Pool opening · New Hampshire
When to Open Your Pool in Dover, NH: Best Dates & Checklist
Last updated: July 15, 2026 · Model window year: 2026 · dates · checklist · FAQ
Plan to open your pool in Dover by May 22. Our model of NOAA 1991–2020 normals puts the local 7-day mean temperature at the algae-growth threshold around June 5 — and pool stores hit their May rush weeks later. Below: today's estimated water temperature, the full opening window, and a step-by-step checklist with what to buy before shelves empty.
Dover opening dates at a glance
| Open by (recommended) | May 22 |
|---|---|
| Opening window | May 15 – June 5 |
| 61°F crossing (7-day mean) | June 5 |
| Closing window | September 15 – September 25 |
| Close by (deadline) | September 25 |
| First freeze, 50% probability | October 10 |
| 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 opening checklist
Built for Dover's window: physical teardown first, a full day of circulation, then chemistry per each product's label. Nothing here requires a pro, but step 1 goes easier with a second pair of hands.
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Pump off and clear the winter cover
Start with the cover: pump the puddles off, sweep the leaves, and fold it back in sections so nothing slides into the water. Everything the cover caught all winter stays out of your chemistry budget.
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Top up the water level
Bring the level up to the middle of the skimmer opening before anything runs. Too low and the pump gulps air; too high and the skimmer door stops doing its job.
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Remove winter plugs and reinstall fittings
Pull expansion plugs and the skimmer guard, then refit return eyeballs, baskets, and ladders. Check each gasket as you go; a cracked one now is a mystery air leak later.
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Reassemble the equipment pad
Reinstall drain plugs on the pump, filter, and heater; lube o-rings with the manufacturer-recommended lubricant; reconnect unions hand-tight plus a quarter turn.
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Prime the pump and run for 24 hours
Pour water into the pump housing, crack the filter's air relief, and fire it up. Give the system a continuous day of runtime before you draw any conclusions about the water.
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Service the filter
Whatever the media — cartridge, sand, or DE — start the season with it clean, following the manual's procedure. A half-clogged filter turns a two-day clearing into a week.
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Brush, skim, and vacuum
Physical dirt leaves physically: brush every wall and step, skim the film, vacuum the bottom. Each scoop of debris removed is sanitizer you don't have to buy.
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Test the water
Test pH, total alkalinity, calcium hardness, stabilizer, and chlorine with fresh strips or a kit — spring readings drift over winter, and everything downstream depends on this baseline.
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Balance, then shock — per product labels
Balance in order (alkalinity, then pH, then the rest), with the label on each container as the only dosing chart. Finish with a startup shock, applied and timed as its label directs.
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Filter until the water clears
The last step is patience: filter, test, repeat until you can read a quarter on the bottom and your readings hold steady in the label ranges two days running.
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Inspect for winter damage
Walk the deck, coping, and tile line looking for new cracks, and watch the pad for drips during the first day of runtime. Catching a weep in May 15 beats a leak hunt in June.
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Photograph the pad and plumb lines
Take phone photos of valve positions, plumbing runs, and the equipment pad while everything is fresh. Fall-you, holding a blowout adapter, will be grateful for the reference set.
What to buy before the rush
The June 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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Robotic pool cleaner
Hands-off floor and wall cleaning while you do the chemistry.
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Pool opening chemical kit
One box covers balancing and startup for most residential volumes.
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7-way test strips
The first thing to run and the last thing to skimp on.
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Start-up shock
The opening oxidizer; dose by the label for your volume.
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Filter cartridge / DE refill
Start the season on fresh media; dirty filters stretch cloudy days.
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Leaf net + wall brush
The debris you remove by hand is chemistry you keep.
How Dover compares locally
Before booking a service slot, compare Dover against its neighbors: Portsmouth (10 mi) models to May 22, Manchester (32 mi) to May 18, against Dover's own May 22 — placing it in the earlier half statewide at the 50th percentile. When autumn planning starts, the closing checklist picks up where this page ends, and the Dover pool season page holds the one-glance summary.
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
Stabilizer: the sunscreen your chlorine needs
Spring sun destroys unstabilized chlorine within hours, which reads as "the pool eats chlorine" when it's really UV. Test cyanuric acid at opening — winter rain and splash-out dilute it — and restore it per the product label before judging your sanitizer consumption.
Cartridge, sand, or DE — the opening difference
Cartridges want a hose-down (or replacement if pleats are fraying); sand wants a long backwash and a check that the bed hasn't channeled; DE wants a backwash plus a fresh label-measured coat. Whichever you run, start the season clean — a filter opened dirty turns the clearing phase from days into a week.
Salt pools: check the cell before the season leans on it
Opening is the natural moment to inspect a salt cell: scale on the plates, connections, and the salinity reading after fresh spring water. Follow the manufacturer's cleaning guidance exactly — over-acid-washing a cell shortens its life more than the scale did. The salt-water opening notes cover the cold-water handoff too.
Making a 41-day season feel longer
The normals give Dover roughly 41 true warm-swim days, so the margins are the strategy: an on-time opening adds usable cool-water weeks up front, a solar cover adds degrees at both ends, and a heater turns the shoulder months from theoretical to Tuesday-night real.
Dover pool opening FAQ
What water temperature causes pool algae?
There's no single magic number, but the practical range is 65–70°F: below it algae barely tick over, above it they bloom, especially in the still, dark water under a cover. Dover reaches that band in the weeks after June 5, which is why the recommended opening lands May 22.
What temperature should it be outside to open a pool?
The industry rule of thumb says open when daytime highs sit consistently around 70°F — before the water itself reaches 65–70°F. We track it more precisely: when the 7-day mean of daily highs and lows crosses 61°F, unheated water is on approach. In Dover that crossing is about June 5, so working back two weeks gives May 22.
Is it cheaper to open a pool early or late?
Early, almost every time. Cold water suppresses algae, so an early opening usually needs only baseline balancing and a label-dosed startup shock. A late opening into 65°F-plus water risks a green start: repeated shocking, clarifier, extra filter runtime, and sometimes a service call — far more than the few extra weeks of pump electricity.
How long after opening can you swim?
Swim when three things line up: the water has gone visually clear, your test kit shows levels holding in label ranges, and the interval printed on any shock product's label has passed. Cold-water openings near May 22 typically get there fastest because there's less to correct.
What chemicals do I need to open a pool?
A test kit or strips, alkalinity and pH adjusters, calcium hardness increaser if your water runs soft, stabilizer (cyanuric acid), your regular sanitizer, and a startup shock. Buy before Dover's rush around June 5, and dose everything strictly by each product's label for your pool volume — category-by-category buying notes live in the opening chemicals guide.
When do most people open pools in NH?
Habit says May: the first warm weekends and Memorial Day carry most of the country's openings, and the whole supply chain groans under them at once. The New Hampshire climate itself asks for May 20 (median across our 4 covered cities) — and Dover specifically for May 22. Being the early neighbor is purely an advantage.
Email me when Dover hits the opening 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.