How To Winterize An Inground Swimming Pool: A Professional Step-by-Step Closing Guide

Winterizing an inground swimming pool protects your structural and mechanical investment from catastrophic freeze damage, which occurs when water expands by nine percent upon freezing. This professional closing protocol outlines how to balance pool water chemistry, completely purge plumbing lines of water, isolate mechanical equipment, and secure a safety cover. Following these precise steps prevents cracked PVC pipes, ruptured filter tanks, and costly spring algae blooms.

Pre-Closing Inspection & Winterization Toolkit

Attempting to winterize a swimming pool without the proper industrial-grade equipment can lead to incomplete line purges and eventual pipe ruptures. Before beginning the winterization process, monitor local weather patterns. Do not close your pool until the daily high water temperature consistently drops below 65 degrees Fahrenheit (18 degrees Celsius). Closing the pool too early, while the water is still warm, will deplete your winterizing algaecide and promote rapid, unseen algae growth under the cover.

Required Materials, Equipment, and Performance Benchmarks

• Essential Chemicals:
  
  
  • 60% non-foaming Polyquat algaecide
  • Non-chlorine oxidizer shock (potassium monopersulfate)
  • Liquid pool shock (sodium hypochlorite)
  • Calcium hardness increaser (calcium chloride)
  • pH decreaser (sodium bisulfate) or pH increaser (sodium carbonate)
  • Non-toxic pool antifreeze (propylene glycol only; never use automotive ethylene glycol)
• Essential Gear and Specialized Tools:
  
  
  • High-volume air blower (such as a Cyclone blower or a 6.5 HP peak wet/dry shop vacuum)
  • Tapered rubber expansion plugs (ranging from size #8 to #11, matching your return inlet diameters)
  • Threaded winter plugs with rubber O-rings (for threaded wall fittings)
  • Skimmer expansion bottles (commonly referred to as Gizzmos, featuring 1.5-inch or 2-inch male threads)
  • Teflon thread-seal tape
  • Spring tension installation tool and hex key for safety cover anchors
  • Submersible pump with a garden hose attachment
• Pre-Procedure Benchmarks:
  
  
  • Estimated Budget: $150 to $350 for DIY materials; up to $600 if purchasing a professional air blower.
  • Estimated Duration: 4 to 6 hours of active labor, spread across a 48-hour chemical stabilization period.
  • Mandatory Prerequisite Knowledge: Knowledge of your pool’s exact gallon capacity and the configuration of your plumbing lines (including the number of suction and return lines).

The Step-by-Step Pool Closing and Decommissioning Protocol

Step 1: Perform Deep Cleaning and Establish Chemical Equilibrium

To prevent scale formation, metal staining, and organic decay over the winter, the water must be structurally balanced. Vacuum the pool floor to remove all organic matter, brush the walls to dislodge stubborn spores, and skim the surface. Organic debris left in the pool will consume your sanitizer rapidly and stain the plaster or vinyl liner.

Test your water using a reliable liquid reagent test kit and adjust the chemical levels to hit these exact parameters:

1. Adjust the pH to a range of 7.2 to 7.6.
2. Bring Total Alkalinity (TA) to 80 to 120 parts per million (ppm).
3. Adjust Calcium Hardness to 200 to 400 ppm. This is critical for preventing the winter water from leaching calcium directly out of your plaster or grout.
4. Add a high-quality metal sequestering agent to prevent copper or iron from precipitating out of the cold water and staining your pool walls.
5. Add your pool shock to raise free chlorine levels to 10 to 12 ppm. Let the pump run for at least 24 hours to fully circulate the chemicals. Once the chlorine level drifts back down to 3 to 5 ppm, add your 60% Polyquat algaecide.

Warning: Never add pool shock and algaecide to the water at the same time. High chlorine levels will break down the active ingredients in the algaecide, rendering it useless before winter even begins. Space these chemical applications at least 24 hours apart.

Step 2: Draw Down the Pool Water Level

Lowering the water level prevents water from entering the skimmer throat and freezing, which can warp or crack the skimmer assembly. The depth to which you lower the water depends directly on the type of pool cover you use.

If you are using a standard solid tarp-style cover held down by water bags, use a submersible pump or your pool pump’s waste setting to lower the water to 3 inches below the bottom of the skimmer mouth. If you are using a mesh safety cover that allows rainwater and melting snow to drain through into the pool, lower the water to 12 to 18 inches below the skimmer mouth.

Monitor this drawdown closely. For pools with vinyl liners, never lower the water level below the shallow-end floor, as the weight of the water is required to keep the liner stretched and secured in its track.

