Closing a salt water pool is the same job as closing any other pool, with one extra step that costs several hundred dollars if you get it wrong: the salt cell has to come out. Water left sitting inside a cell over a hard freeze expands, and cells are not repairable when the housing splits.
This guide covers the full closing sequence in order, what to do with the cell, how low to take the water, and the question almost every new owner asks in October — whether all that salt stops the pool from freezing. It does not. Before you close, check your salt level with the pool salt calculator so you know where you are starting from next spring.
Do salt water pools freeze? Yes — and here is the maths
The assumption is understandable. Salt lowers the freezing point of water, the ocean rarely freezes, and your pool has salt in it. So surely a salt pool is protected?
It is not, because of how little salt is actually in there. Seawater is roughly 35,000 ppm. A salt water pool runs around 3,000 ppm — less than a tenth as salty, and barely above the level at which most people can taste it at all.
| Water | Salinity | Approximate freezing point | Practical effect |
|---|---|---|---|
| Fresh water | 0 ppm | 32°F (0°C) | Baseline |
| Salt water pool | ~3,000 ppm | About 31.7°F (−0.2°C) | Effectively none |
| Brackish water | ~15,000 ppm | About 30.4°F (−0.9°C) | Small |
| Seawater | ~35,000 ppm | About 28.6°F (−1.9°C) | Noticeable |
Freezing point depression at 3,000 ppm ≈ 0.3°F — less than the accuracy of most pool thermometersThree tenths of a degree buys you nothing. Your pool, your plumbing, your pump and your salt cell all freeze at the same temperature they would in a freshwater pool, so they need exactly the same protection. Anyone telling you a salt pool does not need winterising is confusing your pool with the sea.
The same maths explains why pool salt is a poor deicer for a driveway compared with dedicated ice melt — it works, but only within a narrow temperature band. See does pool salt melt ice and using pool salt for snow.
When to close: the 60 degree rule
Close when water temperature is consistently below about 60°F, not when the calendar says September. Two competing risks set the window.
Your generator gives you a useful nudge here. As water drops below about 60°F most units cut output sharply and some display a low temperature warning. Once the cell has effectively stopped making chlorine, the pool is running on residual anyway and there is little reason to keep it open.
Do not leave the generator running into the cold. Producing chlorine at very low water temperature is hard on the plates and achieves very little. Switch it off once the water is below the manufacturer’s stated minimum, even if you are not closing for another two weeks.
What to have ready before you start
Plugs and fittings
Winter plugs for every return, a skimmer Gizzmo or plug, threaded plugs for the pump and filter drains, plus spare union O-rings and a dummy pipe or bypass for where the salt cell lives.
Tools
Shop vacuum or a dedicated line blower, union wrench or channel locks, pool brush and vacuum, leaf net, submersible pump for lowering the level, and PTFE tape or thread sealant.
Chemicals
Liquid chlorine, muriatic acid or soda ash for final pH correction, sodium bicarbonate if alkalinity is low, calcium chloride if hardness is low, non-toxic pool antifreeze, and optionally a winterising algaecide.
Cover kit
Safety cover with anchors and an installation rod, or a solid cover with water bags or a cable and winch, plus a cover pump if you use a solid cover.
Use pool antifreeze, never automotive antifreeze. Automotive coolant is ethylene glycol and is highly toxic to people and pets. Pool antifreeze is propylene glycol based and formulated for this job.
The full closing sequence
Work through in this order. Chemistry is corrected before the water level drops, and the cell comes out before anything gets blown out.
Brush the walls, vacuum the floor, net every leaf and empty the baskets. Organic debris left under a cover for five months is what turns a pool green by March. Give the pool a proper clean, run the filter for a day or two, then clean again just before you close.
You are setting the water up to sit still for months. Aim for the middle of every range so that neither scaling nor corrosion has a head start. Targets are in the table below.
The day before closing, bring free chlorine up manually. The generator is about to be switched off for the season, so the residual you leave behind is all the pool gets. Add it after sunset with the pump running. Do not use the super chlorinate setting for this.
Off at the control panel, then off at the breaker. This prevents anyone accidentally energising the cell circuit when there is no water flowing through it.
Undo the unions, slide the cell out, and look through it at daylight. If the plates are clean, rinse and drain it. If they are scaled, clean it now so it goes into storage ready for spring — how to clean a pool salt cell has the ratio and soak times. Fit the dummy pipe in its place so the plumbing loop is complete for the blow-out.
Backwash a sand or DE filter thoroughly, or pull and wash cartridges and let them dry completely before storing. A filter put away dirty over winter breeds problems and comes out with a bad smell.
