A heatwave adds three things at once: more sun, which destroys free chlorine; warmer water, where algae grow faster; and more bathers, bringing sweat and sun cream that use up chlorine. The result is that chlorine that used to last days is gone in hours.
Why chlorine "disappears" in the heat
What to watch
- Test chlorine and pH every day. With lots of swimming, twice is better. In communities of more than 30 homes, routine checks are twice a day regardless of the weather.
- Add chlorine at dusk, not at midday, so the sun doesn't destroy it before it gets to work.
- Filter for longer. A widely used rule of thumb: as many hours as the water temperature divided by two (at 30 °C, about 15 hours). Include the middle of the day, when the pool is used most and the water warms up most.
- Check the stabiliser. In outdoor pools, cyanuric acid protects chlorine from the sun. The recommended level is 30-50 ppm; regulations allow a maximum of 75. Above that, chlorine stops working properly.
- Watch the water level. A lot of water evaporates in the heat; if the level drops below the middle of the skimmer mouth, the pump can suck in air. You can estimate it with the evaporation calculator.
- Empty the baskets and pre-filter more often: with so much swimming they fill up faster.
- Shower before swimming. Less sun cream and sweat in the water means fewer chloramines and less chlorine used.
Salt pools suffer too
A chlorinator produces chlorine at a set rate; if demand shoots up, it can fall short. Increase the output or running hours and test more often. Also, in salt pools pH tends to rise, and with high pH chlorine works less well.
What about the water temperature?
Regulations only set the temperature (24-30 °C) for heated pools. There's no limit for an outdoor pool, but the warmer the water, the faster algae grow and the more chlorine is used. This is no time to relax on maintenance.
Get ahead of the next heatwave
If you save your pool in My pool, the forecast for your town warns you when 35 °C or more is expected, so you can boost chlorine in time. And the chlorine predictor tells you when you'll run out based on the forecast.
Frequently asked questions
Should I double the chlorine during a heatwave?
Not by eye. Test every day and adjust to keep free chlorine between 0.5 and 2.0 ppm. In the heat you'll normally need quite a bit more product, but the test decides how much.
Do I need to filter 24 hours a day?
Not if the water is fine: filtering for longer is enough. 24-hour filtration is for when the water has already gone cloudy or is starting to turn green.
It turned green in two days. What do I do?
Correct the pH, shock at dusk, brush the walls and floor and filter continuously for 24 hours. The green water guide takes you through it step by step.