How To Close An Above Ground Swimming Pool: The Definitive Professional Winterization Guide

Successfully closing an above ground swimming pool requires a precise combination of chemical balancing, mechanical drainage, and physical protection to prevent structural damage from ice expansion and algae proliferation. By maintaining a pH of 7.2–7.6 and lowering water levels to 4–6 inches below the skimmer, you safeguard the liner and filtration system against the structural stresses of freezing temperatures.

Essential Inventory and Pre-Closing Site Preparation

Preparing for the off-season involves more than just pulling a cover over the water. Above ground pools are particularly susceptible to wind damage and ice expansion because their walls are exposed to the elements on all sides. Before beginning the physical labor, ensure you have all necessary components staged at the poolside to avoid mid-process delays that could leave the pool vulnerable to debris.

Professional Winterization Kit and Tool Requirements

• Water Testing Equipment: High-accuracy liquid reagent kit or digital tester capable of measuring pH, Total Alkalinity, Calcium Hardness, and Cyanuric Acid.
• Winter Chemical Arsenal: High-concentration non-chlorine shock (potassium monopersulfate), long-acting winter algaecide (60% concentration preferred), and a metal sequestrant if using well water.
• Mechanical Plugs and Seals: Expansion plugs (rubberized), threaded return line plugs, and a skimmer winter plate (Aquador or similar) if you choose not to drain below the skimmer.
• Expansion Protection: Heavy-duty air pillows (minimum 4x4 or 4x5 feet) and an ice compensator for the skimmer.
• Pool Cover Hardware: A winter cover rated for your pool's diameter, a high-tension steel cable, a heavy-duty winch, and optional wall clips for high-wind environments.
• Submersible Pump: For removing excess water from the top of the cover throughout the winter.

Budget and Timeline Benchmarks: The entire process typically requires 4 to 6 hours of active labor, spread across two days to allow for chemical circulation. Expect to spend between $150 and $300 on high-quality winterization chemicals and replacement hardware, depending on the size of the pool and the age of your current cover.

The Step-by-Step Mechanical and Chemical Winterization Sequence

Following a specific order of operations is vital. Adding chemicals too early or draining water before cleaning can lead to staining or equipment failure. The following sequence is designed to maximize the lifespan of your vinyl liner and ensure a clear opening in the spring.

Step 1: Final Deep Clean and Structural Inspection

Before adjusting a single drop of water, the pool must be physically pristine. Any organic matter left in the pool—leaves, silt, or insects—will decay over the winter, consuming your algaecide and potentially staining the vinyl liner permanently.

1. Vacuum the floor thoroughly, preferably using a "waste" setting if your multi-port valve allows, to remove fine particulates from the system entirely.
2. Brush the walls and floor to loosen biofilm and microscopic algae spores that hide in the texture of the liner.
3. Clean the water line with a non-abrasive pool surface cleaner to remove oils and scale; this prevents the "winter ring" often seen during spring openings.
4. Inspect the pool’s uprights, top rails, and "boots" for signs of corrosion or stress fractures. Address these structural issues now, as the weight of ice and snow will exacerbate any existing weaknesses.

Step 2: Precision Water Chemistry Balancing

Winterized water behaves differently than summer water. Because the water will remain stagnant for months, you must balance it to the higher end of the scales to account for the natural "drift" that occurs during the cold season.

1. Adjust Total Alkalinity (TA): Target 100–120 ppm. This acts as a buffer for your pH.
2. Adjust pH: Target 7.4–7.6. If the pH is too low, the water becomes acidic and can make the vinyl liner brittle; if it is too high, it leads to scale formation on the pool walls.
3. Calcium Hardness: Maintain 200–400 ppm. In above ground pools, low calcium can actually cause the water to "leach" minerals from the vinyl or metal components.
4. Shock Treatment: Add a double dose of non-chlorine shock. This oxidizes organic contaminants without bleaching the liner or interfering with the algaecide you will add later.

Pro-Tip: Run the filter for at least 6 to 12 hours after adding chemicals to ensure total homogenization. Adding winter chemicals to stagnant water can lead to "hot spots" of high concentration that may damage the pool liner.

Step 3: Draining and Disconnecting Plumbing

Water expands by approximately 9% when it freezes. In a closed plumbing system, this expansion generates thousands of pounds of pressure, which will easily crack PVC pipes, pump housings, and filter tanks.

1. Lower the Water Level: Drain the water to roughly 4 to 6 inches below the lowest plumbing return or the skimmer mouth.
2. Exceptions: If you are using a skimmer faceplate (like an Aquador), you do not need to drain the water below the skimmer, though many professionals still recommend it to reduce the risk of ice pressure on the pool walls.
3. Disconnect Hoses: Remove all flexible corrugated hoses. Drain them completely, roll them up, and store them indoors. Leaving hoses attached to the pool allows ice to form inside them, which can pull on the pool wall and cause a structural collapse.
4. Winterize the Skimmer: Remove the skimmer basket and weir door. Insert a "Gizzmo" or a similar expansion-absorbing tube into the bottom of the skimmer. This device is designed to collapse if water enters the skimmer and freezes, protecting the plastic skimmer housing from cracking.

Step 4: Filtration System and Pump Winterization

The pump and filter are the most expensive components of your pool. They must be completely devoid of water.

