Winter Pool Maintenance: What the Water Does to Your Pool Under the Cover

I want to talk to you about something today that you've probably never heard about. The science is there, ready for anyone to use it. Yet the pool industry completely ignores it. Why? Personally, I think it's because it's never been done this way, so there's real pushback due to fear of doing something new and different. There's some degree of just plain laziness. There's definitely a lot of confusion about the science. And cynically, on my part, it might have something to do with contractors making lots of money from it, year after year, and every year that something isn't done about it. It's probably some combo platter of all of those.

By my estimate, and in my experience, that platter full of excuses is costing nearly every pool owner thousands of dollars every year. That includes your pool. Today, we're going to talk about what happens under your pool cover through the off-season. I'm confident this episode can save you some big money. Let's go.

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

Most pools get closed the same way. In October, the water gets cold enough that somebody decides it's time. The equipment is blown out and drained, a winterizing kit goes into the water, the cover goes on, and see you in April. Then nothing happens, or it looks that way. Snow, leaves, rain, a few months of gray. Nobody tests the water because there's no reason to, and a pool that looks fine under a cover is a pool nobody really worries about. Down there, the water is cold and still has all winter to work on the surface. It pulls calcium out of the plaster, and the plaster etches. The sun shelf starts to pit.

On a fiberglass pool, the gel coat turns rough or pitted. Scale forms. Wrinkles form in the vinyl liner. In April, the cover comes off and what's underneath is a green swamp, with a fine white dust on the floor that, in my experience, is the calcium the water took all winter coming back out as the pool warms up. And the explanation is always the same. That's just how spring goes. It's an older pool, the plaster's getting rough, and some cleaning and some shock will bring it back. Just slam it. The pool goes into the season a little rougher than last year, and the whole thing starts again in October.

Sound familiar? The six months under the cover are where a pool gets damaged, and the chemistry that protects it has almost nothing in common with what keeps it clean in July. Nearly every pool owner you'll ever meet has never heard that, and wouldn't know what to do about it if they did. You don't have to be one of them. I've developed a protocol to mitigate that damage. It's a month-by-month system for managing the water through the off-season, built on the LSI and run on hundreds of pools. The goal is simple. A pool that comes out from under the cover in the spring should look the way it did in September.

And you can do it for your own pool. This is my seventh season managing pools through the winter, and I didn't start out knowing how, which is a big part of why the protocol actually works. The ebook, The Science of Off Season Pool Care, is a guide to that protocol, and the chemist circle is where I check your numbers against it, alongside a group of pool owners working through the same winter. Together, they're the recipe for success. A pool is one of the biggest purchases most families will ever make. These days, it's six figures, and for plenty of people, it costs more than the car in the driveway.

Maybe more than all of the cars in the driveway. And for half of the year, the plan for that investment is cover and ignore. In my experience, from watching hundreds of pools go through many winters, managing the water through the off-season is the one thing an owner can do that prevents about 90% of the damage a pool takes over its entire lifetime. Off season isn't one situation either. In a cold climate, it's a closed pool under a cover. In a mild climate, it's a pool that keeps running with nobody swimming in it. Either way, it's the temperature of the water that decides which chemistry applies. The calendar doesn't.

And the damage comes at two speeds. The fast kind happens in a single winter, and it's the list from the start of this episode scale, etching, a pitted sunshelf, a rough gel coat, white dust on the floor when the pool warms, a wrinkled liner, and a green swamp when the cover comes off. One unmanaged winter is enough to do all of it. The slow kind is every winter added together. It shows up as a replaster every three to five years or a liner that has to be replaced or damaged gel coat, and by then nobody traces it back to the winter. It gets blamed on the plaster, the builder, the age of the pool.

That's just what happens to a pool. They wear out. And the same playbook runs again next October. That's why this winter, the one that's coming up now, is the one that matters. The material the water takes from a surface doesn't come back. So if you wait until the long-term damage is visible, it's already happened. It's too late, and the only repair left is the expensive one. Whether your pool is about to close or it's already under the cover, this is the winter to start. Your pool doesn't stop being an asset worth protecting just because the cover goes on in October.

