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How to Close an Inground Pool for Winter: Step-by-Step Guide

Closing your inground swimming pool properly in late autumn is the single most critical maintenance routine for protecting your investment. Water expands by roughly 9 percent when it freezes, exerting up to 30,000 PSI of outward hydraulic pressure inside closed pipes. A single pocket of trapped water inside an underground PVC line, multiport valve body, or copper heater core can crack fittings and cause thousands of dollars in underground plumbing damage. Winterizing an inground pool requires thorough chemical balancing, line blowout procedures, and secure covering. Calculate your winter chemical additions with our pool chemical calculator and pool volume calculator before shutting down.

14 min read, updated September 2026

Key takeaways

  • Freezing water expands with enough force to shatter rigid Schedule 40 PVC pipes and brass heater headers.
  • Blow out all suction and return lines with a commercial cyclone blower until pipes bubble pure air.
  • Thread rubber expansion plugs into return fittings while lines are still bubbling to create an airtight seal.
  • Always install non-toxic propylene glycol pool antifreeze, never toxic automotive ethylene glycol.

Phase 1: Pre-Closing Water Chemistry Balancing

Balanced water during the 5 to 6 months of winter shutdown protects plaster finishes from etching and prevents scale from plating out on tile lines:

  • Target Parameters: Balance pH to 7.4 to 7.6, total alkalinity to 100 to 120 ppm, and calcium hardness to 200 to 300 ppm.
  • Shock with Liquid Chlorine: Add a strong dose of liquid chlorine shock 48 hours before closing to kill all dormant algae and bacteria. Avoid heavy doses of granular cal-hypo immediately before covering, as undissolved granules can bleach plaster or vinyl liners.
  • Add Winterizing Polyquat Algaecide: Pour a concentrated 60% polyquat algaecide around the perimeter. Polyquat coats algae cells and prevents colonies from multiplying when water warms up in early spring.

Phase 2: Lowering Water Level

How far you lower pool water depends on the type of winter cover you use:

Recommended Water Drawdown Levels by Cover Type

Cover StyleWater Level TargetPurpose
Solid Tarp Cover with Water Bags2 to 4 inches below skimmer mouthPrevents water from freezing inside skimmer throat
Mesh Safety Cover (Rain Permeable)12 to 18 inches below skimmer mouthAccommodates winter rain and snowmelt without overflowing
Automatic Safety Cover1 to 2 inches below skimmerSupports cover fabric without excessive stress

Phase 3: The Line Blowout Procedure

Blowing out plumbing lines requires a high-volume, low-pressure air source, such as a specialized Cyclone pool blower or a powerful 5+ HP wet/dry shop vac:

  1. Connect blower to pump pad: Remove the pump strainer lid and thread your blower hose into the pump intake or discharge port.
  2. Blow out return lines: Force air down the return plumbing. Water will sputter and erupt out of the return jets in the pool. Let it blow until a continuous mist of dry air emerges.
  3. Insert winter expansion plugs: While air is bubbling vigorously, submerge rubber expansion plugs or threaded winter plugs and tighten them securely, locking trapped air inside the pipe.
  4. Blow out skimmer lines: Push air through skimmer pipes. When water clears, pour non-toxic pool antifreeze (propylene glycol) down the pipe and screw in an expansion Gizzmo tube into the bottom threads.
  5. Blow out main drains: Air-lock the main drain by blowing air until large bubbles erupt from the bottom of the deep end, then quickly close the diverter valve to create an air pocket.

Phase 4: Draining Equipment at the Pad

Remove all threaded drain plugs from the bottom of the pump pot, filter tank, heater headers, chlorinators, and booster pumps. Place all plugs inside the pump strainer basket and store the lid loose so gaskets do not freeze and crack.

how to close an inground pool FAQs

What kind of antifreeze should I use in pool pipes?

Always use non-toxic propylene glycol pool/marine antifreeze (typically pink or blue). Never use automotive ethylene glycol antifreeze; it is highly toxic to humans and pets, stains pool finishes, and ruins water chemistry upon reopening.

What is a Gizzmo and why is it necessary?

A Gizzmo is a hollow, threaded plastic tube screwed into the bottom of a skimmer. If ice forms inside the skimmer, the ice expands inward and crushes the hollow Gizzmo rather than cracking the expensive plastic skimmer housing.

Do I have to blow out the lines if I live in the South?

If your region experiences temperatures below 28 degrees Fahrenheit for more than 12 consecutive hours, underground and aboveground pipes will freeze. If you do not blow lines, you must keep your pump running continuously on freeze protection.

Can ice damage a pool safety cover?

Heavy snow and ice are supported by mesh safety covers as long as pool water levels are maintained no more than 18 inches below the coping. If water is drained too low, snow loads can stretch and snap cover springs.