Pool closing · New Hampshire
When to Close Your Pool in Dover, NH: Deadline, Window & Checklist
Last updated: July 15, 2026 · Model window year: 2026 · dates · checklist · FAQ
Plan to close your Dover pool by September 25. The local 7-day mean temperature drops out of the algae-risk zone around September 15, and NOAA normals put the first 32°F freeze near October 10 — winterize between those dates and the water goes under the cover cold, clean, and easy to reopen. Below: today's water estimate, the full closing window, and a step-by-step winterizing checklist.
Dover closing dates at a glance
| Closing window | September 15 – September 25 |
|---|---|
| Close by (deadline) | September 25 |
| First freeze, 50% probability | October 10 |
| Open by (recommended) | May 22 |
| Opening window | May 15 – June 5 |
| 61°F crossing (7-day mean) | June 5 |
| Swim-season length (80°F+ days) | 41 days |
| NOAA normals station | Durham 2 N · 2.5 mi · 119 ft |
A short season cuts both ways: every week opened before June 5 is a real slice of Dover's roughly 41-day warm-swim budget.
Four water checkpoints anchor Dover's year in the model: mid-April at about 42°F, mid-June at 62°F, mid-August near the 69°F peak zone, and mid-October easing through 52°F. The dates in the table are where those numbers cross the thresholds that matter.
The 12-step Dover 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
Give the chemistry a head start — balance to label ranges several days out, while circulation can still mix corrections evenly. Closing-day dosing never distributes as well.
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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
Take the level down only as far as the cover's manual says — usually just below the skimmer for solid covers, higher for many mesh systems. An empty pool is never the goal; shells crack and shift without water's weight.
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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
Give the skimmer something cheap to break: a guard bottle or rated plug absorbs the ice expansion that would otherwise split the housing.
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Add pool antifreeze where blowouts fall short
If any line can't be verified dry, add pool-grade antifreeze per its label. Use only pool antifreeze — automotive products don't belong in pool plumbing.
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Drain the equipment
Open the drains on everything that holds water and let the pad empty completely. Cartridges and small equipment overwinter far better on a garage shelf than outside.
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Set the air pillow and cover
Pillow first, cover second, tension last. A properly centered pillow turns the winter ice sheet from a wall-pressing ram into a harmless dome.
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Remove and store ladders and rails
Pull ladders, rails, and the diving-board hardware; rinse, dry, and store them out of the weather. Anchor sockets get a dab of protectant so spring bolts turn freely.
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Calendar the off-season checks
Set a monthly reminder from September 25 to spring: check cover tension, pump off standing water, and glance at the water level. Fifteen minutes a month protects the whole closing job.
What to buy before the rush
The September crowd empties shelves in a predictable order. This is the short list worth owning before Dover's window opens — nothing exotic, just the stuff everyone needs the same weekend.
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Pool antifreeze
Backup for unverifiable lines; label rate per foot of pipe.
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Winter closing kit
Season-length winter chemicals in one label-dosed box.
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Air pillow
A soft target for the ice sheet, centered under the cover.
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Winter cover
Measure with overlap; the winter workhorse over everything else.
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Cover pump
Solid covers collect rain all winter; this is the drainage plan.
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Expansion plugs + skimmer guard
The blowout's finishing move — one per return, one for the skimmer.
How Dover compares locally
Two nearby data points to triangulate with: Portsmouth, 10 miles from Dover, models its close at September 27 (2 days later); Manchester, 32 miles out, at September 29. Dover's own window ends September 25. For the other half of the calendar see when to open in Dover, or scan the full year on the season page.
The measuring stick here is Durham 2 N — 2.5 miles to the southwest, elevation about 119 feet. Its 1991–2020 record is what the model reads for Dover; your backyard in Strafford County will run a touch warmer or cooler with shade, wind, and pavement, which is exactly the slack the two-week lead absorbs.
Field notes for Dover owners
What comes indoors
Cartridges, the cover pump when idle, chemical containers, and anything with a small motor overwinter better in the garage. Cold cycling is hard on plastics and seals; shelf space is cheaper than replacements. Label a single bin now and spring assembly becomes a scavenger hunt with a map.
Blowout first, antifreeze second
Air is the only guaranteed freeze protection: a line that's verifiably dry cannot burst. Antifreeze (pool-grade, per label) is the fallback for lines you can't confirm — long runs, low spots, water-feature plumbing. Doing antifreeze instead of a blowout, rather than in addition, is how most cracked returns happen.
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 Dover 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.
Dover pool closing FAQ
What temperature should water be to close a pool?
The practical target is water in the low 60s°F or below at closing day. Our Dover model has the sustained cool-down starting September 15; closing between then and September 25 means chemistry stays put and the spring reveal is a mild one.
Can you close a pool too early?
Yes — it's the most common closing mistake. Seal 70°F water under a cover and algae keep growing in the dark all autumn; the spring opening turns green and expensive. In Dover, hold off until the cool-down near September 15 before covering.
Do I need antifreeze in pool lines?
Treat antifreeze as a backup, not a substitute: the real protection is air in dry lines. Where a full blowout isn't possible, pool-grade antifreeze per label is cheap insurance against a cracked pipe — worth it anywhere freezes are routine, and Dover sees them from about October 10.
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?
Two failure modes. Where freezes reach the plumbing, expansion cracks pumps, filters, and fittings from the inside. Where they don't, an unwatched pool simply drifts green and unbalanced by spring. Dover has no published freeze normal to pin the date, so the winterizing above plus forecast-watching covers both risks.
When is the last safe date to close in Dover?
Treat September 25 as the deadline in Dover. It's derived from the 1991–2020 normals: the cool-down plus ten days (the freeze normal, October 10, leaves room to spare). Weather varies year to year, so watch the 10-day forecast in late September — an early cold snap moves the real deadline, and the widget above flags exactly that.
Email me when Dover hits the closing window
Data: NOAA 1991–2020 climate normals via Durham 2 N (2.5 mi); live outlook by Open-Meteo. Full attribution on the sources page. Model assumptions and error bars on methodology.