Every opening guide runs the same steps in roughly the same order, and they're fine. Where they go wrong is the two decisions that decide whether the week is easy: when — they all say wait for warm air, which is backwards — and what the first test means, which they skip entirely in favor of "add shock." A pool that opens clear and holds chlorine is done in a day. The one that doesn't has a specific reason, it's usually the same reason, and it has a number.
Take the sheet outside
Prints as a one-page opening checklist — the order with tick boxes, and the numbers to open at underneath. No email, no signup.
When should you open your pool?
Before the water reaches 60 °F — not after the air does. Algae doesn't read the forecast; it wakes up when the water warms, and it starts growing under the cover, in the dark, weeks before anyone would swim. A pool opened at 55 °F water is a pool that spent the winter cold and gets chlorine before anything can grow. A pool opened at 72 °F air, as the top-ranking guides suggest, has typically had two or three weeks of 60-something-degree water under a cover with no chlorine — and that's the green opening that needs a week of shock. What opening early costs is a few weeks of running a pump in cold water, and it isn't much:
| Open 3 weeks early | Open warm, into a bloom | |
|---|---|---|
| Pump | 4 h/day × 3 weeks — $27 | 24 h/day × 5 days — $39 |
| Chlorine | ~1 ppm twice a week — $4 | Reach 16 ppm and hold it 5 days — 7.9 gal, $48 |
| Total | $31 | $86 and a week you can't swim |
15,000 gal, CYA 40, a 1.5 HP single-speed pump at $0.17/kWh, 12.5% liquid chlorine at $6 a gallon; a green pool at shock level loses about 10 ppm a day while it clears. Cold water below 60 °F has almost no chlorine demand, which is why the early column is so small.
The closing guide makes the same argument in the other direction — close late, open early, and the pool never has warm water without chlorine in it.
The opening checklist
| Stage | Do | The number | ✓ |
|---|---|---|---|
| Before the cover | Pump the water off the cover; clear leaves so nothing falls in | ||
| Fold the cover off, hose it, dry it, store it dry (a bin, not the lawn) | |||
| Equipment | Plugs out of returns and skimmer; gizmo out; skimmer basket in | ||
| Drain plugs back in (they’re in the pump basket); reconnect pump, filter, heater, cell | |||
| Fill to mid-skimmer. Prime the pump, open the air relief, run, walk the pad for leaks | water ½ up the skimmer | ||
| Run continuously; skim, brush, vacuum (to waste if there’s a layer) | 24 h before testing | ||
| First test | Full test: FC, CC, pH, TA, CH, CYA (salt). Write them down | ||
| CYA reads 0? Chlorine demand test before anything else | 10 ppm; holds 30 min? | ||
| Chlorine to target for your CYA; retest in the evening and again next morning | FC = 11.5% of CYA | ||
| Green or won’t hold chlorine → SLAM (green pool guide). Cloudy but holding → cloudy guide | shock = 40% of CYA | ||
| Once it holds | pH, then alkalinity; calcium on plaster; stabilizer only now, in a sock | pH 7.2–7.8 | |
| Salt pools: cell back in, salt to range, cell on last | |||
| Day 2–3 | Clean the filter when pressure climbs; note the clean reading | ||
| Overnight chlorine loss test passes → open. Back to the weekly schedule | ≤ 1 ppm lost |
| Open when | Chlorine target | Shock level | CYA reads 0 |
|---|---|---|---|
| Water under 60 °F | 11.5% of CYA — 4.5 ppm at CYA 40; never under 2 | 40% of CYA — 16 ppm at CYA 40 | Demand test first: 10 ppm, 30 min. Holds → restabilize. Gone → ammonia, keep chlorinating, no CYA yet |
Balance order once the water holds chlorine: pH, alkalinity, calcium, then stabilizer. Doses for your gallons at pooltoolbox.com/dashboard.
Test first, then add — not the other way around
The most-read opening guide says shock first and balance afterward; the next one says balance first and shock last. Both are guessing, because neither has looked at the water. The order that works starts with a full test after 24 hours of circulation, then follows what the test says — and for most pools that means chlorine before anything else, because a pool that's been unsanitized for months needs sanitizing more than it needs its alkalinity at 70. Fix pH only if it's below 7.2 or above 7.8 (chlorine works fine across that band), get chlorine to the target for your CYA, let it circulate, and retest. Stabilizer, calcium and salt wait until the water is holding chlorine, for the reason in the next section. The dashboard turns the first test into doses in the right sequence, so the order isn't something you have to remember.
