Why Swimming Pool Temperature Matters

Swimming pool temperature impacts the water chemistry in a big way. This article will outline a few reasons why.

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Pool Temperature and Water Quality

Let's divide "water chemistry" into two different categories, and start with water quality:

water quality, water balance, LSI balance, orenda chemistry, pool chemistry

Warmer water means more chlorine demand. This is not due to sunlight degradation of chlorine, though summertime does mean more hours of direct sunlight hitting an outdoor pool. The real reasons for higher chlorine demand are because both living and non-living contaminants are more prevalent in warmer water. Algae and bacteria, for instance, are living contaminants that chlorine must kill (sanitization). Warmer water means those microorganisms can reproduce more rapidly, therefore problems like algae are more prevalent in the summertime. If you have a pool, you have probably already know this.

To kill any living organism, chlorine needs a certain amount of contact time (CT). CT is influenced by pH (in a non-stabilized pool), and water temperature. Why? Because warmer temperatures increase chlorine performance. Here's a quote from the water research center:

To determine CT required we need to know:

  1. 1  The minimum temperature of the water during disinfection. The minimum temperature of the water in the chlorine contact chamber must be monitored. Minimum temperature is used because chlorine’s ability to disinfect becomes less with lower temperatures. By using the lowest temperature of the water when determining CT required, we know that the disinfection that occurred was at least as good as the lowest temperature allowed.

Non-living organics, which are the bulk of the pool's oxidant demand, are more prevalent in warm water because more bathers use pools in the summer than the winter. Hotter days mean more swimmers and more non-living organics in the water, like sunscreen and body oils. Between these organics and the living contaminants like algae, warm water has a higher chlorine demand than cold water.

Chemical reactions generally work faster at higher temperatures. Water chemistry is no different. So chlorine will work better and faster, and as a result, chlorine will also get reduced faster.

Chlorine Demand

Non-living oxidants dominate the chlorine demand list. Non-living organics are the most common, followed by nitrogen compounds, then metals. Of these, metals are the easiest for chlorine to oxidize, so they are the first to go. Nitrogen compounds like ammonia and urea are much more difficult to oxidize, and require a breakpoint chlorination process where chlorine combines with these compounds, then destroys them. Here's the breakpoint chlorination curve to remind you:

breakpoint chlorination curve-2

Compared to the oxidant demand, the percentage of actual living contaminants (algae, germs, viruses, etc.) in swimming pools is very small. Yet chlorine is best at disinfection, and comparatively weak as an oxidizer. This is the main reason why our Second Pillar of Proactive Pool Care is about addressing the oxidant demand by complementing chlorine.

Pool Temperature and Water Balance



Water balance is measured using the Langelier Saturation Index (LSI), which tells us the saturation equilibrium of calcium carbonate (CaCO3) in water. Water temperature plays an important role in LSI balance. The colder the water, the lower the LSI. In other words, colder water will be more aggressive, because it requires more calcium carbonate in solution to maintain balance.

Related: LSI Balance and Calcium Management (PIllar 1)

Water Temperature is the most ignored LSI factor

Of the six LSI factors, the easiest and most reliable factor to measure is water temperature. You just need a thermometer; no fancy test kit required. Most people–pool professionals and homeowners alike–are unaware that water temperature makes as big of a difference as it does. Therefore, temperature is the most ignored LSI factor. We were unaware of water temperature's LSI impact too, until we built the Orenda App and saw it for ourselves. Check out the difference that temperature makes in the screenshot below:

cold water lowers the LSI, orenda calculatorJust a 35ºF drop in pool temperature fundamentally changes the LSI to aggressive. It was perfectly balanced at 80ºF, and at 45ºF, the water will etch a surface (and fade a vinyl liner, degrade a fiberglass gelcoat, etc.). Fortunately, water temperature does not drop down so dramatically overnight. It takes weeks, if not months, for a swimming pool to drop 35º, which gives pool owners and operators time to prepare and predict how to handle it. And that brings us to our next point...

Water Temperature is the moving baseline for an LSI strategy

Because we can predict that temperatures drop in the autumn and rise in the spring, we must act accordingly. Use the pool's temperature as the baseline for every chemical treatment. This baseline moves, albeit slowly, and it allows you to adapt your chemistry throughout the seasons.

Colder water will need more calcium and higher pH to stay balanced. Warmer water will not need as much calcium, and you can have less alkalinity too (unless you're using trichlor as your primary chlorine). Since pH will naturally rise every week thanks to physics, consider where you put your calcium hardness and your alkalinity at different times of the year. This, of course, depends on the climate where you live. Pools in the upper Midwest and Northeast will freeze, and therefore must have more calcium hardness than pools in hotter climates.