Under the cover, nothing is moving. The equipment's been blown out, drained, and shut off, so there's no circulation and no filtration. And the water sits against the surface, cold and still, for about 180 days. Just because the water isn't moving, though, doesn't mean nothing is happening. That water keeps reacting with whatever it touches, which is your plaster, your grout, your tile line, your gel coat, your vinyl liner. The LSI from episode 3 tells you which direction that reaction runs. At zero, the water is saturated, in balance with the calcium. Below zero, it's undersaturated and pulls calcium out of the plaster, which is the etching, and some of that calcium can come back out of solution as a fine white dust on the floor when the water warms back up in spring.

Above zero, it's oversaturated and drops calcium onto the surface as scale. The target range, negative 0.3 to positive 0.3, is the band where that exchange is small enough to be acceptable. Water temperature is one of the inputs. The temperature factor is 0.6 at 76 degrees Fahrenheit or about 24 Celsius, and zero at 32 degrees or zero Celsius. If you cool the water from 76 to freezing, temperature alone takes 0.6 off the LSI, a dramatic shift towards corrosiveness, with no change to anything else.

Here's an example. Take a plaster pool closed in range at 65 degrees Fahrenheit or about 18 Celsius. Its LSI is a positive 0.08 comfortably in the range. Do nothing, and by the time the water reaches 40 degrees, or about 4 Celsius, the LSI is negative 0.27. At 34 degrees water temperature, about 1 Celsius, it's a negative 0.36 out of range on the aggressive side. That low LSI condition then exists as long as the temperature stays cold and the damage is cumulative. Every day below zero LSI removes a little more calcium from the surface. Inside the target range, that loss is minor and with intolerance. And outside the range it's faster.

And off season is about 180 days long. Plaster isn't the only surface in that water. Vinyl and fiberglass pools sit under the same cover in the same cold, and the LSI math applies here as well. What they don't have is room for error. Their calcium ranges are lower to begin with. 100 to 150 parts per million for fiberglass and 150 to 250 for vinyl, and lower calcium pulls the LSI down further. Take a fiberglass pool and a vinyl pool, each at the calcium and alkalinity it would normally run in season with a pH of 7.6. Cool them to 34 degrees or about 1 Celsius. And the fiberglass pool's LSI is a negative 0.76, and the vinyl pools is a negative 0.44.

Both are well outside the range, and the plaster pool in the last example was only at a negative 0.36. Same cold, same 180 days, with a bigger miss, and you see it in the spring. Fiberglass gel coat comes out rough or pitted. A vinyl liner comes out worn and wrinkling. Years ahead of schedule for both. Neither one grows back, and neither one can be fixed with chemistry once it's happened. Algae runs on its own timeline. Most algae blooms in a closed pool start in the fall, in pools that were closed while the water was still warm. The bloom starts, the water gets cold, and the green stays, so a pool that gets closed too warm can sit green all winter.

When the water warms in the spring, the algae blooms back again. And if the pool isn't opened in time, it turns into a swamp. In Kansas City, my team closes nearly every pool after October 15th and opens them all before April 15th. Closing early is what causes the green water, and opening late makes it worse. So that timing takes care of a large part of that algae threat. It's one more reason the pool can't be left alone in the winter.

The summer rules can't be winter rules. In season, a pool has one thing that can hurt it fast, and that's disinfection. You hold free chlorine at 7.5% of the cyanuric acid level and pH between 7.4 and 7.6, and the whole system is aimed at warm water, sunlight, and swimmers. Those ranges exist because of chlorine and bathers. Winter has its own set of rules, and the in-season rules don't apply to it. Water temperature decides the job. Above about 50 degrees Fahrenheit or 10 Celsius, chlorine stays on the job alongside the LSI. Below that temperature, the work is LSI alone. A chemist circle member named Dustin is a good example of the warm side.

He runs his pool year-round and doesn't swim in it over the winter. And where he lives in the southern US, the winters are mild. It's a plaster pool, no salt, 35,000 gallons or about 132,000 liters. When he submitted his numbers, the water was 79 degrees Fahrenheit, about 26 Celsius, with a pH of 7.6, and that's an LSI of a positive 0.12. He had never managed his water as it cooled, and he asked what he'd need to adjust. If you leave everything alone and let the water cool to 50 degrees Fahrenheit, or 10 Celsius, the LSI falls to a negative 0.26. That's still in range, but it's near the edge, and a pool that never freezes and never wears a cover still loses almost 0.4 of LSI to temperature alone.

