What Else Could Possibly Go Wrong in Your Pool in Winter? Crystals, Ice, and Other Winter Surprises

Two weeks ago, I talked about what the water does to your pool under the cover in winter, and the protocol that my team runs to keep the chemistry in range so the pool doesn't sustain damage. Last week was hot tubs and why a spa is not a small pool. The timing for those episodes turned out to be appropriate for what we're going to talk about today. Those two episodes contain things that I can explain in a way that hopefully makes sense. This week is everything else. Knowing what's happening during the off season is great, but not everything that happens fits into a fixable category. We like to think we have a handle on things when it comes to keeping a pool or a spa safe during the winter.

What else could possibly go wrong? Well, quite a bit, as it turns out. Some of it is just strange, and some of it is completely avoidable. Some of it I can explain down to the molecule. Some of it I can explain pretty well but can't prove, and well, some of it I can't explain it at all. I'll label the things today as we go. Known means it's settled and you can be confident that you know it. Theory means someone has a reasonable explanation, but nobody's proven it yet. And where I know absolutely nothing, not a clue, we'll call that a shrug. I just don't know, and so I'll tell you.

Here are a few small ones then to warm you up. Ice floats. That's a known. The neighbor's pool looks better under its cover in February than yours does. I have an explanation for that, but I can't prove it, so that's called a theory. And what's living under the leaves on the top of your cover? Shrug. I don't think I ever even wanted to find out.

I'm John Cooper, and with you own a pool, service them. Run a pool company. This show is built for you. Real Chemistry, Real Answers. Every Tuesday.

Cold water is aggressive. The LSI says so, and episode 26 walked you through why a pool sitting at 40 degrees Fahrenheit, or 4 Celsius, spends the winter quietly taking calcium out of its own plaster. That's unknown. Well known, in fact, if you follow the chemistry. And then some pools open in the spring with calcium crystals growing out of the surface. Growing out of a pool that just spent four months dissolving. Dissolving and growing on the same wall at the same time. I don't love it when the chemistry does both. The people who have looked at these crystals most closely, as far as I can tell, are the team at Arenda Technologies, and what they report is the closest thing to a known that this subject has.

The crystals grow out of cement, plaster, pebble tech, the grout between the tiles. They don't grow on the tile face or the light or the plastic fitting, and they only form underwater. They've named at least four types of crystals sandpaper, needle, toenail, and flaky. I'd like to meet whoever named the toenail one. Sandpaper is the hardest to get rid of, and Arenda describes it as notoriously difficult to remove, even with acid washes. Each crystal has a microscopic hollow tunnel running through it, and the lab that looked at the needle ones called their structure skeletal calcite, which is a phrase I'd like to have on my business card.

Why they form is a theory, and Arenda calls it a theory repeatedly. Their own words include we believe or our theory so far, or it's still a mystery. Which I respect. Their explanation is that cold water, low in calcium and sitting still, pulls calcium hydroxide out of the cement. Calcium hydroxide is strongly basic. It gets carbonated into calcite, and the crystal grows, with material traveling up through that tunnel and landing at the tip. If that's right, it makes a prediction, and the prediction is about the LSI. In their cases, the winter LSI was very low, like a negative 1.2 or even below, which is a long way past the negative 0.3 floor.

It also predicts the fix. Load the calcium before winter, 400 or even 500 parts per million or more if there's a mesh cover, because rain and snow melt keep diluting it. That lines up with what my team does with plaster pools, which is about four or five hundred parts per million or more at closing. There's one thing you need to be aware of, however. Orenda sells a product for dissolving these crystals, and they say it can take weeks in cold water. The other explanation comes from Kim Skinner of On Balance in Aqua magazine in January of 2024. She disagrees, and she's pretty specific about it. She says the water isn't the problem, the plaster is.

New plaster leaches calcium hydroxide and calcium chloride, which is the accelerator plasterers use in cold weather, and it keeps leaching for months after the pool is filled, no matter how balanced the water is. That pushes the pH up, sometimes to 9.0 and often above it by spring, and pushes the LSI above positive 0.8, so calcium carbonate drops out of the water and onto the surface. And she rejects aggressive water as the cause because aggressive water takes calcium out of the surface. It doesn't put it back. Her evidence is reasoning in concrete industry literature, and there's no lab testing in the article. To be fair, Arenda has a lab look at the crystal structure, but that's the crystal, not the cause.

