Pool closing · New Hampshire
When to Close Your Pool in Portsmouth, NH: Deadline, Window & Checklist
Last updated: July 15, 2026 · Model window year: 2026 · dates · checklist · FAQ
Circle September 27 on the Portsmouth 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 17 — the live widget below shows how this year is actually tracking against it.
Portsmouth closing dates at a glance
| Closing window | September 17 – September 27 |
|---|---|
| Close by (deadline) | September 27 |
| First freeze, 50% probability | October 8 |
| 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) | 39 days |
| NOAA normals station | Greenland · 3.4 mi · 95 ft |
A short season cuts both ways: every week opened before June 5 is a real slice of Portsmouth's roughly 39-day warm-swim budget.
Put dates aside and follow the water: the Portsmouth curve says roughly 43°F by mid-April, 63°F by mid-June, 70°F in mid-August, then back down through 53°F in mid-October. The summer ceiling sits near 70°F, and every window above is just a line drawn on this curve.
The 12-step Portsmouth winterizing checklist
Sequenced against Portsmouth's September 17–September 27 window: chemistry while the pump still runs, blowouts before the equipment drains, cover last. Labels dictate every dose.
-
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.
-
Deep-clean the pool
Leave nothing organic behind: skim the surface, brush every wall and step, vacuum the floor slowly. What goes under the cover dirty comes out worse — winter only ever compounds what it's given.
-
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.
-
Apply winter chemicals per label
Winter chemicals go in before shutdown, not after: label-dosed, circulated for a few hours, distributed evenly. A floater dropped on still water protects one corner.
-
Lower the water level
Drop the level as your cover manufacturer specifies — typically below the skimmer mouth for solid covers. Never drain a pool fully; groundwater pressure can damage the shell.
-
Blow out the lines and plug returns
The blowout is the whole ballgame: drive air through each line until it runs dry, seat the plug against the airflow, move to the next. A dry line cannot burst, full stop.
-
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.
-
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.
-
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.
-
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.
-
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.
-
Store chemicals properly
Seal opened containers, keep oxidizers and acids separated, and store everything cool, dry, and locked away from kids and pets — exactly as each label describes.
What to buy before the rush
The September crowd empties shelves in a predictable order. This is the short list worth owning before Portsmouth's window opens — nothing exotic, just the stuff everyone needs the same weekend.
-
Pool antifreeze
Pool-rated glycol for the lines air couldn't clear.
-
Winter closing kit
Closing chemistry in one box, dosed by pool volume.
-
Air pillow
Centers under the cover so ice pushes inward, not outward.
-
Winter cover
The one purchase every other closing step depends on.
-
Cover pump
Solid covers collect rain all winter; this is the drainage plan.
-
Expansion plugs + skimmer guard
Cheap rubber that stands between ice and your fittings.
How Portsmouth compares locally
Portsmouth closes in the earlier half of New Hampshire's calendar. Neighbors run close: Dover (10 mi away) models its deadline at September 25 (2 days earlier vs Portsmouth's September 27), while Lawrence (31 mi) shows October 6. The spring mirror of this page is the Portsmouth opening guide, and the season overview draws both windows on a single bar.
The instrument behind this page is Greenland, 3.4 miles southwest of Portsmouth — the closest station publishing complete 1991–2020 daily temperature normals. Thirty years of its readings set every date above; your own yard adds or subtracts a degree with shade, wind, and pavement, which is what the window's width is for.
Field notes for Portsmouth owners
Leaf season vs closing day
If your yard drops serious leaves, the cheap trick is a leaf net over the main cover through the drop, then one bulk removal before snow. Leaves that winter on (or worse, under) the cover steep like tea and hand you stained water and clogged pumps in spring.
Cover pumps die in the cold — plan for it
A cover pump left running into a hard freeze can lock in ice and burn out. On freezing forecasts, pull it, let the storm pass, and put it back for the melt. Automatic models with freeze protection earn their price in exactly one forgotten weekend.
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 Portsmouth 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.
Portsmouth pool closing FAQ
What temperature should water be to close a pool?
Close once water holds below about 65°F — the point where algae go mostly dormant — and before hard freezes. In Portsmouth, the 7-day mean drops through the threshold around September 17, so the window between then and September 27 is the sweet spot for a clean, stable close.
Can you close a pool too early?
Absolutely. A pool closed warm is an algae incubator: no circulation, fading sanitizer, and weeks of growth-friendly temperatures. Portsmouth's water doesn't settle below the risk zone until around September 17 — closing much before that trades a few weekends of maintenance for a rough spring.
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 Portsmouth's freeze clock starting near October 8, uncertainty is the thing to eliminate.
How far should I drain my pool for winter?
Only to the line your cover manufacturer prints — a few inches below the skimmer for most solid covers, close to operating level for many mesh designs with the skimmer plugged. The water you leave in is structural: it holds the shell against groundwater all winter.
What happens if you don't winterize a pool?
Expect equipment damage first — cracked pump volutes, split filter tanks, ruptured heater exchangers — then plumbing leaks that surface at startup. In Portsmouth, normals put the first freeze near October 8; everything after that date is borrowed time for an unwinterized system.
When is the last safe date to close in Portsmouth?
The model draws the line at September 27 for Portsmouth. It isn't arbitrary: the cool-down plus ten days (the freeze normal, October 8, leaves room to spare), and the whole closing sequence needs a working weekend of margin. The one exception that overrides any date — a hard freeze inside the 10-day forecast, which the widget above flags as urgent.
Email me when Portsmouth hits the closing window
Data: NOAA 1991–2020 climate normals via Greenland (3.4 mi); live outlook by Open-Meteo. Full attribution on the sources page. Model assumptions and error bars on methodology.