Combined chlorine

Clearing it

Shock past breakpoint. Add ten times the combined chlorine reading in free chlorine, all at once. Half measures make it worse, because a partial dose creates more chloramines than it destroys.

• Liquid chlorine, not cal-hypo, on vinyl and salt pools
• Dose in the evening so the sun doesn't burn it off before it finishes working
• Retest the next visit, not the same hour

A salt cell's boost or super-chlorinate mode is too slow for this. The cell is capped at its production rate. Manual liquid gets there faster and you control the dose.

Combined chlorine is chlorine that has already reacted with sweat, urine, sunscreen and leaf litter. It's spent. It doesn't sanitize, and it irritates eyes and skin.

The strong chlorine smell people complain about is combined chlorine. A pool that smells heavily of chlorine has too little working chlorine, not too much. Worth knowing when a customer asks.

You need the right kit

Combined chlorine is total chlorine minus free chlorine. Test strips can't do it accurately enough to be worth logging. You need a FAS-DPD drop kit. If you don't carry one, leave the field blank rather than guessing. Blank means you didn't test. A guess becomes a number somebody trusts later.

When it keeps coming back

Combined chlorine that climbs again every week means the pool is producing more waste than the chlorine can burn through. Usual causes: heavy swimmer load, a pool used by small children, poor circulation, or free chlorine sitting too low for the stabilizer level. Check the FC–CYA ratio before assuming it's bather load.