His chlorine job doesn't stop either since he'll hold free chlorine at 7.5% of the cyanuric acid level all winter, so both jobs run at once. In winter, pH follows the LSI. It's set by what the water needs at that temperature. Take a fiberglass pool at 150 parts per million of calcium, the top of its range. At 34 degrees Fahrenheit, or about 1 Celsius, it takes a pH of about 8.4 to put the LSI at zero. An owner trained on the in-season range sees 8.4 and reaches for acid. And in January, under a cover, that's the wrong move, because the LSI is the number that matters. The levers aren't equal.

A tenth of a pH unit moves the LSI by a tenth, and doubling calcium or alkalinity moves it by about 0.3, which makes the pH the efficient lever and calcium and alkalinity the slower ones. Where a pool starts, when you close it, matters as much as what the cold does to it. Take two identical pools, both closed in October with the water at 65 degrees Fahrenheit or about 18 Celsius. One is closed in the middle of the normal ranges and the other at the high end of those same ranges, and the cold takes the same amount of LSI from both. The first starts slightly below zero, and by January it's at a negative 0.56, well out of range on the aggressive side.

The second starts slightly above zero, and by January it's at a negative 0.29, still in range, but barely. Same weather and same winter, and the only difference is where each pool started. Now, fix the first pool in January. Raising the pH brings it back into proper LSI range, and so does adding some calcium, and that second fix does something extra. It leaves the water with a base of calcium to take from instead of the plaster. That's why plaster pools often close at 500 parts per million calcium or more, vinyl at 250 and sometimes up to 300, and fiberglass at 150. None of those numbers is a fixed formula.

They're a system. The pH and how high it goes, the alkalinity, the calcium, the volume after a partial drain for the cover, and the rain and snow melt that dilute everything and change the volume again all interact, and every pool at every point in the winter needs its own answer. A winterizing kit is built the other way around. You've seen them at the pool store or online. It's sized by gallons and nothing else, so every 20,000 gallon pool, or about 75,000 liters, gets the same box, whatever its surface, its starting chemistry, or its temperature. There's nothing in the kit to adjust pH or alkalinity or calcium, and nothing in it accounts for the LSI.

The shock in it is often a non-chlorine oxidizer, which isn't a sanitizer, throws off the free chlorine test, and adds sulfates to the water. Most of what's in the box the pool doesn't even need, and you pay somewhere around 50 to 60 bucks for it. It's generic by design, which is how it sells at volume and why it does nothing to protect the surface of your pool. The gallons are the only thing that box knows about your pool, and even that isn't at all specific.

So what's actually happening to your pool during the winter? Plaster is where the damage is easiest to see, because plaster is mostly calcium and aggressive water goes after calcium. Over a winter, the cold water pulls it out of the surface and the plaster etches. The sun shelf pits, and the surface turns rough, and a rough surface holds stains and algae more easily than a smooth one. When the water warms in spring, in my experience, some of that dissolved calcium comes back out as a fine white dust on the floor. A pool that keeps running isn't exempt because colder water pulls calcium from the plaster whether there's a cover on it or not.

Do that for several winters, and the surface is passed what a brush can fix. And that's the road to a replaster every three to five years. Scale is the other direction. When the LSI runs positive, the water drops calcium onto the surface as a white crust. It's visible, so it gets noticed, and it usually gets blamed on hard water. Fiberglass has no plaster to etch. Its risk is the gel coat, the thin outer layer that gives the pool its finish. When chemistry goes wrong, the gel coat turns rougher pinted, and the repair to that surface is expensive. When winter chemistry is neglected, the vinyl liner wears early and wrinkles, which is the same water doing the same thing to a different material.

My ebook goes through it surface by surface. You'll get the whole deal. Metals work differently. Chrome, stainless, galvanized, and the heat exchanger in the heater corrode from low pH specifically, not from a negative LSI as a whole, so a winter program that keeps pH high protects them too. That applies most to a pool that keeps running where the equipment is still in the water. The loss in the surface adds up and it does not come back. As I mentioned, in Kansas City, my team closes everything after October 15th and opens everything before April 15th. And that window is the calendar for this walkthrough. If your winters are milder, go by your water temperature instead of the month, because temperature is what decides which job you're doing.

Every visit is the same panel. pH, total alkalinity, calcium hardness, cyanuric acid, temperature, and TDS. All six every time. I have six winters of data from my client's pools, and every one of those numbers drifts over the winter. And not only because of temperature, so nothing gets assumed that it will hold from closing. On a closed pool, two things make the math harder. A partial drain for the cover changes the volume, which changes every dose. Then if you have a mesh cover, rain and snow melt refill the pool and they carry almost no calcium or alkalinity. So every inch that comes back in dilutes everything and changes the volume again.