How far depends on your cover and climate — see the section below. Use a submersible pump or the filter’s waste setting, and watch it so you do not overshoot.
Skimmer lines, return lines, main drain, cleaner line, spa lines, heater bypass. Push water out with a blower or shop vac until you see air bubbling from each fitting, then plug it while air is still moving. Details in the section below.
Pull the drain plugs from the pump volute, the filter, the heater and any chlorinator. Tip the pump to get residual water out. Put every plug into a labelled bag and tape it to the pump lid, or you will spend an hour hunting in April.
Non-toxic pool antifreeze goes into skimmer lines and returns after they are cleared, as insurance against the small amount of water that always remains. It is not a substitute for blowing the lines out.
Install the safety cover and tension the springs evenly, or fit the solid cover with water bags or cable and winch. Set up a cover pump on a solid cover, because standing water plus debris is heavy enough to damage the cover and the coping.
Last job, and the one people forget after the cover goes on. Make sure the cell is fully drained, dry and somewhere heated or at least frost-free. Details below.
The salt cell: the step that separates a good closing from an expensive one
Everything else on this list applies to every pool. This part is specific to salt, and it is the reason people who close their own salt pool sometimes end up buying a replacement cell in spring.
Water inside a cell that freezes will crack the housing. Cells are moulded plastic bodies with titanium plates bonded inside. When trapped water expands, the body splits. There is no repair — you buy a new cell, typically several hundred dollars.
Take the cell out, do not just isolate it. Closing valves on either side of the cell traps water inside it. That is precisely the failure you are trying to prevent. Undo the unions and physically remove it.
Rinse and inspect before storage. Rinse with a hose, then look down the length of the cell towards a light. Clean plates are uniformly dark grey. White crusty bridging between plates is calcium scale and should be cleaned now — scale left on all winter is harder to remove and starts next season under-reading salt.
Drain it completely and store it dry, upright. Tip it both ways to empty the channels, leave it to air-dry, and stand it upright on a shelf in a garage, basement or shed that stays above freezing. Do not stack anything on it and do not leave it in a bucket of cleaning solution.
Fit the dummy pipe. Most cells ship with a straight pipe of the same length. It completes the plumbing loop so you can blow lines out properly and so the pad is not left with an open gap all winter.
Protect the cell cable and the control box. Coil the cell cord and tuck it somewhere dry rather than leaving the connector on the ground. If your control box is not weather rated, cover it. Rodents chew cell cords over winter more often than you would think.
Note the cell’s age while it is in your hands. Closing is the natural moment to decide whether it will make another season. How long a pool salt cell lasts covers the warning signs and the replacement maths.
Write down your salt ppm and pool volume in October. In spring, put both into the pool salt calculator and you will know instantly whether the panel is lying about a low reading or the pool genuinely lost salt.
Winter chemistry targets (different from summer targets)
Water that is going to sit for five months without circulation is a different problem from water you swim in. You want it chemically neutral — neither aggressive enough to etch plaster and corrode metal, nor saturated enough to deposit scale on the walls and inside the plumbing.
| Parameter | Winter target | Why this number |
|---|---|---|
| pH | 7.2 – 7.6 | Middle of range. It will drift over winter, so do not leave it at either edge |
| Total alkalinity | 80 – 120 ppm | A little higher than a running salt pool wants, because the cell is off and no longer pushing pH up |
| Calcium hardness | 200 – 300 ppm | Low enough to avoid scale in still water, high enough to protect plaster and grout |
| Free chlorine | Raised above normal at closing | It is the only sanitiser the pool gets until spring |
| Cyanuric acid | 60 – 80 ppm | Protects the residual under a mesh cover that lets light through |
| Salt | Leave as is | Nothing to do — it stays in the water and waits for spring |
Note the alkalinity difference. During the season a salt pool is usually happiest at the lower end, around 70–80 ppm, because the cell continuously drives pH upward and lower alkalinity slows that drift — the reasoning is in how to lower pH in a salt water pool. With the generator off for winter that upward pressure disappears, so a slightly higher buffer is helpful rather than harmful.
Never mix chemicals together. Add each product separately to the pool with the pump running, and leave time between them. Combining acid with chlorine products, even as residue in a bucket, can release chlorine gas. Always add acid to water, never the reverse.
How far to lower the water — and why not to drain it
How low you go is dictated by your cover type and your climate, not by preference.
Mesh safety cover
Typically 12 to 18 inches down, below the skimmer mouth and below the returns. Mesh lets rain and snowmelt through, so the level will rise over winter and you are leaving room for it.
Solid cover
Usually only a few inches below the skimmer. A solid cover keeps water out, so a big drop is unnecessary and leaves the liner or plaster exposed to freeze-thaw damage.
