Proactive Pool Winterization: How to Prevent Crystals

Cold water is more aggressive than warm water, because of how water temperature lowers the LSI. This is why most pool surface damage occurs during the winter. And no, the calcium you see in the spring time is probably NOT scale. Problems can be prevented by taking an LSI-based approach to pool winterization. In this article we will explain how.


How to Prevent Winter Problems like Calcium Crystals

Problems caused by cold water are very common, and we have written quite a few articles about them. Some of them are how to remedy the problems, others just describe the problems. Today we are writing about how to prevent the problems. After all, Orenda promotes proactive pool care.

Prevention is far a more relaxed and affordable strategy than remedy. So when it comes to backyard pool problems that occur over the winter, how you winterize the pool in the fall matters. A lot.

Here is a summary of this entire article: feed your pool enough calcium and alkalinity to maintain LSI balance in the coldest water during the winter. If you have a mesh cover (or no cover), you must take rain and snow into account, because it has no calcium in it.

If your pool will freeze, that means preparing your pool for 32ºF (0ºC), and assume dilution will occur. You can use the Orenda App’s LSI Calculator to discover what your pool will need for the winter. Think of it like feeding a giant bear (the swimming pool) before it hibernates (under the safety cover all winter). If you don’t feed it before the water temperature drops, that bear will wake up in winter to feed, and your pool's surface will be first on the menu. 

Yes, even vinyl liner, fiberglass, and painted pools. The LSI does not discriminate…aggressive water looks for calcium regardless of the pool surface type. The consequences may differ, but there are always consequences to ignoring the LSI.

Related: Four Things to do When Opening a Pool with Calcium Crystals

How does cold water impact water chemistry?

LSI GRADIENT , orenda LSI, orenda startupCold water lowers the LSI, meaning calcium carbonate (CaCO3) is more likely to be dissolved. Cold water, therefore, needs more calcium than warm water if it is going to maintain LSI balance. The opposite also holds true…hot water cannot retain as much calcium and is more likely to deposit carbonate scale. Temperature is why the first places to form scale are always the hottest places—the salt chlorine generator (high pH + heat), the heat exchanger (heat), and the sunny tile line or spillway (wet/dry evaporation + heat).

Remember: water always wants to be in equilibrium on the LSI...but it cannot be greedy. Too much calcium forces water to give away the excess calcium as scale or dust. Too little calcium, and water will stop at nothing to find calcium until it is happily saturated with it.

Cold water needs more calcium. So feed it more calcium in the winter, or it will take it.

What really happens under the pool safety cover?

pool-winterizationLet’s first talk about a “traditional winterization” procedure. All is calm and quiet…the pool is dormant, not circulating, and still. The water level has been lowered, anywhere from a few inches below the tile line to maybe 18 inches (to get below the return inlets). The pipes were blown out and plugged, so it’s just water sitting in a bowl, undisturbed.

As the temperature drops, the water gets hungrier for calcium. Eventually, the LSI drops low enough (below -0.30) that the water “wakes up” to feed on calcium anywhere it can find it. Sorry in-ground pool with a plaster finish…that means your surface. The etching is often visible, but can also be minimal. Why? Because water can easily feed itself, but it stops eating once it’s happy. Remember, water cannot over-saturate itself. It just feeds until it's full.

With vinyl liner and fiberglass pools, however, the cold water never gets the calcium it craves and gets increasingly more aggressive…and stays aggressive throughout the winter. That’s prolonged exposure to hangry water. (By the way, for you non-millennials out there, hangry = hungry + angry). These surfaces fade and suffer permanent damage because of it.

So that's the chemistry going on under the pool cover all winter long. Either the water is balanced, or it's trying to balance itself. Beyond just balance, keeping the water clean is also a challenge.

Winterization Kits ignore the real problem

A traditional winterization procedure typically involves some sort of chemical package, or "kit." Usually this includes sanitizer, a long-lasting algaecide, some antifreeze maybe, and possibly a sequestering agent. A winterization kit is a cocktail of chemicals that don’t address the actual problem we’re writing about here. In fact, none of the items in the winterization/closing kits that we have seen include calcium, nor anything else to balance the LSI. It’s astounding.

So you fill your pool with a combination of all sorts of products designed to keep the pool clean–or at least “not green”–until it is reopened in the spring. But realistically, cold water slows down almost all chemical reactions anyway, so these products, just like Orenda products, do not work well in cold water. That’s a conversation for another time. Let’s get back to calcium saturation.

Calcium deficiency is the real problem

During the winter, water must find LSI balance at the coldest temperature (as low as 32ºF, or 0ºC). If the water is deficient in calcium (or other factors on the LSI like alkalinity and pH), that's a problem. So water steals calcium from the surface–which has a high pH–which increases the calcium hardness and raises the pH. This is why most swimming pools have a very high pH when they are opened in the spring.

But when the water begins warming up again in the spring, that excess calcium and high pH may, in fact, become a problem. Not because you have too much calcium, but because you will have too high of an LSI. There is a difference!

Winter Dust

calcium dust2With a traditional winterization that ignores LSI, the pH will have spiked high during the cold months when calcium was stolen from the plaster. That high pH balances the water when it's at its coldest...but drives calcium out of solution as the water warms up. The result is winter dust. Winter dust is calcium carbonate, just like scale. But unlike scale, winter dust was actually caused by a low-LSI problem during the winter. It only falls out of solution when the water warms up, so it's like a generation of delayed plaster dust.

