Every closing guide on the first page of Google gives the same steps and the same numbers: balance to pH 7.2–7.6, shock with a pound per 10,000 gallons, algaecide, drop the water, blow the lines, cover. The steps are right. The numbers are summer numbers. A pound of shock is a third of a shock for a pool that ran on tablets all season, and water balanced for 85 °F is dissolving plaster at 40 °F under the cover. This guide keeps the steps, fixes the numbers, and prints them on one sheet.
Take the sheet outside
Prints as a one-page closing checklist — the week's order with tick boxes, and the chlorine and water-level numbers underneath. No email, no signup.
When should you close your pool?
When the water is consistently below about 60 °F — not when the calendar says, and not when the air is. Algae is a temperature story: below 60 °F it barely grows, so a pool closed cold with good chlorine opens clear, while a pool closed warm in early September to get it over with spends six weeks growing under a cover with no circulation and no way to test. Later is better, right up until the first hard freeze. If you can't wait — you're leaving, the service company has a date — then close with the chlorine high and accept that spring may need a SLAM.
The other end of the calendar is the same rule in reverse: open when the water is still below 60 °F, before anything has had the chance to grow. The two decisions bracket the season, and both err toward cold.
The closing checklist
| When | Do | The number | ✓ |
|---|---|---|---|
| A week out | Test everything; fix TA and pH first, then CH — aim high side of range | pH 7.8, CH 350+ (plaster) | |
| Read CYA and note it — it sets the chlorine number below | CYA ___ ppm | ||
| 3–4 days out | Brush, vacuum, empty baskets, skim — water should be clear | ||
| Raise FC to shock level with liquid chlorine; hold until it holds overnight | FC = 40% of CYA | ||
| 1–2 days out | Let FC drift to 20% of CYA, then add polyquat 60 (optional); pump 24 h | FC = 20% of CYA | |
| Closing day | Clean the filter: backwash / hose cartridges / recharge DE. Leave the valve open | ||
| Lower the water below the skimmer — by cover type | solid 3–4 in · mesh 12–18 in | ||
| Blow out each line until it spits air; plug returns and skimmer; gizmo in the skimmer | shop vac / blower, not a compressor | ||
| Drain pump, filter, heater, chlorinator, cell — plugs out, store them in the pump basket | |||
| Antifreeze only in lines you couldn’t clear; never in the pump | |||
| Remove ladders, lights on cords, skimmer baskets, cleaner; store the cell indoors | |||
| No trichlor floater under the cover. A closed pool gets chlorine, not tablets | |||
| Cover: solid — pump on top; mesh — straps tight, no sag into the water | |||
| Monthly | Lift a corner: clear water, no smell. Pump the cover; top up under a mesh if the level drops |
| Your CYA | 30 ppm | 40 ppm | 50 ppm | 70 ppm |
|---|---|---|---|---|
| Close at (shock level) | 12 ppm | 16 ppm | 20 ppm | 28 ppm |
| Drift to before polyquat | 6 ppm | 8 ppm | 10 ppm | 14 ppm |
| 12.5% liquid chlorine, 15k gal from FC 4 | 1 gal | 1.4 gal | 1.9 gal | 2.9 gal |
Close when the water is under 60 °F. Shock level is 40% of CYA; scale the chlorine to your gallons. Balance first: pH 7.8, TA 70–80, calcium 350+ on plaster — cold water drops the saturation index.
The chlorine level to close at
More than the kit holds, and it depends on a number the kit doesn't ask for. "Shock" isn't a product, it's a chlorine level — high enough to kill everything in the water, which means high relative to the stabilizer that's holding the chlorine back. That level is 40% of your CYA. A pound of 65% cal-hypo — the standard kit scoop — raises 10,000 gallons by 7.8 ppm. On a pool that ran on liquid chlorine at CYA 40 all summer, that's about half a shock; on a pool fed by a trichlor floater since June, whose CYA is now 70, it's 28% of one. The kit doesn't know which pool it's going into, so it under-doses both.
