Why it matters
TC minus FC equals combined chlorine. Combined chlorine causes the strong chlorine smell and eye irritation, and means the pool needs shocking.
Ideal ranges
| Water type | Ideal | Acceptable |
|---|---|---|
| Pool | TC = FC (combined chlorine = 0) | Combined below 0.5 ppm |
How to test
- 1
FAS-DPD test reads FC and TC separately
Subtract FC from TC to get combined chlorine.
- 2
Strips can show TC but not differentiate well
Use FAS-DPD when troubleshooting.
How to correct high total chlorine
Shock the pool
Raise FC to shock level for your CYA. 10x combined chlorine is the rule of thumb. Hold until CC drops below 0.5.
Verify with overnight FC loss test
If FC drops more than 1 ppm overnight with no swimmers, shock again.
Common causes
- •Heavy use
- •Rain washing in organic debris
- •Insufficient FC for long periods
- •Pool filter clogged
FAQs
Why does my pool smell like chlorine if FC is low?
That smell is chloramine, not free chlorine. Low FC and high combined chlorine. Shock the pool.