In vinyl liners or fiberglass pools, the consequences are different. But rest assured, there is damage occurring to these pools if you are deficient in calcium.

Calcium [Calcite] Crystals

Calcium crystals -calciteCalcium [calcite] crystals are an increasingly common problem in plaster pools that are winterized. We do not know why they crystallize and harden, but we know three things with absolute certainty:

  1. Calcite Crystals form in cold, low-LSI water,
  2. There are at least three distinct types of crystals that we know of, and
  3. The types of crystals we know of grow out of the surface. They are not deposits from the water (like scale or dust would be).

There is a lot more about the crystals that we have yet to learn. For example: why do they harden and crystallize…when other pools in similar conditions just have winter dust? How come some crystals are easy to break down and clean up with SC-1000 or lowering the pH, but some are super stubborn? Why do some forms of calcium crystals form in some parts of the pool, but not necessarily everywhere, while other types of crystals form everywhere?

We just don’t know yet. But we DO know how to prevent both crystals and winter dust.

How to Winterize a Pool, the Orenda Way

We all want to keep a pool from turning green in the winter. But unlike the conventional method, our first priority is the LSI. After that, we can handle the easier task of keeping water from turning green. So our winterization procedure is as simple as balancing the LSI for the expected cold water, and removing nutrients that can flip the pool green.

Related: Orenda Winterization Procedure

Raise Calcium Hardness

winterization chemistry1

Figure 1: Just dropping the temperature on this typical saltwater pool makes balanced water become aggressive.

Find out how much calcium hardness your water will need when the temperature gets to its lowest point. Test your water chemistry and input it in the Orenda app. Then lower the water temperature on the right side of the calculator, and figure out how you want to get the LSI on the right to be green…something between 0.00 and +0.30.

Do not be afraid to raise your calcium to something way higher than you would operate in the summer. 500 - 550 ppm+ is great! If your pool freezes, 400 ppm calcium hardness is the bare minimum in our opinion. Keep in mind, rain and snow will get into your pool (unless you have a solid cover), and this precipitation will dilute your water.

Rain and snow contain zero calcium, and usually an acidic pH. It’s super hangry water (in technical chemistry terms) that can dramatically change your water chemistry. Because of this dilution and cold temperature, do not be afraid to add more calcium in your pool for the winter. The water will need it.

Raise Alkalinity

winterization chemistry 2Figure 2: To compensate for the future cold water, we significantly raise calcium hardness and alkalinity.

When you select a desired calcium hardness level, it should be the highest level you are comfortable with, with a minimum of 400 ppm if your pool can freeze. After that, if your LSI (at the lowest expected temperature) is not +0.00 to +0.30, start adding alkalinity on the calculator. 

Important note: Do not add sodium bicarb and calcium chloride on the same day. Perhaps add calcium the same day you add PR-10,000, a week before closing day. Then add bicarb on the last day with CV-600 enzymes before you put the cover on. Or the other way around, whatever you choose. Always pre-dissolve dry chemicals in a bucket to make sure they don’t just fall to the floor and stay there.

Temperature Swings

winterization chemistry3

Figure 3: Temperature will eventually rise back up in the spring, which WOULD be a scale-forming condition.

Before you ask, yes, we are aware that pools are closed with warmer water than it will be in the winter. And yes, that’s absolutely a valid point. So let’s talk about it.

If you want, you can ease into winterization with multiple visits to the pool as the weather continues to cool. Consider a “winter watch” program if you are not already doing so. All the calcium and bicarb does not necessarily need to be added in September and October. You just need to get them in the water ahead of the temperature drop. Multiple visits have benefits in themselves, as you can test the water and gauge progress.

On the flip side, when the water warms up, don’t freak out about super high LSI with 80ºF water, because 

  1. the pool was most likely diluted by rain and snow over the winter,
  2. if it wasn't, the pool is about to be diluted when we refill the pool in the spring, and
  3. water will not go from the 30ºs to the 80ºs overnight. We have time to ease out of winterization just like we eased into it.
winterization chemistry4

Figure 4: Dilution lowers the calcium, CYA and TDS, but we can also reduce alkalinity and pH with acid.

Changing habits can be painful, but certainly not as painful as dealing with calcium crystals or failing liners.

Clean and Not Green

  1. To keep the pool clean, we recommend removing phosphates with PR-10,000 about a week before closing the pool. When the dust settles, vacuum the pool and clean the filter (you should clean the filter during winterization anyway as a best practice).
  2. When the water is lowered below the return inlets, and the lines are blown out, purge the pool with CV-600 enzymes. The exact dose for your purge (in fluid ounces) is found on the results page of the Orenda App’s LSI calculator.  If you want to add a sanitizer, feel free, but shocking is unnecessary. Just be sure that what you add is non-stabilized chlorine, like liquid chlorine or cal hypo.

So that’s how we keep pools from turning green during the winter, and it has proven to be very effective.


LSI First, range chemistry second. Predict the cold and prepare for it, or else your pool will suffer the consequences. That is the proactive way to winterize a pool.

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