By The Zagosa Team. Published .
Reviewed or updated .
Most shock products are designed for water sitting at 25 to 38C. In a UK garden hot tub held at 38C, that is no bother. But lift the cover, let the surface drop to 25C in a stiff breeze, or run a cold-soak tub at 28C, and the same dose works half as fast. You can be left with leftover oxidiser an hour later.
Why it matters
Non-chlorine shock at 38C is gone within 15 minutes. At 25C it can take an hour. At 15C (a refilled but not yet heated tub) it takes hours.
Through that lag the bathing band has not yet returned. Getting in before the shock has dissipated brings skin tingling and mild eye irritation, which is why winter dosing should always be early evening rather than late.
How to adjust the routine
In December and January, dose with the cover off and the jets 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 tubs held at lower economy temperatures (28 to 32C), cut the dose by around 25% but shock more often. Two half-doses a week clear the load better than one full dose at a temperature where it cannot work fast.
Why a missed winter shock hurts more
Bather load is lower in winter, which is exactly why owners reckon they can skip the shock. But the cover is on more, the air cannot vent the gases off, and combined chlorine builds faster.
On top of that, cold water slows oxidation across the board, so any chloramines that do form hang around longer.
The upshot: skip winter shocks for a month and you get a tub that smells off, with combined chlorine well above 1 ppm, needing two or three rounds of shocking to recover.
Common questions
Is there a cold-water-tolerant shock?
Sodium percarbonate-based shocks handle cold water better than peroxymonosulphate-based ones. They are uncommon in domestic UK hot tub ranges but worth hunting out for a winter routine.
Can I skip winter shocking if I am not using the tub?
Drop to fortnightly, but do not skip entirely. A tub left with rising combined chlorine through January is far harder to recover in March than one that got a token shock every two weeks.