The right water temperature for your pool
For most people, water around 26–28 °C is pleasant for family use, and a little cooler for exercise swimming. The higher the temperature, the more closely you need to watch the chemistry.
Comfortable temperatures by use
| Use | Typical range | Comment |
|---|---|---|
| Exercise swimming | About 25–27 °C | The body warms up from the effort |
| Family and play | About 26–28 °C | Pleasant for most ages |
| Young children, older people, relaxing | About 28–30 °C | Less movement, more need for warmth |
| Cooling dip | Below about 24 °C | Pleasant for a few minutes in a heatwave, cold for staying in |
In an unheated outdoor pool, in most of Greece the water is usually pleasant from June to September. The pool's position, shade, wind and a cover can shift these limits by weeks. For a longer season, see the options for pool heating.
How it affects the chemistry
Warm water is pleasant for you, but also for microorganisms. In practice:
- Chlorine is used up faster. With heat, strong sun and more swimmers, free chlorine drops below 1 mg/l more easily. Test more often and prefer smaller, more frequent doses.
- Algae multiply. In water above about 28 °C, a gap in chlorination of even one or two days is enough for it to start turning green. Prevention is explained in pool algae.
- Scale forms more easily. Calcium carbonate dissolves less in warm water, so in hard water white deposits appear more easily at the waterline and in the electrolysis cell.
- In cold water everything slows down. Algae grow slowly, chemicals act more slowly and many salt electrolysis units reduce their output.
In addition, a PVC liner softens when the water stays very warm for long periods. Membrane manufacturers usually give a maximum recommended temperature; if you heat the pool, do not set the thermostat above it.
How to keep the heat in the water
Most of the heat in an outdoor pool is lost from the surface, mainly through evaporation at night. That is why the most effective measures do not produce heat; they keep it in:
- A cover at night: a solar or summer pool cover is the first thing worth having, with or without heating.
- Wind protection: a fence or planting on the windward side reduces surface cooling and evaporation.
- Sun and colour: a spot that gets sun for most of the day helps a lot, a darker liner a little; see the role of liner colours.
- Heating in moderation: with a heat pump, schedule it to run in the warm hours of the day, when the air is warmer and efficiency is better.
How to measure it
A simple floating thermometer is enough. Measure at the same time every day, preferably early in the morning when the water is coolest, so you see the real trend. A heat pump's reading refers to the water entering it and may differ slightly from the surface, especially after a sunny afternoon. When the temperature changes suddenly, for example after a storm or a large top-up with cold water, test pH and chlorine again.
Questions
Why does the pool turn green more easily in August?
Because warm water, strong sun and many swimmers use up chlorine quickly and favour algae. Test more often, run the pump for more hours and keep free chlorine steady at 1–3 mg/l.
Does a pool with 1.10 m walls warm up faster?
Yes, it holds less water for the same surface area, so it gains temperature faster, but it also loses it faster without a cover. See what else changes with pool depth.
See also
Pool heating
Heating a prefab pool: solar cover, solar collectors, heat pump, heat exchanger. What suits your pool volume,…
Pool cover
Solar summer covers, winter covers, roller covers and safety covers: what each does for temperature,…
Pool algae
Pool algae: how to recognize green, yellow and black algae, how to treat them with gentle brushing on the…