Shocks work more slowly in cold water. Here is how UK winter temperatures change the routine, and why a missed weekly shock matters more in January.
Most shock products are designed for water at 25 to 38C. In a UK garden hot tub kept at 38C, that is fine. But when the cover comes off and the water surface drops to 25C in the wind, or in a cold-soak tub kept at 28C, the same dose works half as fast and you can be left with leftover oxidiser an hour later.
Non-chlorine shock at 38C dissipates within 15 minutes. At 25C, it can take an hour. At 15C (a refilled but not yet heated tub), it takes hours.
During that lag, the bathing band has not yet returned. Bathing in not-yet-dissipated shock causes skin tingling and mild eye irritation, which is why winter dosing should always be early evening, not late.
In December and January, dose the shock with the cover off and the jets running on full for the whole 30 minutes, not just the first 15.
Wait double the bottle's stated re-entry time before bathing. If the label says 15 minutes, give it 30. If it says 20, give it 40.
For hot tubs being held at lower economy temperatures (28 to 32C), reduce the dose by about 25% but increase the frequency. Two half-doses a week clear the load better than one full dose at a temperature where it cannot work fast.
Bather load is lower in winter, which is why owners think they can skip the shock. The cover is on more, and the air does not get to vent the gases off, so combined chlorine builds faster.
Additionally, cold water makes oxidation slower across the board, so any chloramines that do form linger longer.
The net effect: skipping winter shocks for a month leaves a tub that smells off, with combined chlorine well above 1 ppm, and that takes two or three rounds of shocking to recover.
Sodium percarbonate-based shocks tolerate cold water better than peroxymonosulphate-based ones. They are uncommon in domestic UK hot tub products but worth seeking out for a winter routine.
Drop to fortnightly, but do not skip entirely. A tub left with rising combined chlorine through January is much harder to recover in March than one that received a token shock every two weeks.