1. The Pump: Remove all drain plugs from the pump housing (usually two: one near the basket and one near the motor). Turn the pump on for a split second to sling any remaining water out of the impeller, then disconnect the pump and store it in a dry, climate-controlled area.
2. The Filter:
   
   
   • Sand Filters: Set the multi-port valve to "Winterize" (or between settings) and remove the large drain cap at the bottom. Leave the cap off all winter.
   • Cartridge Filters: Remove the cartridge, clean it thoroughly with a filter soak, and store it indoors. Leave the drain plug out of the filter tank.
   • DE Filters: Backwash the system, then disassemble the grids and hose them down. Store the grids in a safe area where they won't be punctured.
3. Chlorinators and Accessories: If you have an inline chlorinator, drain it and remove any leftover tablets. Remove the pressure gauge from the filter and store it inside; these are highly susceptible to freezing and will fail if even a drop of water remains in the bourdon tube.

Step 5: The Air Pillow and Winter Cover Installation

The air pillow is not there to keep the cover up; it is there to act as an "expansion joint." When the water in the pool freezes, it expands inward. The air pillow compresses, absorbing that pressure so it doesn't push outward against the pool walls.

1. Inflate the Pillow: Fill the air pillow to about 60–70% capacity. Do not over-inflate; the pillow needs "give" to work correctly and to prevent it from popping when snow piles on top.
2. Centering: Tether the pillow in the exact center of the pool using thin nylon strings attached to the pool’s top rails.
3. Apply the Cover: Pull the winter cover over the pool, ensuring it rests on top of the air pillow. The cover should drape loosely over the sides; it should not be taut like a drum.
4. Secure the Cable: Thread the steel cable through the cover loops and the winch. Tighten the winch until the cover is secure against the pool wall.
5. Wind Protection: If you live in a windy area, use winter cover clips or "wall-bags" (weighted tubes) to prevent the wind from getting under the cover and lifting it.

Warning: Never use bricks, cinder blocks, or heavy abrasive items to hold down a pool cover. If they fall into the pool, they will tear the liner, and the weight can cause the pool wall to buckle inward.

Critical Chemical Thresholds and Specifications

The following table outlines the technical parameters required for a successful dormant period. Adhering to these metrics prevents the most common "green pool" scenarios in the spring.

Common Winterization Failures and Remedial Actions

Even with careful planning, environmental factors can compromise your pool closing. Recognizing these failures early can save the structural integrity of the vessel.

• Failure: The Air Pillow Moves to One Side or Deflates
  
  
  • Root Cause: Improper tethering or a slow leak in the valve.
  • Actionable Fix: Use a "patch kit" for small leaks and ensure the pillow is tethered from four points (North, South, East, West) rather than just two. If it has already moved, use a telescopic pole to gently reposition it before the first hard freeze.
• Failure: Excessive Water Accumulation on the Cover
  
  
  • Root Cause: Heavy precipitation or melting snow causing the cover to "dip" and pull on the pool walls.
  • Actionable Fix: Deploy a cover pump immediately. Never allow more than 2 inches of water to sit on the cover. The weight of the water can create enough tension to pull the pool walls inward, leading to a total structural collapse.
• Failure: The Winter Cover Slips into the Pool
  
  
  • Root Cause: Cable winch failure or improper sizing of the cover (too small for the pool diameter).
  • Actionable Fix: Pull the cover back into place immediately. If the water has been contaminated by debris, you must re-treat with a dose of liquid chlorine or algaecide before re-securing the cable. Ensure the cable is threaded through every single grommet.
• Failure: Evidence of "Ice Heaving"
  
  
  • Root Cause: Water level was left too high, or the air pillow was not used, causing ice to push against the skimmer or top rails.
  • Actionable Fix: Do not attempt to break the ice, as sharp edges can slice the liner. Add a pool-safe antifreeze to the skimmer (if not already drained) and ensure the Gizzmo is properly seated.

Frequently Asked Questions

When is the best time to close an above ground pool?

The ideal time is when the ambient water temperature stays consistently below 65 degrees Fahrenheit (18 degrees Celsius). Closing earlier, while the water is still warm, allows algae to grow under the cover because the chlorine and algaecide will be consumed much faster in warm water.

Can I use automotive antifreeze in my pool plumbing?

No, never use automotive antifreeze. It is toxic to humans and will damage your pool liner. If you must use antifreeze for your plumbing lines, use only non-toxic, propylene glycol-based "Pool and Spa Antifreeze" which is designed for this specific purpose.

How do I prevent the cover cable from rusting?

The cables provided with most winter covers are plastic-coated steel. To prevent the winch and the cable ends from rusting, apply a small amount of water-resistant grease or WD-40 to the winch mechanism before tightening. This ensures you can easily release the tension in the spring.

Should I keep the pool light inside the pool?

If your above ground pool has a "through-the-wall" light, it is safest to remove it, wrap it in a plastic bag, and submerge it at the bottom of the pool (weighted down) or pull it up onto the deck. Leaving it in its housing at the water line subjects it to the crushing force of expanding ice.

Do I need to keep adding chemicals during the winter?

In most cases, no. If the pool was balanced and shocked correctly at the time of closing, and the water remains below 60 degrees, the chemicals will remain effective. However, if you have a particularly long or warm spring, you may want to peel back a corner of the cover and add a quart of algaecide in early March.

Protect Your Investment This Winter

A meticulous winterization process is the most effective way to extend the lifespan of your above ground pool and ensure a stress-free spring opening. By prioritizing chemical balance and mechanical protection, you safeguard your backyard oasis against the harshest winter elements.