So you can adjust your alkalinity, calcium hardness and pH. But pool temperature, apart from cranking up the pool heater, is something you cannot easily adjust, especially in the off-season. Instead, we at Orenda strongly suggest predicting the worst-case scenario for your off-season. If you're in a climate where the pool is guaranteed to freeze, that means setting your LSI parameters to be balanced at 32ºF (0ºC), so when the temperature drops that low, you have enough calcium and alkalinity to keep the water happy. If you don't, there will be consequences...

Cold water consequences

We have other articles about cold water and the LSI. Since water becomes more aggressive as the temperature drops, ignoring water temperature leads to costly consequences. Almost all pool damage occurs during the winter. Most often, we see these consequences in swimming pools with cementitious finishes (plaster, quartz, pebble, etc.). There are consequences in vinyl liner and fiberglass pools too: degradation and fading of the surfaces. But let's hone in on the cementitious pools, because those types of finishes chemically interact with water when the water temperature drops.

Calcite crystals underwater

The most common winter issues we see are calcium-related. Most people see calcium deposits and immediately think "SCALE!!" But winter damage is hardly ever scale. These calcium deposits are usually calcite crystals or winter dust. These issues occur because of a low-LSI violation, and water had to seek equilibrium by dissolving calcium from the cement pool surface. When it did, the pH of the water spikes high (because calcium hydroxide in the cement has a very high 12.6 pH), and the calcium hardness rises in the water slightly. The water finds equilibrium at the coldest point, and stops eating the walls.

Winter dust

In the case of calcite crystals, we believe the stagnant water provides a unique environment where calcification "grows" out of the cement surface as water eats away at it. The high pH and stagnant water seem to allow the calcium to re-calcify after it has been dissolved from the plaster surface. In the case of winter dust, the problem occurs in the spring as the water temperature begins to rise again. The pool will go from its coldest point, up 30ºF or more, which makes a dramatic difference on the LSI. The water becomes over-saturated with calcium because the temperature is warmer now, so it deposits calcium in the form of winter dust.  By the time the pool is opened, there could be winter dust everywhere.

The bottom line: cold water will be hungry for calcium, and if you don't feed the pool enough calcium before the temperature drops, it will feed on your pool surface. If you have a vinyl liner or fiberglass pool, the water will degrade those surfaces looking for calcium on the other side. This is why it's so important to winterize a pool properly, with the LSI in mind. If you are debating whether or not to have winter pool service, we recommend you do.

Orenda Procedure: How to Winterize a Swimming Pool

Warm water consequences

how to prevent scale in pool heaters

On the opposite side of the temperature spectrum, we have scale-forming problems. The higher the temperature, the more likely calcium carbonate is to fall out of solution as carbonate scale. This is why you first see scale in a warm spa, or on the sunny tile line, or spillway. Calcium will always precipitate in the hottest places first. The higher the temperature, the higher the LSI. So water temperature determines where carbonate scale will form first.

Calcium Flakes

If you have a saltwater pool, your salt chlorine generator will scale before you ever see scale in the pool itself. The consequences of this are what we call "snowflaking" (see photo). White chunks of calcium carbonate flake off the salt cell and blow into the pool through the return inlets. This is a very common LSI violation. Temperature is one of the factors that cause saltwater calcium flaking. Salt cells have very high pH inside, as well as heat caused by electrolysis.

why salt cells form calcium flakes

Similarly, heat exchangers will scale too. Again, because of the high temperature. LSI violations occur locally, not everywhere in the pool system at once. So carbonate scale will precipitate wherever there is an LSI violation.  

If you have "scale" on the bottom of your pool, it is probably not scale. Sure, it is probably calcium carbonate, but you must ask how it got there. Most often, it's discoloration/uneven carbonation from abusing acid, which created a localized low-LSI violation, which rebounded into a high-LSI violation, and white dust or calcification occurred. The reason we know it wasn't scale is because the bottom of your pool is typically colder water than the surface. Since scale always precipitates in the hottest places first, the bottom of the pool is not a likely place.

But most often, if you have a scale issue, it will present itself on the tile line and spillway (if you have one). To soften and remove it, it's just a matter of balancing the LSI, raising the water level, and letting SC-1000 go to work.

Orenda Procedure: How to Soften and Remove Scale

How to deal with changing water temperature

As we mentioned before, water temperature is the moving baseline for a good LSI strategy. No matter what it is, or where it is going in the next few weeks, it is wise to take temperature into account. Deal with it by using a thermometer and simply inputting the temperature into the Orenda App. It's really that easy.

For winterization, plan ahead, knowing that the temperature will drop. Plan for the coldest water and ease into winter with the LSI in mind. The same thing goes for the springtime when the temperatures are rising. As long as you use a thermometer and include temperature in your chemistry strategy, you're doing your part to make sure that pool stays LSI balanced year-round. If you have questions, leave a comment below, or you can request training with us.


Water temperature is the easiest of all the LSI factors to measure, yet it is ignored by many. Simply knowing the temperature and inputting it into the Orenda Calculator™ is beneficial to know how balanced your water is. Then, expect temperatures to change over time, so that you can modify your strategy in different times of the year. 

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