UV, heat, and bather load all conspire to burn through chlorine in July. Here is the science in plain English, and three changes that fix it.
Chlorine in a UK hot tub is a perishable thing. Heat speeds it up, sunlight breaks it down, and warm-weather bathing loads add to the work. In July you can use noticeably more chlorine than in February for the same number of bathers, and the readings drop faster between sessions.
Direct sunlight. Free chlorine is broken down by UV in minutes when the cover is off. Even partial sun on a removed cover is enough to halve a fresh dose.
Warm bather skin. Higher air and skin temperatures mean more sweat and oils per session, which means more organic load for the chlorine to oxidise.
Warmer water. Above about 35C, chlorine kinetics speed up. It works faster but also exhausts faster against the same amount of organic matter.
Add a low level of cyanuric acid (CYA) for outdoor tubs that get any direct sun. 20 to 30 ppm in summer protects free chlorine from UV without blunting it the way higher levels do.
Shock at the end of the day, not the start. Evening shocks have all night to do their work without UV undoing it.
Replace the cover on the cool end of the dosing window. The 30-minute wait after a top-up should still be cover-on, with the jets running.
Do not switch to a stabilised tablet without checking the existing CYA. Stabilised chlorine adds CYA every time. In a small tub the level can climb past 50 ppm in a fortnight, and at that point free chlorine becomes sluggish.
Do not just dose more. Repeated overdosing builds combined chlorine and irritates skin without solving the underlying loss.
Do not assume the tub has a fault. UK summers genuinely behave differently from winters in chlorine terms, and a small adjustment to your routine usually fixes it.
Dilute by partial drain and refill. There is no chemical that removes CYA economically.
Yes, and it works. A shade structure over an outdoor tub cuts UV exposure significantly and softens the summer chlorine demand.