| Reading | It means | Go to |
|---|---|---|
| Clear, FC 0, CYA about what you closed at | The normal case — chlorine drifted to nothing in the cold | Chlorine dose to target, then balance |
| Clear, FC 0, CYA reads 0 | Either CYA is below the test's floor or it turned to ammonia | The demand test, below |
| Chlorine vanishes within an hour of adding it | Ammonia or algae is consuming it on contact | Keep chlorinating — the table below says how much |
| Green, or slippery walls | Algae established under the cover | SLAM — shock level from your CYA |
| Cloudy but chlorine holds overnight | Filter or precipitate, not sanitizer | Cloudy water: which cause |
| CH well down (rain under a mesh cover) | Diluted; matters on plaster only | Calcium dose |
| Salt down | Same dilution; the cell won't run until it's back | Salt dose |
Why the chlorine won't hold: stabilizer turned into ammonia
This is the failed opening nobody explains. A pool sits under a cover for months with no chlorine, and a particular bacterium eats the cyanuric acid, leaving ammonia behind. Come spring, the CYA test reads zero, the water may even look fine — and every dose of chlorine disappears within the hour, because ammonia consumes chlorine before anything else can. Owners add shock, add more shock, buy the double bag, and conclude the pool is cursed. It isn't; it's arithmetic. Burning ammonia takes about 7.6 ppm of chlorine per ppm of ammonia-nitrogen, closer to 10 in a real pool, and each ppm of CYA that vanished left 0.33 ppm of it:
| CYA lost over winter | Ammonia left behind | Chlorine to burn it | 12.5% liquid, 15,000 gal |
|---|---|---|---|
| 10 ppm | 3.3 ppm N | 25–33 ppm | 3–3.9 gal |
| 20 ppm | 6.5 ppm N | 49–65 ppm | 5.9–7.8 gal |
| 30 ppm | 9.8 ppm N | 74–98 ppm | 8.9–11.7 gal |
| 40 ppm | 13 ppm N | 99–130 ppm | 11.9–15.6 gal |
Stoichiometric breakpoint (1.5 Cl₂ per NH₃) to the practical 10:1 that side reactions push real pools toward. Nothing removes the ammonia except chlorine — or water.
So a pool that closed at CYA 40 and opens at zero may be carrying 99–130 ppm of chlorine demand — 12–16 gallons of liquid chlorine on 15,000 gallons, poured in over a couple of days in large doses until a dose finally holds. It's a lot, and at a few dollars a jug it's still far cheaper than the drain it would otherwise take. The one thing not to do is add stabilizer first: the bacteria are still in the water, and fresh CYA is fresh food. Chlorine until it holds, then restabilize. And if the demand is beyond what you'll pour, a partial drain removes ammonia in proportion — the dilution calculator covers the risks.
If CYA reads zero at opening, don't dose to target — dose a probe. Add enough liquid chlorine to raise FC by 10 ppm (the chlorine calculator gives the amount for your gallons), run the pump, and test again in thirty minutes. If most of it is still there, the water is fine: the CYA was simply below the test's floor, and you can restabilize. If it's gone, you've found the ammonia, and you know the shape of the next two days before you've bought a single bag of shock. Ten ppm is more than a normal dose on purpose — a small probe can vanish into ordinary demand and tell you nothing.
Refilling, and how long it takes
The water goes back to the middle of the skimmer mouth before the pump starts — below that, the skimmer gulps air and the pump runs dry. How much depends on how the pool was closed, and under a mesh cover a wet winter may have done it for you:
| How it was closed | Water to add, 15 × 30 ft pool | Hose at 8 GPM | Cost |
|---|---|---|---|
| Solid cover (closed 4 in down) | 1,122 gal | 2.3 h | $12.34 |
| Mesh cover (closed 15 in down) | 4,207 gal | 8.8 h | $46.28 |
450 ft² of surface; water at $11 per 1,000 gal including sewer. The cost to fill calculator prices it at your rates and flags the sewer credit. Fresh water brings no stabilizer and, usually, some calcium and alkalinity — another reason the first test comes after the fill, not before.
Is an opening kit worth it?
The kits hold a shock, an algaecide, a clarifier and an "oil absorber" or enzyme, and sometimes a stain-and-scale bottle. A clear pool that holds chlorine needs one thing from that list — chlorine — and the bag of cal-hypo in the kit is a fixed scoop that has no idea what your CYA is. A pool that doesn't hold chlorine needs, from the table above, five to twelve gallons of liquid chlorine, which no kit contains. Buy liquid chlorine by the case in spring, and put the kit money into a proper test kit if you don't own one — the FAS-DPD drop test is what makes "does it hold?" a measurement instead of a guess. Algaecide is insurance against low chlorine, clarifier treats a filter problem, and an enzyme digests oils that chlorine already oxidizes; none of them is wrong, and none of them opens the pool.
Above-ground pools, salt pools, and the first week
An above-ground pool opens the same way with less plumbing: hoses back on the wall fittings, pump and filter out of the garage and reconnected with the drain plugs in, air pillow out, and the level up to mid-skimmer. The cover comes off wet and heavy; two people and a dry driveway beat one person and a hedge. A salt pool goes back in its normal order except for the cell: reinstall it, but leave it off until the water is balanced, the salt is in range and the water is above about 60 °F — most cells won't generate in cold water anyway, so the first days of chlorine are liquid regardless. The salt calculator sizes the top-up from the reading.
Then the first week is just the weekly schedule run a little more often: chlorine and pH daily for a few days while the water finds its level, the filter cleaned once the winter's fines have loaded it, and the overnight chlorine loss test the morning you'd like to call it open. Lose a ppm or less overnight and it is.