| CYA at closing | Shock level | Kit's 1 lb per 10k gal reaches | Liquid chlorine to get there, 15,000 gal |
|---|---|---|---|
| 30 ppm | 12 ppm | 7.8 ppm (65%) | 1 gal of 12.5% ≈ $6 |
| 40 ppm | 16 ppm | 7.8 ppm (49%) | 1.4 gal of 12.5% ≈ $9 |
| 50 ppm | 20 ppm | 7.8 ppm (39%) | 1.9 gal of 12.5% ≈ $12 |
| 70 ppm | 28 ppm | 7.8 ppm (28%) | 2.9 gal of 12.5% ≈ $17 |
Shock level from the same FC/CYA relationship as every calculator here; liquid chlorine to raise a 15,000-gal pool from FC 4, at $6 a gallon. The shock calculator does it for your gallons and your reading.
The method, then, is a small SLAM three or four days before the cover goes on: raise FC to shock level with liquid chlorine, keep the pump running, and top it back up each evening until the level holds overnight — that's the proof the water is clean, and it's the same test the green pool guide uses to end a SLAM. Then stop adding chlorine and let it fall to about 20% of CYA before the cover goes on. At that point the pool has a full reserve of chlorine, no organics for it to fight, and water too cold for algae. That's a closed pool. Everything else in the kit is addressing a problem this step already solved.
A closed pool has no circulation, so chlorine added on closing day doesn't mix — it sits where it was poured, and a trichlor floater or "winter pill" under a cover is a puck of concentrated acid and chlorine parked over one patch of liner or plaster all winter. Bleached liner patches and etched plaster in spring are the usual result. Dose to shock level while the pump still runs, let it mix for days, and put nothing that dissolves slowly under the cover. If the pool needs chlorine in February, lift the corner and pour liquid, then walk it around.
Balancing the water for winter — not for summer
Every closing guide tells you to balance to the summer ranges and stop. But whether water scales or dissolves plaster depends on temperature, and the saturation index that reads balanced at 85 °F reads aggressive at 40 °F with nothing else changed. Here's the same water — pH 7.5, TA 70, CYA 40 — as it cools, at three calcium levels:
| Water temperature | CH 250 ppm | CH 350 ppm | CH 450 ppm |
|---|---|---|---|
| 85 °F | -0.24 | -0.1 | +0.01 |
| 70 °F | -0.4 | -0.26 | -0.15 |
| 60 °F | -0.51 | -0.37 | -0.26 |
| 50 °F | -0.62 | -0.48 | -0.37 |
| 40 °F | -0.73 | -0.59 | -0.48 |
Calcite saturation index; balanced is −0.3 to +0.3, shaded cells are aggressive. From the saturation index calculator, which takes temperature as an input for exactly this reason.
Water at CH 300 that sits at -0.16 in August is at -0.66 by January — plaster-etching territory, for four months, with nobody watching. Vinyl and fiberglass shrug at a negative index (they have no calcium to lose), so for those pools the summer targets are fine and the calcium row is irrelevant. For plaster, pebble and tile, close on the high side of every range: pH toward 7.8, calcium 350 or more, alkalinity 70–80. At pH 7.8 and CH 350 the same water sits at -0.3 at 40 °F — balanced. The calcium calculator gives the dose; do the TA and pH first, because raising calcium doesn't move either of them, and the acid you'd add to lower pH would take the alkalinity with it.
A typical kit for a 15,000-gallon pool holds a pound or two of cal-hypo or non-chlorine shock, a quart of algaecide, a stain-and-scale sequestrant, sometimes an enzyme and a "winter pill" floater, for about $50. The water needs chlorine to shock level — 1.4 gallons of 12.5% liquid at CYA 40, $9 — and, if you want the belt-and-braces, a dose of polyquat 60 algaecide, the only algaecide worth using (copper stains, quats foam, and both are what the "winter algaecide" usually is). The sequestrant treats metals you probably don't have; the enzyme digests oils the chlorine already oxidized; the floater is the liner-bleacher above. The kit isn't harmful. It's $41 of products for a pool that a $9 chlorine dose already closed.
