After any session with multiple bathers or extended use, hot tub water needs an oxidation treatment to destroy combined chlorine and organic compounds. This restores clear, odour-free water for the next use.
Key Facts
- Test combined chlorine after heavy use — if above 0.5 ppm, a shock treatment is required.
- Non-chlorine shock (MPS) is the fastest option: it works without raising FC and the spa can be used in 15–30 minutes.
- Chlorine shock provides deeper treatment but requires a minimum wait of 4–8 hours.
- Remove the spa cover during shocking to allow off-gassing of volatile chloramines.
What Heavy Use Does to Spa Chemistry
Every bather introduces nitrogen-containing organic compounds into the water: sweat, body oils, hair products, and sunscreen. In a small hot tub volume (300–500 gallons), even two or three bathers in an extended session can consume most of the free chlorine and leave behind significant combined chlorine. Combined chlorine is the source of eye irritation, skin irritation, and the "hot tub smell" that many users notice after a well-used spa. It also reduces the effective sanitation of the remaining free chlorine.
Shock Types for Hot Tubs
Non-chlorine shock (potassium monopersulfate / MPS) is the standard choice for hot tubs after a soak session. It is an oxidiser that destroys organic contaminants and combined chlorine without raising free chlorine levels. Add MPS according to the product label, run jets for 15 minutes, and the spa is typically ready for use within 30 minutes. Chlorine shock (sodium dichloro granules or liquid chlorine) is used when the water needs a more thorough reset — it raises FC significantly and requires 4–8 hours with the cover removed before the spa is safe for re-entry. Use chlorine shock at least weekly regardless of use.
After the Shock
After adding any shock product, run the jets on high speed for 15–20 minutes to mix the chemical and accelerate off-gassing of chloramines. Leave the cover open or slightly ajar during this period — closing a cover over a freshly shocked spa concentrates volatiles under the cover and drives them back into the water on the next opening. For chlorine shock, wait until FC drops below 5 ppm before re-entry. Test and record the post-shock chemistry to establish the baseline for the next use cycle.
Examples
Six people used the spa for 90 minutes for a birthday party. After they exit: FC tests at 0.5 ppm (almost depleted), CC at 1.8 ppm (high), strong chloramine odour present. Add two doses of MPS (non-chlorine oxidiser) and run jets for 20 minutes with cover off. Test 30 minutes later: FC still 0.5 ppm (MPS does not raise FC), CC now 0.3 ppm (below threshold), odour eliminated. Add chlorine granules to raise FC to 4 ppm. Cover off for 15 minutes. Ready for next use.
Common Mistakes to Avoid
- Closing the spa cover immediately after adding shock — this traps volatile chloramines under the cover instead of allowing them to escape.
- Using non-chlorine shock as a substitute for weekly chlorine shock — MPS oxidises organics but does not kill bacteria as effectively as free chlorine.
- Not testing combined chlorine after heavy use — FC can look fine while CC is elevated enough to cause significant irritation.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference