Pool closing · Colorado
When to Close Your Pool in Centennial, CO: Deadline, Window & Checklist
Last updated: July 15, 2026 · Model window year: 2026 · dates · checklist · FAQ
Circle September 30 on the Centennial calendar. Closing earlier traps warm, algae-friendly water under the cover; closing later gambles the plumbing against the first freeze, which the 1991–2020 normals place near October 8. The window opens September 20 — the live widget below shows how this year is actually tracking against it.
Centennial closing dates at a glance
| Closing window | September 20 – September 30 |
|---|---|
| Close by (deadline) | September 30 |
| First freeze, 50% probability | October 8 |
| Open by (recommended) | May 16 |
| Opening window | May 9 – May 30 |
| 61°F crossing (7-day mean) | May 30 |
| Swim-season length (80°F+ days) | 89 days |
| NOAA normals station | Denver Centennial Airport · 1.8 mi · 5883 ft |
Centennial banks only about 89 days of 80°F-plus afternoons — early opening weeks are the cheapest season extension available.
At roughly 5883 ft, Denver Centennial Airport runs cooler than lowland forecasts for the same region — trust these local numbers over metro-wide weather graphics.
Four water checkpoints anchor Centennial's year in the model: mid-April at about 46°F, mid-June at 65°F, mid-August near the 72°F peak zone, and mid-October easing through 53°F. The dates in the table are where those numbers cross the thresholds that matter.
The 12-step Centennial winterizing checklist
A closing is a plumbing project with a chemistry warm-up. Start a few days ahead of your target date, keep every dose per its product label, and don't skip the photographs — spring-you reassembles from them.
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Balance the water a few days ahead
Do the chemistry midweek, close on the weekend: alkalinity and pH into label ranges with days of circulation left to spread them. Winter locks in whatever state the water holds on closing day.
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Deep-clean the pool
Make the last cleaning the best one of the year: full skim, full brush, careful vacuum. Debris left behind steeps all winter and greets you as April's water problem.
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Service the filter one last time
Backwash sand or DE, or pull and rinse cartridges, per the manual. A filter stored dirty cakes over winter and starts spring half-clogged.
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Apply winter chemicals per label
Add a winterizing kit or your usual closing chemicals exactly as their labels direct for your volume, with the pump still circulating so everything distributes before shutdown.
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Lower the water level
Your cover's manual sets the number — commonly a few inches under the skimmer for solid covers, barely below normal for mesh. Stop there. The remaining water isn't laziness; it's ballast holding the shell in the ground.
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Blow out the lines and plug returns
Air through every line — skimmer, returns, cleaner — until each blows dry mist, plugging returns while the air still pushes. Nothing else on this list protects as much plumbing per minute.
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Protect the skimmer
Install a skimmer guard bottle (or a plug rated for your skimmer) so ice that forms there crushes the sacrificial piece instead of cracking the skimmer body.
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Add pool antifreeze where blowouts fall short
Doubt is the criterion: any run you can't confirm dry gets pool-grade antifreeze at the label's per-foot rate. The automotive jug from the garage is for cars — it has no business in pool plumbing.
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Drain the equipment
Nothing on the pad should hold water overnight: pull the drain plugs from pump, filter, heater, and feeder, stash them all in the pump basket, and carry the portable pieces indoors.
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Set the air pillow and cover
Center an inflated air pillow, then fit the cover and secure it with water bags, cable, or straps as designed. The pillow gives ice a place to push besides your walls.
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Stage the cover pump
Solid covers need drainage all winter: set a cover pump or siphon before the first storm, not after. Standing water strains seams and invites a mid-winter emergency.
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Note this year's dates
Jot down when Centennial's water actually cooled and when you closed. A two-line note beats memory next September 20 — your own yard runs a few days off any model, including ours.
What to buy before the rush
Every item below sells out somewhere in Colorado every September. Stocking the short list before the rush costs nothing extra and saves the mid-project store run — the chemicals guide explains what each category actually does.
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Winter closing kit
The under-the-cover chemistry, measured for your gallons.
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Air pillow
A soft target for the ice sheet, centered under the cover.
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Winter cover
The one purchase every other closing step depends on.
