Closing and Winterizing Your Pool

You close a pool the same way you close a book. You put it down, you walk away, and when you pick it back up in the spring, it's exactly how you left it. That's the story everyone tells themselves about pool closing. And it's wrong. Nothing about the word closed applies to what's actually happening under that cover for the next five months. The water doesn't know it's October. It doesn't know you stop testing it and it keeps responding to temperature and to whatever's dissolved in it, and to light if light's getting through. Whether you're watching or not.

Welcome to the Pools Scientific Podcast, elevated pool care through science and technology. I'm John Cooper, and whether you own a pool, service them, or run a pool company, this show is built for you. Real Chemistry, Real Answers. Every Tuesday.

Closing weekend is usually the last time most pools get balanced correctly all year, and it's also the beginning of the single longest stretch that same water will go untested. Whatever balance you set that day is what the water runs on for the next five months, with no one there to adjust it, and most people set it in the wrong order to begin with. It's the same water and the same cold for every pool. What changes the math is what you do before that cover ever goes on.

Every pool needs the same things done to it before any cover goes on, whether it's plaster, vinyl, fiberglass, or above ground. None of this was settled practice a lifetime ago. Backyard pools barely existed in this country before World War II, and then the number exploded, something like 11,000 of them by 1947, over 46,000 built in a single year by 1959. Gunnite, pneumatically applied concrete, is what made that boom possible. It was cheap enough that tens of thousands of ordinary families could put a pool in their own backyard instead of just visiting one at the country club. Nobody had generations of experience closing a concrete pool for a Midwest winter, because until that boom, almost nobody had a concrete pool to close in the first place.

The whole practice was built from scratch, mostly the hard way, through cracked pipes and etched plaster that taught people what not to do the following year. Skim, vacuum, and brush right before the cover goes on, not three days before, since every leaf and every bit of organic debris you leave in that pool sits at the bottom for five months, decomposing, pulling oxygen out of the water, and feeding whatever wants to grow in the dark. A clean pool closes clean, and a pool closed with a season's worth of leaves on the bottom is running its own private experiment in low oxygen decomposition. Balance the water before the cover goes on, not after, and in the right order.

Alkalinity first, since it's the buffer everything else leans on. PH second, and calcium last, since adjusting pH against unstable alkalinity means chasing a number that won't hold. And calcium is the one input you can't easily pull back out once it's in the water. The equipment is the part of closing that hurts people every single year, and it's also the part that costs the most when it goes wrong. Blowing out your lines means clearing the water sitting in the pipes so it doesn't freeze, expand, and crack the pipe from the inside over the winter. And that part of the job isn't actually dangerous. What people used to do is the pipe itself can handle a lot.

Schedule forty PVC is rated for water pressure most pool systems never come close to even touching. But compressed air is a completely different animal, since air is compressible and stores energy the way water doesn't. Pool equipment itself, pumps, filters, heaters, are typically rated around 50 psi at most, and manufacturers like Hayward specifically warn against purging lines with a compressed air compressor at all, calling for a low pressure, high volume blower instead, something under 5 psi. A shopvac motor running in reverse or an air compressor with no regulator can blow well past what that system was ever built to hold, and people have been hurt by plugs firing out of return lines under pressure they had no way to see building.

My team uses a thing called a cyclone blower. It moves a lot of air at a low pressure, so it clears returns and main drain lines even when they're underwater. Every year we see who can make the tallest geyser out of a blowing skimmer. The record's about 60 feet, or about 18 meters. If you need one, they're available on our website at poolsscientific.com/slash supply. Get it wrong in the other direction with too little clearing rather than too much force. And you're not looking at an emergency room trip necessarily, but you're looking at a cracked pipe or a split heater core from leftover water freezing that you won't find out until it fails in February or floods the equipment pad when you start your pump up in the spring.

That's two separate ways to lose on this job. One to your body, ouch, and one to your wallet, double ouch, and both of them are avoidable in the same way. If you're doing this yourself, use a low pressure blower built for the job like the cyclone, never a compressor without a regulator, and never stand directly in front of a fitting while it's under pressure. If any part of that last sentence made you uncertain, that's your answer. This is genuinely worth paying a professional for. Having it done right is cheap against the alternative, a trip to the emergency room on one end, or a repair bill running into the thousands on the other.