Warm climate, no freeze
Often no drop at all. You are covering to keep debris out, not to protect plumbing from ice.
Never fully drain an inground pool to winterise it. The water in the pool counterbalances groundwater pressure outside the shell. Drain it and a vinyl liner can shrink and shift, a fiberglass shell can lift, and a concrete pool can crack or float out of the ground. If a full drain is genuinely required for repair work, that is a job for a professional who will check the hydrostatic relief valve and the water table first.
The same principle applies during the season if you ever need to dilute the water — see how to lower the salt level in a pool for how to do partial drains safely.
Blowing out the lines properly
Every line that holds water needs clearing. Freezing water in a buried return line is one of the most expensive repairs in pool ownership because it means digging up the deck.
Connect your blower or shop vacuum on blow to the pump side, and push air through each line one at a time with the others valved off. A wet/dry vac works on small pools; a dedicated blower is faster and better on long runs.
When you see a steady stream of bubbles from a return, screw the winter plug in while air is still moving. That leaves the line air-filled rather than letting water siphon straight back.
Blow the main drain line until bubbles rise, then close the valve while air is flowing. This leaves an air pocket that keeps water from backing up the line.
A Gizzmo screws into the skimmer’s threaded drain and absorbs the expansion of any ice that forms. An empty plastic bottle is the old workaround, but a Gizzmo is designed for it and costs very little.
Anywhere with a loop or a low spot holds water. The dummy pipe you fitted where the cell was needs blowing through too, along with the heater, any in-floor cleaning system and spa jets.
Pour non-toxic pool antifreeze into the cleared lines as insurance. It is a backup for the residual you cannot see, not a replacement for doing the blow-out properly.
If you are at all unsure about the plumbing layout, this is the one part of closing worth paying a professional for once. Watch what they do, photograph the valve positions, and you will be able to do it yourself every year after that.
What actually happens to the salt over winter
Nothing. This is the reassuring part.
Salt does not evaporate, does not break down, does not get used up and does not freeze out of the water in any meaningful quantity at pool concentrations. The sodium chloride you added in June is still dissolved in there in February. There is no winterising step for salt itself and nothing to remove.
What does change is concentration, and only because the volume of water changes:
Water you pump off the cover takes salt with it if the cover leaked or you pumped from the pool by mistake. Water pumped off the top of a solid cover is rainwater and takes nothing.
Rain and snowmelt through a mesh cover dilutes. The same pounds of salt in more gallons means a lower ppm. This is the biggest genuine winter loss for most pools.
Lowering the level in autumn removes salt. Drop 15 inches of water and you take that proportion of your salt with it. Factor it in mentally so the spring reading does not surprise you.
Evaporation concentrates rather than dilutes. Water leaves, salt stays. In a dry winter climate the level can drop and the ppm can actually rise.
Do not trust the spring panel reading. Cold water conducts less electricity, so a generator’s conductivity-based salt reading runs low in April even when the salt is fine, and scale on the plates makes it worse. Clean the cell, let the water warm, test independently, and only then buy salt. The full explanation is in how to open a salt water pool, and testing methods are in how to test salt level in a pool.
Warm climates: you probably should not close at all
In regions that never see a sustained freeze — much of the southern and south-western United States — full winterisation is usually the wrong move. Keeping the pool running at reduced hours costs a little in electricity and saves you an opening and a closing.
| Task | Freezing climate | Mild climate |
|---|---|---|
| Remove the salt cell | Yes, always | Not necessary, but switch the generator off below its minimum temperature |
| Blow out the lines | Yes | No |
| Lower the water level | Yes, per cover type | No |
| Run the pump | No — system drained | Yes, at reduced hours |
| Sanitiser | Winter chlorine dose, then nothing | Low generator output, or liquid chlorine as needed |
| Cover | Safety or solid winter cover | Optional, mainly for debris |
The one thing that applies everywhere: once water temperature falls below the manufacturer’s stated minimum, switch the generator off and dose with liquid chlorine instead. Cold water makes the cell inefficient and forcing it to run is hard on the plates. How to handle that is covered in can you put chlorine in a salt pool.
Even an occasional freeze night matters. If a hard freeze is forecast in a normally mild area, run the pump continuously overnight — moving water is far more resistant to freezing than still water — and be ready to remove the cell if the cold spell is going to last.
Closing mistakes that show up in April
Leaving the salt cell plumbed in. The single most expensive winterising mistake in salt pool ownership. Water inside the cell freezes, the housing cracks, and you open the season with a several hundred dollar bill.
Closing while the water is still warm. Above about 65°F, algae carries on growing under the cover with no sanitiser being made. Wait for the water to cool.
