic material left in the pool feeds algae all winter.
3. 3

   Balance the chemistry

   Alkalinity, then pH into the CDC's 7.0–7.8 range, then chlorine. Test rather than assume — this is the last reading you will take for months.
4. 4

   Add winterizing chemicals

   A polyquat algaecide and an oxidizer, dosed for your actual volume, with the pump running. A closing kit bundles these and is worth the small premium the first year.
5. 5

   Run the filter for a few final hours

   So the winterizing chemicals distribute fully before circulation stops for the season.
6. 6

   Lower the water to the stated level

   Your manufacturer publishes a winter water level. Follow theirs rather than a general rule — it differs by pool type and wall height.
7. 7

   Disconnect and drain all equipment

   Pump, filter, every hose. Tip them, shake them, and store somewhere that does not freeze. This is the step that prevents the most common winter loss.
8. 8

   Fit and secure the winter cover

   A winter cover, not a solar cover. Secure with its cable and winch or straps so wind cannot lift it, with enough slack to sit on the water rather than stretch across it.
9. 9

   Float an air pillow, if you freeze

   Inflated to about 60 percent and tethered in the center. Expanding ice pushes against the pillow rather than the wall. Over-inflating bursts it, which is the most common mistake here.
10. 10

    Check on it through the winter

    Clear standing water and heavy snow off the cover. Accumulated weight is what tears covers, and a torn cover in January means a green pool in April.

Soft-side pools: a different answer

If you have an inflatable or a small soft-side pool, none of the above applies in the same way. Empty it, dry it thoroughly, fold it and store it indoors. A soft-side pool left up through a freezing winter is a pool you are gambling with, and the odds are poor.

Dry is the operative word. A pool folded away damp develops mildew along the folds, which weakens the material exactly where it is already stressed. Give it a genuinely dry day.

What closing costs

The chemicals are modest — a closing kit plus the cover, which is a multi-season purchase. The water you lower is real but small at the national average of $6.06 per 1,000 gallons. The full picture is in the annual cost guide, which includes closing and opening as line items most budgets leave out entirely.

Set against that: the cost of a new pump, or a new liner, or a spring recovery that takes a full shock treatment and a week of continuous filtration. Closing properly is the cheapest insurance available in pool ownership.