Once the lines are clear, winterizing plugs go into every return. Gizmos go into every skimmer, and antifreeze goes into the skimmer wherever it's appropriate for your setup, because in our climate, a heater or filter housing still holding water when that first real cold snap comes through is a repair bill waiting to happen, sometimes a four-figure one. ASTM rated safety covers, mesh or solid, meet a real engineering standard. ASTM stands for the American Society for Testing and Materials, and their standard for pool safety covers means that the cover has actually been load tested to bear real weight, not just marketed as safe. A standard winter tarp simply isn't built to that standard, the same way an auto cover left in the closed position for five months isn't either, since auto covers are built and tested for daily use, not for sitting closed under snow load for months at a time.

Our standard practice on pools with an auto cover is to leave that cover open for the entire off season and install a mesh safety cover on top of it instead. It's a bit of a cost up front to have a second cover installed, but from a safety standpoint it's worth every penny. From that point on, you treat it as a mesh pool cover, chemistry and structure both.

Children and pets pay for the gap between a rated cover and something that only looks like one, and they don't pay for it the same way. A covered pool reads a solid ground to a child who has no way of knowing the difference between a rated cover and a tarp, and a non-rated cover gives way with no warning, leaving no time between standing on it and being underneath it. Pets go through the same gap faster and get found later. A dog or cat crosses a covered pool the way it crosses a patio, no hesitation, no test step first, and a pet that goes through a non-rated cover usually can't get purchased to climb back out from under the material the way a person might.

They can't call for help and their whereabouts get checked far less often during the winter months. By the time anyone notices, it's often too late, and that's exactly why this deserves its own attention rather than a line tacked onto someone else's warning. A mesh safety cover is ASTM rated. It bears real weight and it still lets light through, which matters because sunlight reaching the water all winter keeps algae in the phosphate load that feeds it as live concerns, not something you get to stop thinking about until spring. A solid safety cover is also ASTM rated and bears real weight, but it blocks light completely and no light means no photosynthesis.

That sounds like a win and it mostly is, except it trades one problem for another. Organic material trapped underneath decomposes in the dark with the oxygen it needs already used up. And standing melt water on top of the cover needs a cover pump running or it becomes its own weight problem before it becomes anything else. Water bags aren't a cover type. They're an anchoring method for tarp on top. The water-filled bags hold the edges down, and I do not recommend this setup at all. Two problems stack on top of each other here. The tarp doesn't stay taut and sags toward the middle. So anything that lands on it over five months, leaves, branches, dirt, dead animals, they all sit there till spring.

Skimming it off means pushing that same debris straight down into the water underneath, so the tarp delays the mess instead of preventing it. Worse, a loose tarp held down by water bags is close to the worst possible thing for a person or an animal to end up on top of, since it doesn't hold weight like a rated cover and it doesn't stay open like no cover at all. It gives way, and the same loose material that couldn't support anything on top of it wraps around whoever or whatever just went through, taking away the one thing that might have saved them, the ability to swim out. A hard-sided pool with no cover at all is safer than a water bag tarp cover, since at least there's a clear water and a clear edge instead of fabric closing around you exactly when you need to move.

If the choice is a water bag cover or nothing, choose nothing and net the debris out through the winter as needed. Cold water pushes a pool's LSI, the saturation index, negative on its own, since temperature is a direct turn in the calculation. A pool that tested perfectly balanced at 80 Fahrenheit or about 27 Celsius in September can drift meaningfully negative in January at 40 Fahrenheit or about 4 Celsius, even if nothing else about the water changed. What happens next depends entirely on what that particular surface has to work with underneath the water, and it's not the same answer twice. Everything up to this point applies no matter what your pool is made of.

What happens underneath that cover for the next five months does not. Four kinds of pools. Above ground pools are almost always vinyl over a steel or resin frame, thin walled and small enough in volume that temperature swings fast in both directions. In ground vinyl trades the frame for a liner draped over a fixed structural wall, which solves the volume problem but introduces a new one, since that liner depends on the water itself for support in a way none of the other surfaces do. Fiberglass is a single molded shell, smooth and consistent, with its own particular relationship to calcium that has nothing to do with a liner at all.

And plaster is the one surface in this lineup that isn't just holding water, it's made of cementitious material, which turns out to matter more for winter chemistry than anything else in this episode. Same cold, four completely different pools underneath it. Let's start with the one that behaves the least like the other three.

