When to Close
Timing your pool closing correctly is the first decision — and one most pool owners get wrong by closing too early.
The 60°F Rule
Close when water temperature drops consistently below 60°F. Below this threshold, algae cannot grow. Closing with warm water (above 65°F) traps algae-friendly conditions under the cover all winter, and you will open to a green pool no matter how many chemicals you added at closing.
In most of the northern US, this means waiting until late October or November. Check a pool thermometer, not just the air temperature — water retains heat longer than air.
Step 1 — Balance Chemistry Before Closing
Your closing chemistry is the chemistry that will protect your pool surfaces for the next 4–6 months. Unbalanced water sitting under a cover all winter etches plaster, stains vinyl, or builds up scale on every surface it touches.
| Parameter | Target Range | Why It Matters |
|---|---|---|
| pH | 7.4–7.6 | Low pH etches plaster; high pH causes scale and dulls surfaces |
| Total Alkalinity | 100–120 ppm | Buffers pH from swinging during the off-season |
| Calcium Hardness | 200–400 ppm | Low CH causes plaster leaching; high CH causes scale deposits |
Adjust all three parameters and run the pump for several hours to fully circulate before proceeding. Do not skip this step — unbalanced water does real damage over a long winter.
Step 2 — Shock the Pool
Shocking before closing eliminates bacteria, oxidizes organic matter, and ensures you're sealing the pool with a strong sanitizer baseline.
Double-Dose Shock Protocol
Use 2 lbs of cal-hypo per 10,000 gallons. Do this 2–3 days before your closing date — not the day of closing. You need to let FC drop back down to 1–3 ppm before adding algaecide, because high free chlorine destroys algaecide on contact, wasting the product entirely.
Step 3 — Add Winter Algaecide
This is the product that holds algae off all winter long. Don't skip it. A single application of the right algaecide is far cheaper than opening to green water and spending hours and chemicals remediating an algae bloom.
Polyquat 60 — The Right Product
Use long-life polyquat 60 algaecide. Dose: 1 quart per 10,000–20,000 gallons. Polyquat is non-foaming, non-metallic (won't stain), compatible with all sanitizers, and long-lasting. Avoid copper-based algaecides for winterizing — they can stain surfaces over a long off-season.
Add after FC has dropped to normal range. Pour around the perimeter of the pool with the pump running to distribute evenly. Run the pump for 1–2 more hours after adding.
Step 4 — Add Metal Sequestrant
Metals — primarily iron and manganese — oxidize when chlorine is added in spring and stain pool surfaces brown, green, or black. A metal sequestrant binds these metals in solution so they can be filtered out instead of depositing on surfaces.
- Especially critical for well water users — well water often contains elevated iron and manganese.
- Also recommended for pools with older copper plumbing or equipment.
- Add the manufacturer-recommended dose and run the filter for several hours after adding.
- Some metal sequestrants need to be reapplied mid-winter for long closings.
Step 5 — Lower Water Level and Blow Lines
Frozen water expands. Any water left in pipes, equipment, or fittings above the frost line will freeze, expand, and crack. This is one of the most expensive winterizing mistakes to fix.
Lower the Water Level
For inground pools: lower water 4–6 inches below the skimmer mouth. This prevents ice from forming inside the skimmer (which cracks it). For above ground pools: lower below the return jets.
Blow Out the Lines
Use a shop vac or air compressor to blow all water out of return lines, skimmer lines, and main drain lines. Plug each line from inside the pool after blowing. Leave no standing water in any line that runs through the ground or above frost depth.
Add Pool Antifreeze
Add pool-grade propylene glycol antifreeze to skimmer lines and any lines that could not be fully blown out. Use only pool/RV antifreeze — never automotive antifreeze.
Step 6 — Cover
The cover is your last line of defense against debris, UV, and temperature swings all winter. Choosing the right type and installing it correctly matters.
Safety Covers (Recommended)
Anchored to the deck with straps and springs. Keeps debris out, allows rainwater to drain through the mesh, and provides a safety barrier. Higher upfront cost but far better protection. Lasts 10–15 years with proper care.
Winter Tarp Covers (Budget Option)
Lower cost but collect water and debris on top, which can stress the cover and allow contamination if the cover sags into the water. Require water bags or anchors around the perimeter. Need to be checked and maintained throughout winter.
Regardless of cover type, ensure it is properly secured before winter storms. A loose cover that blows off in November undoes all your chemical work and leaves your pool exposed all winter.
Winterizing Chemicals — Quick Reference Checklist
| Step | Action / Chemical | Key Detail |
|---|---|---|
| 1 | Balance pH, TA, CH | pH 7.4–7.6 · TA 100–120 · CH 200–400 |
| 2 | Shock (cal-hypo 2 lbs/10K gal) | 2–3 days before closing; let FC drop before algaecide |
| 3 | Polyquat 60 algaecide | 1 qt per 10–20K gal; add after FC normalizes |
| 4 | Metal sequestrant | Full dose; critical for well water |
| 5a | Lower water level | 4–6 inches below skimmer (inground) |
| 5b | Blow out lines | All returns, skimmer, main drain |
| 5c | Propylene glycol antifreeze | Pool-grade only — never automotive |
| 6 | Install and secure cover | Safety cover preferred; check anchors |
Not sure what your pool needs before closing?
PoolDiag AI checks your chemistry and tells you exactly what to add — amounts, order, and timing. Free.
Try PoolDiag Free →Frequently Asked Questions
Do I need to drain my pool for winter?
No — most inground pools should not be fully drained. Hydrostatic pressure from groundwater surrounding the shell can push an empty pool up or crack it. Just lower the water level 4–6 inches below the skimmer. Above ground pools are different — those can and sometimes should be fully drained depending on climate and construction.
When should I close my pool?
When water temperature is consistently below 60°F — typically October–November in most of the US. Don't close based on air temperature alone; water retains heat and may still be above 60°F when nights feel cold. Closing too early with warm water guarantees an algae problem by spring.
Can I use regular antifreeze in pool lines?
Never. Only use propylene glycol pool antifreeze. Automotive antifreeze (ethylene glycol) is highly toxic to pets, wildlife, and humans — it has no place in a pool system. Look for products labeled specifically for pools or RVs, which use safe propylene glycol formulations.
Do above ground pools need winterizing?
Yes, if temperatures in your area drop below freezing. Drain the water level below return jets, blow out lines if possible, and add antifreeze to any lines that weren't fully cleared. Some above ground pools are fully drained and the liner and structure stored — check your manufacturer's specific recommendations.
How long does winter algaecide last?
Polyquat 60 algaecide typically remains effective for 90+ days. For pools closed 5 or more months — common in the northern US — consider adding a second dose through a small hole in the cover around January or February to maintain protection through the long off-season.
Related Guides
- Complete Pool Closing Checklist — full winterization process beyond just chemicals
- Pool Shock Guide — shock types, doses, and timing explained
- Pool Algae Treatment Guide — if you open to green water next spring