Anytime the water level moves, the dosing needs to be recalculated. So let's go month by month. In October, the water's in the 60s Fahrenheit, about 16 to 21 Celsius, and this is still the warm side of the rules. To keep that fall algae bloom from happening, free chlorine stays at 7.5% of the cyanuric acid level, with the cyanuric acid at or under 50. And the LSI is already doing the second job. Closing should happen when the water temperature is below the 60 degree Fahrenheit or 16 Celsius mark, so the pool goes under the cover cooler, with the numbers leaning to the high end of their ranges.

Calcium gets loaded at closing so the water has something to pull from other than the surface. Plaster often goes to 500 parts per million or even more, and new plaster most of all. Vinyl closes at 250 and sometimes up to 300 to make that pool's LSI work, and fiberglass closes at 150. How high the pH needs to be depends on the pool, since every pool is different. The closing LSI and the water level get written down, because everything after this gets compared against them. In November, the water temperature's usually in the 50s, or about 10 to 15 Celsius, and chlorine still has a job until the water drops below 50.

The LSI now drives the decision, so whatever moved since October gets corrected. A pool that never drops below 50 stays on this side all winter, chlorine and LSI together, and that's dusty. Situation. December puts the water in the 40s, about 4 to 9 Celsius, and chlorine's job fades, so from here the work is LSI alone. If there's ice, it gets augered through because a sample from the top of the ice isn't a sample of the water, and a corrective chemical dumped on the ice never reaches the pool. Ice can also change the TDS fairly dramatically, especially in salt pools, since freezing water pushes the dissolved salt out of the ice and into the liquid that's left, and the TDS in that liquid goes up.

TDS is one of the six numbers that go into the LSI, and on a salt pool it makes a real difference, so it gets measured every time, because assuming is how you end up with the wrong LSI. January is the coldest month, with the water around 34 to 40 degrees, or about 1 to 4 Celsius, and the lowest temperature factor of the year. This is where the pH target runs highest, sometimes well past 8. In my team's data, pH tends to drift up on its own over the winter, so the correction that comes up most is muriatic acid, back down to where the LSI wants it.

Sometimes alkalinity needs a push to bring the LSI up as the temperature falls, and calcium or bicarb is occasional when one of them has moved a lot. Most months is pH, and the others are the exception. February is when the thaw cycles start, so volume and dilution change, and any thaw means a retest. It's also where the turn begins. With calcium loaded, a rapidly rising LSI as the water warms precipitates the excess to the floor of the pool. So February's dosing usually calls for a fairly aggressive drop in pH to keep the LSI out of scaling territory. In March, the process reverses. The water warms from the mid-30s toward the 50s, about 2 to 10 Celsius.

The temperature factor rises, and the LSI climbs with no input from anyone. The risk flips from etching to scale, so pH is tightly controlled, and the LSI is allowed to drift up but not out of range. And the job is to catch it before it reaches a positive 0.3. April is when everything opens, typically before the 15th. Chlorine management returns as the pool goes back to the warm side, and my team opens most of the pools it manages in that window. I know all of this is confusing. Managing a pool in the off-season is a completely different animal than running one during swimming season. It takes an experienced eye, and that's why a membership in the chemist circle matters so much.

Every one of those months has its own numbers to check against, and the circle is where you send yours and get a response written for your water with a group of pool owners around you working through that same winter. If you have a question about your own pool in the meantime, send it through the Ask John page at poolsscientific.com.

Off season maintenance isn't really a thing in this industry. In most cities, you can't hire a company to do it, because the pool companies there don't do it, and it was never part of how the business works. Pool businesses work in the swim season. It's weekly visits from May through September with chemistry aimed at keeping people safe in warm water. And then in October, the pool closes. The closing visit happens, the winterizing kit goes in, the cover goes on, and as far as the business is concerned, the pool is finished until spring. Some pool companies hang Christmas lights to stay in business. The winter science isn't part of the standard training either, and the homeowner doesn't ask because the homeowner doesn't know there's a question to ask.

