The vast majority of damage done to a pool occurs during the winter, or initial filling/startup. And not just damage to the the pool plaster, but pool equipment can be corroded too. Etching and corrosion are more likely to happen in cold water than warm. This article will explain, with the help of the Orenda App's LSI Calculator. It's all about the LSI.
Cold water lowers the LSI and makes pool water more corrosive
Remember the Langelier Saturation Index (LSI)? It's the index that measures the saturation balance of calcium carbonate (CaCO3). Rather than just something you can test for, the LSI accounts for water temperature, pH, alkalinity, calcium hardness, stabilizer and TDS, and needs to be calculated. Our test kits can measure most of those factors...but how often do you actually test water temperature? Oh, you don't? Until we understood the LSI, neither did we. Well as it turns out, water temperature matters. A lot. And it's the most predictable factor of them all.
Temperature really matters to the LSI because calcium is more soluble in cold water, unlike most other solids. If you think about something like sugar, water can hold more of it when heated up...but the opposite is true for calcium.
External Resource: For more info about the relationship of LSI chemistries, read this.
If water temperature is lower, the LSI will be lower, and therefore the water will be more aggressive. With seasonal changes in temperature, the LSI change can be severe. For example, let's say the normal temperature of the pool during the summer is 84ºF. What happens in the winter, when the water gets down to 32ºF and freezes?
Related: Three Types of Calcium Dust
Calcite crystals can form in the winter, and came from the plaster surface! That's calcium that belongs in your plaster...but since the water was cold and the LSI was low, the water extracted this calcium right out of the walls. As of now we still do not know enough about these crystals (plural, because there are several varieties of calcium crystals) to know why they harden and remain on the surface. Some are brittle and break off easily...whereas other types of crystals are stubborn and almost impossible to remove without physically grinding them down. Crystals have become an ever-growing problem in cold climates like the American Northeast and upper Midwest.
Another form of winter damage is what we call winter dust. It is characterized by the formation of calcium carbonate dust that only appears when the water gets warmer. The cold water extracts calcium from the cement in the plaster surface to reach equilibrium on its own. But as the water warms up, the LSI changes, and at some point it precipitates calcium carbonate as dust. It's similar to scale in that regard, but caused originally by a low-LSI violation in the winter. Here are some photos of winter dust:
But winter is not the only time the pool feels cold water. What about startups? What about an initial fill from the hose, which is usually cold ground temperature (~50-60ºF)? For these situations, let's use Orenda's LSI Calculator app.
Changing nothing but temperature from 84º to 50º drops the LSI from nearly perfect to corrosive.
Pool Startups almost always involve cold water
Unless you're filling the pool with water from a truck that has been sitting in the sun for hours, most fill water comes through pipes in the ground (garden or fire hoses). Depending on where you live, ground temperature can vary some, but usually it is around 55º-65ºF. If you have no water thermometer, we recommend you buy one and use it before every fill.
We have written extensively on the importance of a good pool startup and how to prevent pool plaster problems. In short, the LSI really matters, so you have to account for factors like temperature when deciding what to add to the water that initially fills a new or freshly resurfaced pool. It's critical to the integrity of the surface.
Closing and Winterization, the Orenda Way
If you're in a climate that freezes, pool owners deserve proper water chemistry to compensate for the cold temperature. Bringing the temperature down on the LSI calculator, for instance, shows us that we need a higher-than-normal pH and/or alkalinity, or calcium during the winter to prevent corrosion and etching. The best one, in our opinion, is the most consistent of the three chemistries: calcium hardness. Colder water keeps calcium in solution easier (hence the cold temperature lowers the LSI, making calcium carbonate more soluble). If you winterize a pool, chances are you are lowering the water level anyway, which will allow for dilution in the spring. You need more calcium than 400 ppm in our opinion; an opinion backed by the LSI, and results in the real world.
If you have a mesh cover (or no cover), rain and snow will eventually end up in the pool, and can drastically change the chemistry. Combine that with dead leaves and pine needles on a cover or in the water, and the pH is even more acidic. Remember, rain has zero calcium hardness and a low pH...it makes your pool more aggressive. Combined with cold temperatures, and you have a recipe for etching and crystals. Calcium Hardness is your best friend to get through the winter.
We all need to rethink how we treat pools year round, and make sure we are within the safe range of the LSI throughout the year. LSI violations are the number one cause of plaster and equipment damage. At the risk of repeating ourselves too many times, let's leave it here: pay attention to the LSI year round.