Neither article shows me a controlled test of why they form. I'll add something for the plasterers because it matters. Accelerator, otherwise known as calcium chloride, in cold weather is a trade-off, made so a pool gets finished before a freeze. And in the space of full disclosure, on balance promotes a startup method built around a positive LSI. They agree on one thing. It starts with calcium hydroxide leaving the cement. They disagree about whether the water pulled it out, which is Arenda, or the plaster let it go anyway, which is Kim Skinner. The annoying part is that a high pH when you open the pool fits both explanations, so it basically settles nothing.

What would settle it is the chemistry at closing, the age of the plaster, and whether loading calcium actually prevents it. My pools get their calcium loaded at closing, and crystals are extremely uncommon. That's suggestive. It isn't proof, because I don't have a group of pools that I didn't load to compare against, and I'm not about to run that experiment on a customer. So which one is it? Shrug with footnotes.

Winter dust is a different animal from the crystals. It shows up on older plaster, plaster that's long since cured, in water at forty degrees Fahrenheit or colder, four degrees Celsius. And the white dust doesn't appear until spring when the water starts to warm up. Arenda's explanation has to do with the warming, and they hedge it with we think. I hedge it too. In my experience, that is, however, what it does. But theory. Gypsum is a known, and you've heard this one in episode seven. Calcium sulfate, the crystals that crunched underfoot and cost a four-year-old commercial poolets a hundred and twenty thousand dollar plaster job and one of the technicians a hundred dollar boot.

The sulfate came from heavy use of monoprosulfate nonchlorine shock and sodium bisulfate dry acid. And if you remember, sulfate doesn't break down and doesn't filter out. The only way out is dilution. Acid does nothing to it because sulfate doesn't care about pH, which we discussed in episode 17. And on vinyl and fiberglass pools, I have actually seen formations on the fittings a few times, mostly the drain covers or return fittings. They're not cement, and Arenza says winter calcite crystals only grow on cement. So neither theory covers them. I've never identified what they are. Big shrug. One where I know the mechanism, and not the whole recipe, was at a client's pool a couple of winters ago.

They have a fiberglass pool. My team's last off season visit was in March, and the pool was pristine with the LSI managed the way we manage it. The alkalinity was at 70, the calcium was at 145. At opening in mid-April, just a month later, there was enough dust on the bottom to write my name in. The numbers at opening told the story. The pH was 9.2. The alkalinity was 390 parts per million, up from 70. And the calcium was 22 parts per million, down from 145 in March. I tested it three times that day just to be sure, and got the same numbers every time. That is what happens when the pH and the alkalinity get pushed way up in water that has plenty of calcium in it.

The calcium has nowhere to go but out, and it lands on the floor as calcium carbonate dust, which that's what the dust is. So what pushed it up? Eventually, my tech was shown six empty eight-pound bags of baking soda, forty-eight pounds of bicarb, added by our client after our March visit. I think that would do it. And I don't know that baking soda was the only thing that went in. And neither does anyone else. Well, I guess the client does. It could have been soda ash. It could have been any of a dozen other things. The mechanism is a known. The final recipe, however, is a big shrug.

By my math, the LSI of that water at opening was somewhere around positive one, which is past the positive 0.8 in the Kim Skinners theory. Same fingerprint, completely different cause. A high pH and a positive LSI at opening tell you something went wrong. They don't tell you exactly what. The reason I could work out what happened that quickly is that I had the March numbers. I knew what the pool looked like before and what it looked like after. Most of the time when crystals or dust shows up, nobody has a before. Somebody opens the pool and there's a yar. As for the customer, and it was a long story, we parted ways.

Everybody knows that ice breaks pipes. Fewer people think about what it does to a pool wall. Water expands about nine percent when it freezes. That's a known, and it's one reason a pool in January can struggle. But expansion isn't the main thing I see. The main thing I see is movement. A sheet of ice on a pool doesn't just sit there politely. Over the winter it moves back and forth, up and down, and when it moves, it hits the walls and it grinds on whatever it's sitting on. Slowly. Glacial almost, which is the word that seems to fit here. On a sun shelf, that grinding is very hard on the plaster.

In my experience, ice under about an inch thick or two and a half centimeters isn't really a big deal. Over an inch, though, I start seeing physical damage from the ice layer moving. That's from watching it happen on plaster pools in my area. So officially the one inch number is a shrug with a lot of field hours behind it. At opening, it looks like two things. Tile popped or cracked at the waterline, on the main walls and around the shelf. And on the shelf itself, plaster flaked or chipped off in patches. A sun shelf is a shallow ledge, so it holds a shallow sheet of water, and a shallow sheet freezes solid.

