Water temperature
Last reviewed August 11, 2026
Warm water burns chlorine faster, grows algae faster, and scales faster. The same pool with the same chemistry behaves differently in August than it does in March.
Measure the water, not the day
On an unheated pool the water tracks the recent air temperature closely enough that a weather reading gets you within a couple of degrees, which is inside the tolerance the LSI needs.
A pool at 88°F in October has October chemistry everywhere except the number that drives scaling. That's how a heated pool scales through a mild autumn while the readings look fine.
Why it's in the LSI
Warm water holds less calcium in solution, so the same calcium reading is more scale-forming in summer than in winter. Temperature is one of the five LSI inputs and it's the one nobody can change.
The practical effect: a pool that sat balanced all spring can drift into scaling by July with nobody touching it. In hard-water markets that's the normal summer pattern, and it's managed by pulling pH down. See reading the LSI.
What changes with the season
| Water temp | What to expect |
|---|---|
| Under 60°F | Chlorine demand drops off. Algae mostly stops. LSI trends negative. |
| 60–80°F | Normal operating range. |
| Over 85°F | Chlorine burns faster, algae moves fast, LSI trends positive. |
| Over 90°F | Expect to dose more chlorine and watch scaling closely, especially on salt. |
Spas run 100°F and up, which is why spa chemistry moves so much faster than pool chemistry and needs testing more often.
Logging chemistry on every stop? PoolSpartan keeps the readings, works out the LSI as you type them, and sends the customer a service report when you close the visit. Free to start. We only get paid when you do.
Sign up · free