Lowering the water, and how far
The water has to drop below the skimmer mouth so ice can't form inside it and split it, and below the returns if they can't be plugged from the deck. How far depends on the cover. A solid cover sheds rain and snowmelt to its own pump, so the water only needs to be 3–4 inches below the skimmer. A mesh cover lets precipitation through, and a wet winter can bring the level back up into the skimmer, so it starts lower — 12–18 inches — and you check it monthly. On a tile line, keep the water below the tile so freeze-thaw can't pop it. And don't overdo it: an inground pool drained far below its normal level is a shell with groundwater pushing up on it, and a liner left out of the water relaxes and wrinkles.
| Cover | Lower by | Water out, 15 × 30 ft pool | To put back in spring |
|---|---|---|---|
| Solid cover | 4 in below the skimmer | 1,122 gal | $12.34 |
| Mesh cover | 15 in below the skimmer | 4,207 gal | $46.28 |
450 ft² of surface; 0.623 gal per ft² per inch. Refill priced at $11 per 1,000 gal including sewer — the cost to fill calculator has your rates. Under a mesh cover most of it comes back as rain.
Blowing out the lines
This is the step that decides whether spring involves a plumber. Water expands about 9% as it freezes, and it doesn't compress; a pipe full of it has nowhere to go but apart, and the crack is usually under the deck. The lines must be empty, and "drained" isn't empty — low points hold water. So each line is blown clear from the equipment pad, one at a time: a shop vac on blow, a leaf blower with an adaptor, or a purpose made blower — not a compressor, which delivers pressure rather than volume and can pop a fitting.
- Open the pump lid and the filter's air relief; take the drain plugs out of the pump, filter, heater, and chlorinator or salt cell, and put them all in the pump basket so they're found in spring.
- Set the multiport to winterize or recirculate — between positions, so the gasket isn't compressed all winter.
- Blow each return line from the pad until it bubbles steadily at the pool, then plug it while the air is still flowing. Repeat for each return, the main drain (air-lock it: blow until bubbles, then close its valve fast — the trapped air keeps water out), and finally the skimmer.
- In the skimmer, thread in a gizmo or drop in a plastic bottle: something that crushes instead of the skimmer if water gets in and freezes.
- Antifreeze goes only in lines you couldn't clear, and only pool antifreeze (propylene glycol) — never in the pump or filter, and never the automotive kind.
Above-ground pools skip most of this: disconnect the hoses at the wall fittings, plug the return and the skimmer, and take the pump and filter indoors after draining them. Put the air pillow under the cover — it's there so ice pushes on the pillow instead of on the wall, not to keep the cover up.
A salt cell full of water splits its housing when it freezes and its plates scale if it sits in stagnant salt water; take it out, rinse it, and store it indoors — a dummy cell or the spacer it came with goes in its place. A heater holds several gallons in its exchanger: open both drain plugs and blow through it, because a cracked exchanger is the most expensive freeze damage a pool can have. Pumps with a variable- speed drive prefer to spend the winter dry and, if they're easy to unbolt, inside. And a pool light isn't in danger, but a cord on the deck is: coil it, or bring the fixture up onto the deck and cover it.
Covers, and checking on it
The cover's job is to keep debris out and, on a solid cover, light out with it. Solid covers need a pump on top and a check after every storm; a foot of water on a cover is a ton on the anchors. Safety covers (mesh) take the winter without attention but let sun in, which is why the chlorine reserve matters more under mesh. Either way, once a month: lift a corner. Clear water with no smell means the closing worked; a green tint in a warm spell means a jug of liquid chlorine poured in and walked around the edge. A pool that's checked in February opens in a day. One that isn't is the green pool guide's spring reader.
Last thing: write down the readings you closed with, especially CYA. Stabilizer doesn't survive every winter — under a cover, bacteria can convert it to ammonia, and a pool that closed at CYA 40 can open at zero, with an ammonia demand that eats chlorine on sight. Spring starts with a full test and the dashboard, not last autumn's numbers.