Above ground pools drain differently than everything else in this episode, since the concern isn't a skimmer opening at all, it's the returns. You have to drain below those, regardless of what kind of cover is going on top. Most above ground pools close with a tarp cut to the shape of the pool, with a cinch rope or cable that runs under the top cap and pulls the whole thing tight across the surface, keeping leaves and debris out. That tarp still comes in the same mesh or solid choice as everything else in pool covers, and it carries the same trade-off. Mesh lets rain drain through, solid holds it on top instead.

A solid tarp holding rain and debris on its surface needs that water and debris cleared off before you pull the cover, since otherwise it's going straight into the pool the moment the tension comes off. Ask me how I know. This is still a standard tarp, not an ASTM rated safety cover, and the same drowning risk for kids and pets applies here exactly the way it does with any other non-rated cover. Most above ground pools run a vinyl liner, which means the same chemistry story applies here as it does for in-ground vinyl, no cementitious reserve, nothing built into the material itself to lean on when the water goes hungry for calcium during a cold water excursion.

What makes an above ground pool its own case is volume. A smaller body of water swings faster, both up and down, than a large in-ground pool does, since there's simply less water to absorb a cold snap or a warm spell before the temperature and the LSI that comes with it moves. That means less margin for air going into the close, not more. There's a structural side to above ground closing that doesn't come up with an in ground pool at all. An in ground pool sits surrounded by earth, which insulates the shell and slows how much ice actually forms. An above ground pool has nothing but a thin wall between the water and the outside air on every side, so it builds a thicker ice layer than an in ground pool ever will in the same cold snap.

That ice expands as it forms, and if it has nowhere to go, it pushes outward against the wall itself. That's what a crush device is for, a pillow or similar floating mechanism sitting in the water, giving the ice something to compress instead of against the wall. Skip it in a genuinely cold climate, and wall blowouts happen more often than they need to. Proper winterization isn't just a chemistry question for an above ground pool, it's a structural longevity question for the pool itself. We've put together a full guide just on above ground closing, including crush device sizing and tarp tensioning, and we'll tell you where to find it at the end of this episode.

In ground vinyl pools share the same no reserve chemistry with its above ground cousin, but trade the steel frame for a fixed structural wall, and the water's job changes completely. Vinyl has a hard structural limit that the other surfaces don't. You can't drain a vinyl pool more than about halfway down the side walls, since the weight of the water is what keeps the liner's corners tucked into place where the floor meets the wall. Drain it too far and you're not managing chemistry anymore, you're risking the liner itself. Under a mesh safety cover, drop the water a few inches below the skimmer opening. Under a solid cover or an auto cover, drop it only to the bottom of the skimmer opening and no further.

If skimmer throat cover plugs are available for your setup, it's actually preferable to skip the drain down altogether and keep the pool at full level since those plugs seal the skimmer throat itself. Gizmos and antifreeze go in every skimmer regardless, and winterizing plugs go in every return the same as any other pool. The chemistry follows the same no reserve story as above ground. Vinyl's calcium hardness target sits at 150 to 250 parts per million, and that range understates the real cold water risk, since a reading that looks fine at 80 Fahrenheit or about 27 Celsius doesn't account for how much further that same water's LSI will fall by January, with no buffer standing behind it.

Holding calcium hardness at the higher end of that range, going into a close, is preferred since there's no material reserved to make up for the difference later. The written guide for vinyl liner pools goes deeper into this than we have room for here, and we'll point that out to you before we close out.

A fiberglass pool trades vinyl structural drama for a single molded shell, and that changes the water level question completely. Under a mesh safety cover, fiberglass drains to the bottom of the skimmer opening, same as vinyl. An auto cover changes the entire approach. Don't drain a fiberglass pool with an auto cover at all. Plug the skimmers with skimmer faceplate plugs instead because that water needs to stay in the pool to support the weight of the auto cover itself. And if you drain it down, the cover has nothing underneath it to hold it up when the rain or snow starts accumulating on top, which is exactly how a cover rips out of its tracks.

That's expensive. There's a structural risk here that has nothing to do with the water inside the pool at all. Most fiberglass pools are built without a main drain at the bottom, which means groundwater building up underneath and around the shell over a wet winter has nowhere built in to go, and that hydrostatic pressure is a real problem, not a theoretical one. Better pool builders install a sump opening along with an upper pool drainage system specifically to manage this. And if your pool has one, it's worth checking on periodically through the off-season rather than forgetting about it until spring. If water is accumulating in that sump, dropping in a small sump pump is usually all it takes to relieve the pressure.