Closing a dirty pool. Leaves and organic debris consume the chlorine residual within weeks and then rot all winter. Clean thoroughly, then clean again the day you close.
Skipping the final chlorine dose. The generator is off for five months. Whatever residual you leave is all the pool gets, so leave a generous one.
Fully draining an inground pool. Hydrostatic pressure can lift or crack the shell. Lower the level, never empty it.
Losing the drain plugs. A trivial problem that costs a whole Saturday in spring. Bag them, label them, tape them to the pump.
Storing the cell wet or outdoors. A cell left in an unheated shed with water in the channels can still crack. Drain it, dry it, bring it inside.
Leaving a solid cover unpumped. Water and debris on a solid cover get heavy fast, and a collapsed cover dumps everything you were trying to keep out straight into the pool.
Frequently asked questions
Do salt water pools freeze?
Yes. At roughly 3,000 ppm, a pool is about a tenth as salty as seawater, which lowers the freezing point by around three tenths of a degree Fahrenheit. That is far too little to matter, so a salt pool needs exactly the same freeze protection as a freshwater one.
Do I have to remove the salt cell for winter?
In any climate that freezes, yes. Water trapped inside the cell expands as it freezes and can split the housing, which is not repairable. Remove it, rinse it, drain it completely, fit the dummy pipe, and store it indoors above freezing.
Do I need to remove the salt from the pool before winter?
No. Salt stays dissolved in the water all winter and is still there in spring. It does not evaporate or break down. The only real losses come from water leaving the pool or fresh water diluting it.
Should I add chlorine before closing a salt water pool?
Yes, with liquid chlorine, the day before you close. The generator is switched off for the season and cannot maintain a residual, so the level you leave behind has to carry the pool through winter. See how to shock a salt water pool for dosing.
How low should the water level be for winter?
With a mesh safety cover, usually 12 to 18 inches down, below skimmer and returns. With a solid cover, often only a few inches below the skimmer. In non-freezing climates, often no drop at all. Never fully drain an inground pool.
Can I leave the cell in and just close the valves?
No, not where it freezes. Closing valves traps water inside the cell, which is exactly the failure you are avoiding. In a frost-free climate you can leave it plumbed in, but switch the generator off once the water is below its minimum operating temperature.
When should I close a salt water pool?
When the water is consistently below about 60°F. Closing earlier while the water is still warm invites algae under the cover; closing later risks a freeze reaching the plumbing and the cell.
Will winter rain ruin my salt level?
It dilutes it, but usually less than the spring panel reading suggests. Most of the apparent drop is cold-water conductivity error plus scale on the plates. Test with strips or a meter before buying anything, and use the pool salt calculator to work out the real shortfall.
Do I need to winterize an above ground salt water pool?
Yes, and the cell rule is identical — remove it and store it indoors. Above ground pools have shorter plumbing runs, which makes the blow-out simpler, but the pump, filter and hoses still need draining. See can an above ground pool be salt water for the equipment considerations.
Should I clean the salt cell before storing it?
Inspect it and clean it if you see scale. Storing a cell clean means it comes out in spring ready to use and gives an accurate salt reading from day one. Scale left on all winter hardens and becomes harder to remove.
Can I use antifreeze in the lines instead of blowing them out?
No. Pool antifreeze is insurance for the residual water you cannot remove, not a substitute for clearing the lines. And never use automotive antifreeze, which is toxic.
What if I get a surprise freeze before I have closed?
Run the pump continuously through the cold snap. Moving water resists freezing far better than still water, and continuous circulation is the standard freeze protection strategy. If the freeze will be prolonged, close properly or at minimum remove the salt cell.
Sources and further reading
Healthy Swimming — disinfection and water chemistry guidance for pool operation.
Why is the ocean salty? — seawater salinity figures behind the freezing point comparison.
Saline water — salinity classifications from fresh through brine.
Owner’s manuals — manufacturer winterising instructions and temperature limits for salt chlorine generators.
One extra step, five months of peace of mind
Closing a salt water pool is a normal pool closing plus one thing: get the cell out, drained, dry and indoors. Do that, leave the water balanced and chlorinated, clear the lines properly, and the pool that comes out from under the cover in spring will be the pool you put away in autumn.
Note your salt reading before you close, then use the pool salt calculator in spring to work out exactly what the pool needs. When the cover comes off, how to open a salt water pool picks up exactly where this guide leaves off, and cleaning the salt cell is the natural companion job.
For the rest of the season, maintaining a salt water pool, how often to add salt and the right salt level for your pool cover the routine — and if you are still deciding whether salt is right for you, read salt versus chlorine and what a salt water pool really costs.
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