That's the way it's always been done, and I've been talking about it since the beginning of this podcast. Nobody is served by it anymore, and winter is where it's easiest to miss. The fast damage and its causes are months apart, and for the slow damage, years apart. I've spoken with people throughout this industry for years, the ones who see what pool companies do from the inside, and ask them whether anyone else runs an off-season program built on the LSI. Nobody could name one. As far as they know, and as far as I know, my team is the only company in the country doing it. I could be wrong, but to me that's a shame.

Year round in cold climates is a missing piece of this industry, and it should be as ordinary as a weekly visit in July. Pool owners deserve that level of care. Until it is, it falls to the owner. This is not where you want to make a mess of your pool. The mistakes don't announce themselves, they cost real money, and one winter is enough to do visible damage. The good news is that an owner can do this with the right guide and a second set of eyes, and doing it puts you ahead of almost every pool owner out there because they don't even know it exists. The guide is the ebook, The Science of Off-Season Pool Care.

I wrote it over a period of about two years. It's a working protocol written for the person who owns the pool. It explains the LSI in plain English, goes through plaster, fiberglass, and vinyl one at a time, and lays out the routine month by month, which takes a decent test kit and about 30 minutes. The links in the show notes at poolsscientific.com on the podcast page. The second set of eyes is the chemist circle. You send in your real numbers from your real pool, I verify and validate them, and you get a response written for that water. This is the key to success of the whole program because every pool is different.

The system has a lot of variables, and a winter leaves no room for guessing. It's also more than a place to send numbers. Winter is the stretch of the year when a pool owner has nobody to ask, and the circle fixes that. It's a group of pool owners from all kinds of backgrounds with all kinds of pools, and they talk to each other. They talk pools, they talk about their kids and their pets and whatever else is going on in their lives. I'm in there too, answering questions as they come up. There are a lot of good online groups for pool owners, and they're great for company.

Most of what you get though is somebody saying, Here's what worked for me, and maybe it'll work for you. Winter is a bad season for that kind of trial and error because a wrong guess takes months to show up on your surface. In the circle, the answer comes from a chemist, me, built on the science and your numbers, and nobody's guessing. I've managed this program for years across hundreds of pools. My clients have a pool company in their backyard through the winter, but most don't. Most pool owners never have anyone who knows their pool and checks its numbers. In the circle, though, you do. It doesn't stop when swim season does, and the details are at poolsscientific.com.

The ebook is the method, and the circle confirms you're applying it correctly. Together, they're the recipe, and they're what keeps a winter from turning into a repair bill. 95% of my clients here in Kansas City have opted in to this program. I don't push it. Once an owner understands what cold water does to a surface though, and sees that it's testable chemistry, the choice makes itself. It isn't okay to neglect a pool for half the year, and you know now what's happening under the cover. My team didn't know how to do this at the beginning. I didn't either, but I followed the chemistry and the science, and that kept the program on the tracks through the learning curve.

The first two or three off seasons were spent finding the extremes on either end of what worked and what didn't. My team struggled with calcium dust and with scale. We worked out when and why algae does what it does. We tested different chemicals to see what each one did for the overall health of a pool. That meant learning how to explain the program too, so a pool owner could see it as protection for their investment. Because that's exactly what it is. Over the last five years or so, that early learning curve became an efficient program that doesn't make a claim that the chemistry can't back up.

Every pool that's been in the program for four or five years has no new damage, 100% of them. Where a pool started with some damage, it hasn't spread or worsened. The pools that were new five or six years ago still look brand new, with no plaster damage, no gel coat damage, and liners that are pristine. The chemistry works, and the learning curve that my crew went through is why. And the clients that participate now have the confidence that their pool, that huge investment, is actually protected. Whether your pool is about to close, already under the cover, or running all winter, the next step is the same.

Test your water and run a full panel. PH, total alkalinity, calcium hardness, cyanuric acid, temperature, and TDS. Calculate the LSI and write all six numbers down because everything after that is comparing against them. Then bring those numbers to the chemist circle, where you get an answer from a chemist and not a guess, in a group of pool owners, and really cool people that are working through the same winter. If anything doesn't make sense, or you have a question about what we talked about in this episode today, the care of your pool through the winter, send it through the Ask John page at poolsscientific.com. I see every one.

A quick production note before I go today. If you happen to hear some ruckus in the background today, specifically the dogs barking, that's Poppy and Daisy, my two westies. They decided they wanted to be a part of the show today. And so they were. Thanks for listening today. 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.