Step 3: Drain and Winterize Filtration and Heating Systems

Water trapped inside your mechanical equipment will freeze, expand, and crack cast-iron pump volutes, plastic filter tanks, and copper heater headers. Turn off all electrical breakers powering the pump, filter, heater, and automation systems.

Locate the drain plugs on your equipment:

1. The Pump: Remove the drain plug at the bottom of the strainer basket housing and the drain plug on the pump volute. Place the pump lid and basket in a safe storage area.
2. The Filter: For a sand filter, unscrew the large drain cap at the bottom of the tank to allow water to drain out. Set the multiport valve handle to the "Winterize" or "Closed" position. For a D.E. (diatomaceous earth) or cartridge filter, open the air relief valve on top, unscrew the bottom drain plug, and remove the internal cartridges or D.E. grids. Hose them off thoroughly, let them dry, and store them in a dry space.
3. The Heater: Remove the heat exchanger drain plugs (usually located on both the front and back headers). Blow compressed air through the pressure switch line to ensure no water remains in the internal heat exchanger tubes.
4. The Salt Cell: Unplug and unscrew the salt chlorinator cell from the plumbing line. Store the cell indoors to protect its delicate internal precious metal plates from freezing temperatures.

Store all removed drain plugs inside the empty pump strainer basket. This ensures you will not lose them when it is time to reopen the pool in the spring.

Step 4: Purge and Plug the Plumbing Lines (The Air Blowout Process)

This is the most critical phase of pool winterization. You must purge all water from your underground PVC pipes using a high-volume air blower connected to your filtration plumbing.

First, connect your air blower to the pump’s strainer basket drain plug opening or to the threaded opening of the pump's outlet pipe. Open all valves on your suction lines (skimmer and main drain) and return lines. Turn on the blower.

[ Air Blower ] | v [ Pump Strainer Basket ] / \ v v [ Suction Lines ] [ Return Lines ] - Skimmers - Pool Return Wall Jets - Main Drain (Bottom) - Step Jets

Begin with the return lines:

1. Keep all return valves open. You will soon see water bubbling vigorously out of the return jets in the pool.
2. Blow out the return jet closest to the pump first. Once only air is blasting out of the jet, insert a rubber expansion plug or a threaded winter plug into the return fitting. Tighten it securely until a hermetic seal is created.
3. Move to the next closest return jet, repeat the process, and continue until you reach the furthest return jet. Plugs must be seated tightly while the air blower is running to prevent water from slipping back into the line.

Next, blow out the suction lines:

1. Close the main drain valve and open the skimmer valve.
2. Once air blows cleanly through the bottom of the skimmer throat, insert a Gizzmo expansion bottle into the skimmer's bottom threaded port. Wrap Teflon tape around the Gizzmo threads to ensure a watertight seal. The Gizzmo is a hollow plastic tube that will collapse to absorb expansion forces if ice forms inside the skimmer throat.
3. Open the main drain valve and close the skimmer valve. Air will push water down the line until a massive column of air bubbles erupts from the main drain grate at the bottom of the pool.
4. Let the main drain bubble for 60 seconds to clear the line. Then, quickly close the main drain valve at your equipment pad. This locks a columns of compressed air inside the vertical pipe, creating an "air lock" that prevents water from rising back up the line from the bottom of the pool.

Pro-Tip: If your pool has a slide, waterfall, or deck chlorinator, make sure those auxiliary plumbing lines are blown out individually as well. Pour one gallon of non-toxic propylene glycol pool antifreeze directly into each skimmer line before sealing it with a plug or Gizzmo. This adds an extra layer of safety against pipe fractures should any condensation collect in low points.

Step 5: Secure the Winter Pool Cover

With the plumbing isolated and the water balanced, you can now install your winter cover. This step prevents wind, leaves, sunlight, and debris from entering your pool, while also acting as an essential safety barrier.

If using a safety cover:

1. Use a hex key to raise all the brass anchors embedded in your concrete pool deck. They should sit roughly 1/4 inch above the concrete surface.
2. Lay the safety cover out across the pool surface.
3. Using the installation rod, attach the heavy-duty spring loops of the cover strap over each brass anchor.
4. Adjust the tension straps so that the springs are compressed between one-third and one-half of their total length. The cover should be taut, resembling a trampoline, with no sagging areas.

If using a standard solid tarp cover:

1. Lay the tarp flat over the pool surface, allowing it to rest naturally on the lowered water level.
2. Fill vinyl double-chamber water bags to roughly 75% capacity (allowing room for freeze expansion) and place them end-to-end around the entire perimeter of the pool.
3. Ensure there are no gaps between the water bags where wind could slip under the cover and dislodge it.

