By The Zagosa Team. Published .
Reviewed or updated .
Dose chlorine at bedtime and find the strip reading zero at breakfast? That is what the trade calls chlorine demand. The water is using chlorine up faster than it can hold onto it. Four causes are common, and each leaves its own fingerprint.
The four usual suspects
Organic load. A busy bather session leaves sweat, oils and dead skin behind. Chlorine goes after those first and gets consumed.
Chloramines. If combined chlorine was already high before the session, the existing combined chlorine locks up new chlorine into yet more chloramine rather than free chlorine.
Biofilm in the pipework. Pipes left unpurged for months hold bacterial colonies that eat chlorine the instant it circulates.
Metals in the fill water. Iron and manganese react with chlorine on contact. Hard-water postcodes are not usually affected, but soft-water rural supplies sometimes are.
Working out which one it is
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 yet the demand is still huge, run a pipe purge product overnight, then drain and refill. Biofilm is almost certainly the culprit.
If the water shows a faint orange tinge once the chlorine drops, you have iron in the fill. A sequestering agent plus a hose-end pre-filter on the next refill will sort it.
Stopping it happening again
Shock weekly even when nothing looks wrong. Combined chlorine builds quietly 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 a tub once it climbs above that.
Purge the pipes at every refill. A 60-pence dose of pipe cleaner beats a sleepless night chasing a reading.
Common questions
Should I keep dosing until the reading holds?
Once. Then test again in two hours. If it has dropped once more, stop and tackle the cause. Repeat dosing without fixing the cause just piles up combined chlorine.
Is the water safe to use during chlorine demand?
No. If free chlorine is below 1 ppm and combined is above 0.5, the water is not adequately sanitised. Sort it before you bathe.