That solid sheet sticks against the walls and the tile and the floor of the sun shelf and it pushes. And the water that soaks into the shelf plaster freezes and thaws in the surface over and over, which is slow damage of its own. None of that is a mystery the way the crystals are. I've seen damage when it wasn't prevented. On a plaster pool, I like to drain below the shelf, with a solid cover just below it, with a mesh cover three to four inches below the shelf, because rain and snow melt pass through the mesh and the level climbs over the winter. And I want the water to be still below the shelf when it does.

Keeping the level down also keeps ice off the waterline tile, which is the other thing I'm protecting. The level doesn't hold itself. My team checks it on the monthly visit and pumps it down when it's necessary. You'll hear people argue about this. Some pool owners and pros leave a few inches on the shelf and they say they've never had a problem. And others say the tile line is the only real risk. I haven't seen a test from anyone. My theory is that part of the argument is thickness of the ice, because under an inch I don't see it much either. That's a guess, so let's label that a theory.

Episode 27 was hot tubs. I hope you got a lot out of it. A spa that's attached to your pool, especially one that sits above it and spills over into it, needs a different winter than the pool does. A spa left full for the winter is a very large, very expensive ice cube tray. The main reason I drain a plaster spa is the same reason I drain below a sun shelf. Ice moves and when it moves on plaster, it grinds. The second reason is the plumbing. Water left in the jets and lines freezes and expands in places that can't flex, and what I've seen is cracked plumbing, cracked jets, and cracked fittings.

Inside the spa I've seen plaster and tile broken up. Well, not for many of my clients, of course, just to be clear. A raised spa is the worst version of this. It's more exposed to the cold air on all sides, and its plumbing sits outside the ground's warmth, above the freeze line. So I drain it completely. The water goes out to waste, the lines get blown out, and the spillway and its plumbing get blown out with them and then plugged. Then I blow air through the equipment drains until the floor is dry. A dry floor matters because ice sitting on plaster is the thing that hurts it, and a spa with a film of water on the floor still has ice to sit on.

The spa's own cover goes on and then it stays on all winter. So make sure you check it. A drained spa that's collecting rain or snow isn't a drained spa anymore, so the cover and the floor are worth looking at during the winter. You'll hear opposite answers about this too, both said with confidence. One of them says to leave the water in the spa so it doesn't pop out of the ground. For a raised spa sitting above grade, I don't worry about that, and I haven't seen it happen on any spa I've ever drained. That one's a known from experience. A deep spa in the ground with a high water table might be a different story, and I suppose there it could happen.

It is a real risk, but a very limited one. The drained and dry spa is the one that makes it to April in one piece. Ice damage to a sun shelf or a spa is slow and it's quiet, and it sits under a cover until April. In my experience, every case of it I've seen was preventable with water level management. That's one of the best arguments I know of for having someone look at a pool in the middle of the winter. The level rises with rain and snow melt. A shelf that was safely dry in November might not be in January. Somebody has to see it, and somebody has to pump it down.

My team's off-season program includes that on the monthly field visit. If you're doing it yourself, though, it's the same job. On a plaster pool with a sun shelf, check the level after a storm and keep it below the shelf, just below with a solid cover and three to four inches below with a mesh cover. On a drain spa, check the cover and the floor. A lot of the winter damage I see is simply a pool that nobody looked at for five months. That kind of damage can't be fixed with chemistry. It can only be prevented by someone looking. That's a known. Big time. But crystals are the other kind.

Nobody has solved those, and until somebody does, I'm measuring the chemistry at closing and writing it down. Because the before is the part almost nobody has. If crystals show up, photograph them, keep a small sample, and don't act on a guess, including mine. And if you've seen a formation on a pool service, a drain cover, a fitting, anything strange or out of the ordinary, send me a photo through the Ask John page at poolsscientific.com. I'd like to see it. We can figure it out together. Most places where you can ask a pool question will give you an answer every time, whether or not anyone actually has one.

The chemist circle works the other way. You send your readings and you get a response written for your water, built from your own numbers and the LSI map. And when the honest answer is that nobody knows yet, that's the answer that you'll get. There's a group chat too where members talk about pools and plenty of other things, and I'm in it regularly. Ice floats, still known. And whatever's living under the leaves on your cover is still a shrug. Real answers for real pools, no guessing. 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.