How to Winterize a Swimming Pool

Closing a Pool for the Winter

Based on 10,000 gallons (37,855 Liters)


Traditional pool winterization is about cleaning the water, loading it with chemicals to prevent algae over the winter, maybe putting in some anti-freeze, blowing the water out of the pipes to prevent freeze damage, and putting on the pool safety cover. Unfortunately most of the damage done to swimming pool surfaces is occurs during the winter, and these traditional steps do not address the crux of the problem: The LSI.

The Langelier Saturation Index (LSI) shows us how cold water makes water more aggressive and hungry for calcium (low-LSI). Traditional winterization ignores temperature, but not the Orenda winterization program. Water temperature matters! So let's be proactive and winterize by preparing for the cold water temperatures. This means feeding the pool the calcium and alkalinity that it needs to survive the winter without feeding on the pool surface, causing problems like calcite crystals, winter dust, fading liners, and etching.

This procedure addresses both the proper balance of water (LSI), and the sanitization of water to prevent it from turning green and nasty.

Related: How to Implement the Orenda Program

You will need:

  • Work gloves, a good attitude, and a badass theme song as you walk into the backyard with confidence
  • A test kit and thermometer
  • Non-stabilized chlorine (liquid sodium hypochlorite or cal hypo shock)
  • PR-10,000 Phosphate Remover
  • CV-600 or CV-700 Enzyme
  • A small amount of SC-1000 Scale & Metal Control
  • Calcium chloride and/or sodium bicarbonate
  • A clean bucket to dilute and pre-dissolve chemicals

At least 24 hours prior to closing day


  1. Use the Orenda App LSI Calculator to balance the water at the coldest future winter temperature.
    • If your pool will freeze, you will need a bare minimum of 400 ppm calcium hardness, but we recommend closer to 500 ppm.
  2. Pre-dissolve the prescribed amount of calcium chloride in a bucket of water with a few ounces of SC-1000, and add to the pool. Related: How to add calcium chloride.
  3. Add up to 8 oz. of PR-10,000 phosphate remover.
  4. Purge the pool with 1 quart (32 fl.oz.) of CV-600 or CV-700 enzymes.

Closing day

  1. Test water chemistry again, and make sure you have enough calcium, alkalinity and pH to sustain LSI balance in the winter. Keep in mind that with pools that are uncovered or have mesh covers, the pH will naturally rise to its pH ceiling, thanks to physics. Pools with solid covers will not be able to off-gas CO2, and therefore may have a lower pH in the winter.
  2. Since you raised calcium on the previous visit to the pool, raise alkalinity as needed by pre-dissolving sodium bicarbonate in a bucket of water and add it to the pool. Related: How to add sodium bicarbonate.
  3. Vacuum the pool and clean the filter
  4. Do the physical winterization procedures: lower the water level, blow out plumbing lines, cover the pool, etc.. But avoid adding anti-freeze because enzymes will conflict with them.


Additional Notes

Keeping a pool from flipping to green is easier than you might think. Algae does not grow well in cold water, so the key is to remove micronutrients in the fall with phosphate remover, and chlorinate on closing. No algaecides or sequestering agents are needed for our winterization procedure. Sequests and chelants (like SC-1000) do not activate in water temperatures below about 60ºF. The only SC-1000 recommended is a small amount to assist in pre-dissolving calcium chloride (because dissolving calcium chloride gets hot, SC-1000 activates quickly). 

The priority is to prepare for the cold water temperature. If you are able to ease the pool into the winter over multiple visits, that is even better. Same with easing the pool out of the winter in the spring.  See our spring opening procedure for more information.


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