Here in Australia, we have the luck of not having to deal with extreme drops in temperature or even have to deal with our swimming pools freezing. This is one of the reasons why many pool winterization guides online can wouldn’t make much sense to us here down under. We don’t have to worry about blowing out our pool lines or adding anti-freeze to our pools to deal with the winter.

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Here’s the situation. You’ve perfectly balanced your pool water before closing it for the winter and your slipped on your pool cover, protecting it from rain and debris and you’re expecting that it will be stable for the next few months. It is worth noting that in a perfect vacuum, this will be correct, but we don’t live in a vacuum and one of the things that we tend to forget is that temperature actually plays a large role in our pool water chemistry and that’s well reflected in the LSI or Langelier Saturation Index.

We’ve gone through LSI before on one of our blog posts and basically, it’s the overall condition of your pool water. Just to refresh your memory, the four main components that add up the LSI are your Calcium Hardness, Total Alkalinity, Total Dissolved Solids (TDS) and your Temperature. Over winter, a well-balanced and protected pool should maintain the first three variables since they don’t move much, but temperature can actually drop quite low over the coming months and can turn your pool water from perfectly balanced to perfectly corrosive.

Here’s an example taken from our explainer article on LSI.

Perfect / Near Perfect LSI

pH: 7.3
Temperature: 84F or 0.7 Temperature index
Calcium Hardness: 200 or 1.9 Calcium index
Total Alkalinity: 100 or 2.0 Alkalinity index
TDS Level: < 1000 or TDS index of 12.1

Use the formula pH + Temperature Index + Calcium Index + Alkalinity Index – TDS index.

We’ll get: 7.3 + 0.7 + 1.9 + 2.0 – 12.1 = -0.2 which falls within acceptable levels when we’re talking about home pool LSI index. We shouldn’t be expecting any corrosion or scaling to happen to our pool.

Now, let’s take into account winter temperatures. For simplicity’s sake, let’s take the winter temperatures here in Wyoming, NSW which averages around 50F and plug that into the computation.

pH: 7.3
Temperature: 50F or 0.3 Temperature index
Calcium Hardness: 200 or 1.9 Calcium index
Total Alkalinity: 100 or 2.0 Alkalinity index
TDS Level: < 1000 or TDS index of 12.1

Use the formula pH + Temperature Index + Calcium Index + Alkalinity Index – TDS index.

We’ll get: 7.3 + 0.3 + 1.9 + 2.0 – 12.1 = -0.6

As we can see, at colder temperatures, the LSI is already at -0.6 which is already at the corrosive level. And once this starts to happen, some funny things can happen to our water chemistry. This is one of the reasons why even if your pool is well balanced and protected during the start of winter, we still recommend doing periodic testing of your pool water over the break.

Countering this corrosion factor is actually quite simple. It just requires a bit of pool chemical gymnastics, but just with a quick look we can tell that there’s only one level that we can adjust without messing everything up and that’s your calcium hardness levels. I mean, we can’t anticipate the temperature drop by raising our alkalinity levels because that will just pull everything else out of alignment and for sure we can’t do anything about the temperature unless we leave our pool heaters and pool heat pumps on over the winter.

This leaves us with the calcium hardness levels, which we can adjust to be on the high side before closing our pool. While this may put us on the borderline of scaling, this is much safer and easier to deal with than risking corrosion. If we take the example above, we can safely raise our calcium hardness levels to about 400ppm and stay within the optimal LSI levels Pre-Winter, During Winter, and once we open the pool. And once we re-open the pool, if the calcium levels are still high, we can simply lower it.

P.S. Don't worry too much about scaling over the winter, remember, one of the factors for scale formation is high temperature.

Closing your pool for winter in Australia can be as simple as balancing your pool water, applying some algaecide, and some phosphate remover and we should be good with some minor monitoring over the winter. The time it starts to get complicated is when the weather starts to get cold, and this is when we should compensate by increasing our calcium levels.

Another practical advantage to this (and we have tested it here at the office!) is that pool water (or any water for that matter) with high calcium hardness levels WILL NOT FREEZE, we’ve tried this by raising the calcium hardness of a water sample and placing it in the freezer. This is a rather extreme example but if a glass of water with high calcium hardness won’t freeze inside a freezer, then a large body of water like your pool won’t freeze!

We also have a number of blog posts dealing with pool maintenance during winter / off-season which we have listed below:

• Looking for a winterizing guide? Check out this article here.
• Confused about winter pool chemicals? Here's every pool chemical you'll need for winter.
• Have a salt water chlorinator? Read up on the relationship of salt water chlorinators and cold weather.