Chlorine demand explained: the four things that quietly burn through a full dose by morning, and the test that proves which one is to blame.
If you doses chlorine at bedtime and the strip is showing zero by breakfast, you have what the industry calls chlorine demand. The water is consuming chlorine faster than it can hold it. There are four common causes, and each leaves a different fingerprint.
Organic load. A heavy bather session leaves sweat, oils, and dead skin in the water. Chlorine attacks them first and is consumed.
Chloramines. If you had high combined chlorine before the session, the existing combined chlorine ties up new chlorine into more chloramine instead of free chlorine.
Biofilm in the plumbing. Pipes that have not been purged in months hold colonies of bacteria that consume chlorine the moment it circulates.
Metal contamination from the fill water. Iron and manganese react with chlorine on contact. Hard-water postcodes are not usually affected. Soft-water rural supplies sometimes are.
Test combined chlorine, not just free. If combined is above 0.5 ppm, chloramines are the cause. Shock with a non-chlorine oxidiser and run the jets for 20 minutes.
If combined is low but the demand is still huge, run a pipe purge product overnight, then drain and refill. Biofilm is almost certainly your problem.
If the water has a faint orange tinge after the chlorine drops, you have iron in the fill. A sequestering agent and a hose-end pre-filter on the next refill will fix it.
Shock weekly even when nothing looks wrong. Combined chlorine builds silently between heavy sessions.
Keep the cyanuric acid level below 30 ppm in a hot tub. CYA stabilises chlorine in outdoor pools but blunts it in hot tubs above that level.
Purge the pipes at every refill. A 60-pence dose of pipe cleaner is cheaper than a sleepless night chasing a reading.
Once. Then test in two hours. If it has dropped again, stop and address the cause. Repeat dosing without addressing the cause just builds combined chlorine.
No. If free chlorine is below 1 ppm and combined is above 0.5, the water is not adequately sanitised. Resolve before bathing.