Water Chemistry Targets and Equipment Storage Specifications

The following table provides the precise chemical thresholds and equipment configurations required to successfully maintain structural integrity throughout winter.

Cold-Weather Complications and Field Solutions

Even when you follow winterization protocols closely, unforeseen environmental factors can cause issues. Below are common winter failures, their root causes, and how to fix them.

Scenario 1: The Skimmer Housing Cracks Near the Throat

• Root Cause: Rainwater, melting ice, or snow accumulated inside the skimmer throat because no Gizzmo was installed, or the installed Gizzmo cracked under stress. The freezing water expanded outward, cracking the rigid plastic skimmer housing.
• Actionable Fix: For minor hairline fractures, dry the area thoroughly and apply a generous coat of two-part marine-grade epoxy or plastic welding cement to seal the leak. For severe structural cracks that cause active water loss behind the pool wall, the concrete deck must be cut back, the soil excavated, and the damaged skimmer unit cut out and replaced with a new assembly.

Scenario 2: Severe Algae Bloom Discolors the Water Under the Cover in Early Spring

• Root Cause: The pool was closed too early in autumn when the water temperature was still above 65 degrees Fahrenheit, or the pool was opened too late in spring. The winterizing algaecide dissipated, and warm sunlight filtering through a mesh cover allowed algae to multiply.
• Actionable Fix: Do not wait to open the pool. Peel back the cover, add liquid chlorine shock directly to the pool to raise free chlorine to 15 ppm, and brush the walls. Run your filter system continuously for 24 to 48 hours until the dead algae turns grey/white and is filtered out or vacuumed to waste.

Scenario 3: The Pool Filter Tank Splits Along the Middle Weld Seam

• Root Cause: The filter tank's drain plug was never removed at closing, or the multiport valve was left in the "Closed" position instead of the "Winterize" or "Special" position. This trapped water inside the lower half of the tank, which expanded when freezing temperatures arrived.
• Actionable Fix: A split filter tank cannot be repaired or patched, as it operates under intense pressure (often 15 to 30 PSI). The entire filter tank must be replaced before restarting the system to avoid dangerous structural failures.

Scenario 4: Return Line Plugs Slide Out or Lose Their Airtight Seal

• Root Cause: The rubber expansion plugs used were too small, worn out, or oily. Alternatively, they were not tightened enough to withstand the pressure of the remaining water or air in the lines.
• Actionable Fix: Remove the loose plug and inspect the opening for damage. Re-insert a new, clean rubber expansion plug of the correct size, and tighten the wing nut until the rubber bulges significantly to form a tight seal. Wrap Teflon tape around threaded plugs to ensure a secure, dry fit.

Frequently Asked Questions

At what temperature should I winterize my pool?

You should winterize your pool when the water temperature drops consistently below 65 degrees Fahrenheit (18 degrees Celsius). Closing the pool when the water is warmer than this will cause your winterizing chemicals and algaecides to break down quickly, leaving the pool vulnerable to algae growth before freezing temperatures arrive.

Can I use automotive antifreeze in my pool plumbing?

No, you must never use automotive ethylene glycol antifreeze in your pool plumbing. Ethylene glycol is highly toxic to humans and pets, and it can ruin vinyl pool liners. Always use non-toxic propylene glycol, which is designed specifically for pool plumbing and RV fresh-water systems.

How far down should I drain my pool for winter?

The correct water level depends on your pool cover. For solid tarp covers, drain the water to 3 inches below the skimmer mouth. For mesh safety covers, which allow rainwater and snowmelt to pass through, drain the water 12 to 18 inches below the skimmer mouth to prevent the rising water level from touching the cover over the winter.

Do I need to run my pool pump during winter?

No, once your pool is fully winterized, the plumbing lines blown out, and the equipment drained, the power to your pump and filter system must remain off for the winter. Running the pump without water in the lines will quickly damage the pump seal and motor.

Should I put chlorine tablets in a floater under the winter cover?

No, you should not leave a chemical floater in the pool over the winter. If the floater gets stuck against a wall or sinks to the bottom, the highly concentrated chlorine will bleach and ruin vinyl liners, or damage plaster finishes. Instead, use a specialized winter chemical kit containing slow-release, non-chlorine oxidizers and algaecides.

Protect Your Pool Investment for the Winter Season

Properly winterizing your pool preserves its structural integrity and ensures a smooth, hassle-free opening when warm weather returns. If you need assistance with this technical process, contact a certified pool care professional to handle your winter blowout and cover installation.