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Cover pump
Keeps rain and melt off a solid cover all season.
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Expansion plugs + skimmer guard
The blowout's finishing move — one per return, one for the skimmer.
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Pool antifreeze
For lines you can't verify dry — pool-grade only, per label.
How Centennial compares locally
Two nearby data points to triangulate with: Highlands Ranch, 7 miles from Centennial, models its close at September 30 (the same day); Aurora, 11 miles out, at October 1. Centennial's own window ends September 30. For the other half of the calendar see when to open in Centennial, or scan the full year on the season page.
Every number on this page traces to one instrument cluster: Denver Centennial Airport, 1.8 miles southeast of Centennial's center at an elevation near 5883 feet. NOAA computed its 1991–2020 normals from roughly three decades of daily readings — long enough that one strange spring in Arapahoe County barely moves the dates.
Field notes for Centennial owners
The warm spell after you closed
A 78°F week in October doesn't mean reopening. Water under an opaque cover warms far less than air suggests, and a closed, balanced pool tolerates a warm stretch fine. Check the cover pump has somewhere to send rain, enjoy the weather, and leave the plumbing sealed.
Salt cells overwinter indoors
Remove the cell at closing, inspect and clean per the manufacturer, and store it inside with the unions capped. A dummy cell or spacer keeps the plumbing sealed. Cells left in outdoor plumbing through freezes are a common — and completely avoidable — spring casualty.
Gas heaters get the manual, not a guess
Every heater brand sequences its winterizing differently — drain plugs, blower considerations, gas supply, control settings. The generic advice (drain fully, close the valve) is right but incomplete; ten minutes with your model's manual protects the most expensive component you'll winterize.
Closing for a real winter
A Centennial closing has to hold for months of freeze-thaw, not a few frosty mornings. Spend the effort where winters bite: prove every line dry, drain every vessel on the pad, guard the skimmer, and tension the cover for wind that will actually come. The reward is a spring opening that's a rinse, not a rebuild.
Altitude closing notes
Elevation compresses Centennial's closing window: at about 5883 ft, radiational cooling can drop a clear night below freezing while afternoons still feel like pool weather. Trust the first-freeze normal (October 8) over the vibe, stage the blowout gear early, and treat any clear-sky cold front in September as your cue.
Centennial pool closing FAQ
What temperature should water be to close a pool?
Below roughly 65°F, and trending down. Water closed warm keeps feeding algae under the cover for weeks; water closed in the 50s goes dormant almost immediately. Centennial's cool-down lands near September 20 in the normals, which is why the window opens there.
Can you close a pool too early?
You can, and warm-water closings are why "we opened to a swamp" stories exist. The fix is patience: let Centennial's water cool through the mid-60s°F — around September 20 by our model — then close inside the window that ends September 30.
Do I need antifreeze in pool lines?
It depends entirely on your confidence in the blowout. Lines that blew fully dry need nothing; anything uncertain — low runs, water features, a stubborn cleaner line — gets pool-grade antifreeze at the label's rate per foot. With Centennial's freeze clock starting near October 8, uncertainty is the thing to eliminate.
How far should I drain my pool for winter?
Follow the cover's instructions first: solid covers usually want water a few inches below the skimmer; some mesh setups run higher with the skimmer sealed. The hard rule is never empty — hydrostatic pressure can lift or crack an empty pool, a far worse outcome than any freeze.
What happens if you don't winterize a pool?
In a freeze climate, physics wins: water expands about 9% when it freezes, cracking pump housings, filter tanks, heat exchangers, and pipes. With Centennial's first 32°F night arriving near October 8 in the normals, an unwinterized pad is a spring repair bill waiting to be discovered.
When is the last safe date to close in Centennial?
September 30, by our model — the cool-down plus ten days (the freeze normal, October 8, leaves room to spare). Later closes happen, but they happen in gloves. The winterizing steps above take a weekend; leave yourself at least that much runway before Centennial's first freeze-risk stretch.
Email me when Centennial hits the closing window
Data: NOAA 1991–2020 climate normals via Denver Centennial Airport (1.8 mi); live outlook by Open-Meteo. Full attribution on the sources page. Model assumptions and error bars on methodology.