Skip that check in the wet winter, and the worst case isn't a crack shell, it's a pool that actually pops out of the ground. Fiberglass carries a calcium hardness floor near 100 parts per million, and floor is the right word, since going below it means the gel coat surface itself starts giving up calcium to satisfy hungry water, and fiberglass has even less to spare than vinyl does. That's a slower, quieter failure than a cracked liner, and it usually isn't caught until the surface already shows it. Fiberglass has enough detail of its own, especially around that autocover question, to earn its own full written guide as well.

It's coming at the close of this episode too. Plaster gets the deepest water level math of any surface here because it's the one surface with a real chemistry answer to lean on. And that changes what the water level is actually protecting. Under a mesh safety cover, drop the water about six inches below the skimmer opening. Under a solid cover, drop it only to the bottom of the skimmer opening. Auto covers are where plaster gets genuinely complicated and it's rare enough that most people never run into it. Most plaster pools are tiled and custom shaped without a skimmer body built in to take the plug the way a fiberglass or vinyl skimmer can.

So the fiberglass approach doesn't transfer cleanly here. But leaving a plaster pool full under an auto cover all winter risks freezing and damaging the skimmer body itself, gizmo or no gizmo, since the skimmer wasn't built to sit full through a hard freeze the way the rest of the shell was. This one is case by case, which is exactly why the lab's standard recommendation for a plaster pool with an auto cover is the same mesh over auto cover approach from earlier. Leave the auto cover open for the winter, install a mesh safety cover on top of it, and drain the pool the same way as any other plaster pool under mesh, about six inches below the skimmer opening.

Plaster is a cementitious surface, which means it carries its own calcium reserve built into the material itself, not only in the water sitting on top of it. When the water goes hungry for calcium during a cold water excursion, the way every pool's water does as temperature drives LSI negative, plaster can lean on that reserve and lose a small amount of surface calcium without anything visibly wrong happening. Repeated years of this is exactly how a plaster surface etches out early. So it's not permission to ignore the numbers, but it does mean plaster carries a built-in margin that above ground vinyl and fiberglass simply don't get. The target through the off-season is a neutral to slightly positive LSI, not just balanced at closing and left to drift.

Closing balanced and then letting the numbers fall wherever cold water takes them over five months is exactly how that etching happens gradually, one winter at a time, until it's suddenly visible on a surface that looked fine the year before. Running calcium on the high side of the range is the right call here, since it gives that reserve more to work with as the water pulls at it all winter. But that same high calcium is exactly what makes pH the number to watch closely, because cold water doesn't just create etching risk, it creates scaling risk too, and pushing calcium up while pH drifts high is how you trade one plaster problem for another.

Target calcium hardness for plaster sits at 200 to 400 parts per million, balanced in the standard order before that cover goes on. Alkalinity first, pH second, calcium last. Every surface before this one has to get its water balance exactly right going into winter because there's nothing else to fall back on. Plaster does get a little room to be human about it. There's a written guide built around exactly this buffer mechanism as well, and it's part of the same set waiting for you at the end.

A pool sitting under a cover isn't sitting still. It's doing chemistry every day for five months, whether you're watching or not. Four pools, four different answers, but the same five months of cold water sitting on every one of them. An above ground pool hangs on the returns being cleared and a crush device doing its job against ice building up on a wall with nothing insulating it. A vinyl liner has to respect its water level as a structural limit before it's ever a chemistry number. Fiberglass needs its sump checked as carefully as its calcium, since the water under the shell can do damage the water inside never could.

And plaster, more than any of them, forgives a slip if you go into the winter with calcium on the high side. PH watched closely, and the LSI held neutral to slightly positive instead of just balanced on the day you closed it. If you want the full depth on your specific pool, we've put together four written guides, one each for above ground, vinyl, fiberglass, and plaster, covering everything in this episode and more. Links are at poolsscientific.com/slash resources. Closing right gets a pool through the first cold snap, but it doesn't tell you what's happening in month three, and most of what goes wrong over a winter goes wrong quietly, long before anyone pulls a cover in the spring to find out.

How to properly maintain your pool through the off season is more important than just closing it right, and it saves you more money than you can possibly imagine. That's where we're going to go next, because the water sitting under that cover right now isn't done, even if nobody's watching. Thank you for listening. I'll see you next Tuesday.

Thank you for listening to the Pools Scientific Podcast, an independent production of John Cooper and Pools Scientific. Show notes, including full source citations for this and every episode, are at poolsscientific.com. We'll see you next Tuesday.