The two shock products do similar jobs but with very different downtime. Here is which one to reach for in each common UK hot tub scenario.
Chlorine shock and non-chlorine shock both oxidise organics and break down chloramines. The difference is what they leave behind. Chlorine shock leaves free chlorine, so the tub is unusable for hours. Non-chlorine shock leaves only oxygen and tiny amounts of sulphate, so you can be back in the water within 15 minutes.
After a long period of disuse (more than two weeks). The free chlorine residual it leaves resets the sanitiser baseline cleanly.
When there is visible biological growth (a green tinge, a slimy waterline). You want the lingering high free chlorine to keep killing as it works through the system.
During an emergency such as a known faecal incident. The dose is much higher (10 to 20 ppm), the wait is at least 30 minutes, and the kill needs to be unambiguous.
Routine weekly maintenance. Knocks down chloramines, lifts oxidation, and lets the tub be used the same evening.
After a normal session of two to four bathers. Half a normal weekly dose immediately after, with the cover off for an hour, prevents the build-up of combined chlorine.
In a bromine tub. Non-chlorine shock reactivates bromide back into bromine, refreshing the residual without adding any chlorine.
Neither product is a substitute for the routine residual. Free chlorine or bromine still has to be present between sessions. Shock is a top-up, not a replacement.
Neither replaces a refill. Once total dissolved solids are too high or biofilm has taken hold, no amount of shock fixes it.
Neither is a quick fix for low pH. Shocking with low pH gives you a localised bleach reaction and can damage the shell. Always balance pH first.
Yes. Most pros use non-chlorine shock weekly and a chlorine shock once a month or after heavy use. The two are complementary.
Fifteen to twenty minutes with the jets running. Longer if the bottle says so. The active ingredient is potassium peroxymonosulphate, which dissipates very quickly.