Do I Need Pool Closing Chemicals? Complete Winter Guide

Step-by-Step Pool Closing Chemical Process

1. Balance Your Water Chemistry First
   Before adding any closing chemicals, test and adjust your water chemistry. Use a reliable test kit like the Taylor K-2006 to measure:
   • pH: Adjust to 7.2-7.6 using muriatic acid to lower or sodium carbonate to raise
   • Total Alkalinity: Target 80-120 ppm for most pools
   • Calcium Hardness: Aim for 250-350 ppm for plaster/concrete pools, 175-225 ppm for vinyl/fiberglass pools
   • Free Chlorine: Should be 4-6 ppm before adding closing chemicals (use the all-in-one pool calculator to find the right FCFree Chlorine — The chlorine actively sanitizing your water right now. This is the number you keep an eye on. how much you need → for your CYACyanuric Acid (stabilizer) — Sunscreen for your chlorine — it keeps sunlight from burning it off. The catch: the more you have, the more chlorine you need to keep. learn more →)
   Balanced water prevents staining, scaling, and corrosion during the closed season.
2. Shock the Pool 24-48 Hours Before Closing
   Add liquid chlorine or calcium hypochlorite shock to achieve the shock level for your CYA (about 40% of CYA). This eliminates any bacteria or organic contaminants that could cause problems during winter. Raise free chlorine to shock level for your CYA (our all-in-one pool calculator sizes the dose). Allow the chlorine level to drop to 4-6 ppm before proceeding to the next step (typically 24-48 hours depending on sun exposure and temperature).
3. Add Winter Algaecide
   Apply a high-quality winter algaecide. A non-metallic polyquat such as Leslie's Algae Control (60% polyquat) provides long-lasting protection without adding staining copper to your water. Copper-based winter algaecides like BioGuard Arctic Blue Algae Protector also work but should be used with caution if you have metal-staining concerns. Follow the manufacturer's dosage instructions, typically 1 quart per 15,000-20,000 gallons. Add the algaecide with the pump running and circulate for at least 2 hours to ensure even distribution.
4. Blow Out Plumbing Lines
   Before adding antifreeze, use a shop vac or air compressor to blow water out of all plumbing lines - skimmers, returns, main drains, and any accessories. This is the most critical freeze protection step. Water left in pipes will freeze and crack them regardless of antifreeze.
5. Apply Pool Antifreeze to Plumbing Lines
   After blowing out lines, add pool antifreeze (propylene glycol) as backup protection. You'll need approximately 0.9 gallons of antifreeze per 10 feet of 1.5-inch pipe (1.5-inch pipe holds about 0.092 gallons per linear foot). Pour antifreeze into skimmer lines, return lines, and any other plumbing that cannot be completely drained. Never use automotive antifreeze, which is toxic and will damage your pool equipment.
6. Add Enzyme Products (Optional but Recommended)
   Enzyme-based products like Natural Chemistry Pool Perfect break down organic contaminants that accumulate during winter. Add according to package directions, typically 1 liter per 20,000 gallons. This step helps prevent waterline scum and reduces spring cleanup.
7. Consider a Winter Chemical Kit
   Many manufacturers offer complete winter closing kits that include algaecide, oxidizer, and metal sequestrant. These kits are sized for different pool volumes and take the guesswork out of chemical selection. Popular options include the In The Swim Winterizing Kit or the Pool Mate Winter Chemical Kit.

Regional Considerations for Closing Chemicals

Cold Climate Pools (Hard Freeze Areas)

In regions where temperatures consistently drop below 32°F, you'll need more aggressive winterization:

• Use maximum dosage of winter algaecide
• Ensure complete plumbing protection with antifreeze
• Consider adding a metal sequestrant if you have hard water
• Lower water level below skimmer and blow out all lines

Mild Climate Pools (Occasional Freeze)

In warmer regions with occasional freezing, you can use lighter chemical treatments:

• Standard algaecide dosage may be sufficient
• Focus antifreeze on exposed plumbing only
• Maintain higher water levels if no freeze risk
• Consider keeping equipment running during cold snaps

Critical Safety and Application Tips

Never mix pool chemicals together. Add each chemical separately with the pump running, allowing 2-4 hours between different chemical additions. Always add chemicals to water, never water to chemicals, and wear appropriate safety equipment including goggles and gloves.

Test your water chemistry again 24-48 hours after adding closing chemicals to ensure everything is properly balanced. Make final adjustments before completely shutting down your circulation system.

What Happens If You Skip Closing Chemicals

Pools closed without proper chemicals often experience:

• Severe algae blooms that require extensive SLAMShock Level And Maintain — raise free chlorine to a target based on your CYA and hold it there until the algae is gone. It's a process, not a one-time dose. the SLAM walkthrough → treatment in spring
• Metal staining from unbalanced water chemistry
• Scale buildup on pool surfaces and equipment
• Freeze damage to inadequately protected plumbing
• Significantly higher opening costs and chemical requirements

The relatively small investment in closing chemicals saves hundreds of dollars in spring cleanup and potential equipment repairs. Proper winterization with the right chemicals ensures your pool opens clear, clean, and ready for another swimming season.

For the full breakdown of safe chlorine levels by CYA level, see our pool water chemistry guide.

Recommended for this job

Pool Safety Cover Hex Key Tool (2-Pack) Hex keys to install or remove a pool safety cover. View on Amazon → Pool Eyeball / Return Jet Fitting Removal Tool (ATIE) Pops out & reseats Hayward SP1419 return-jet eyeballs — handy for closing. View on Amazon → Taylor K-2006C Complete FAS-DPD Pool & Spa Test Kit The FAS-DPD kit pool pros trust — reads chlorine accurately even at shock/SLAM levels, plus pH, alkalinity, calcium hardness and CYA. View on Amazon → PolyQuat 60 Algaecide (Non-Foaming) Non-foaming, copper-free backup View on Amazon →

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