The Dichlor Habit: Combined Chlorine, CYA, and a Low ORP That Looks Fine

Reading them as a system, not as six independent pass/fail checks:

• FC 2.4 ppm looks fine in isolation - and that is exactly the trap. Free chlorine is mostly not hypochlorous acid when cyanuric acid is present. At CYA 68 ppm, only a small single-digit percentage of the free chlorine exists as HOCl/OCl⁻; the rest is bound to the cyanurate ring and is a reservoir, not a disinfectant.
• CYA 68 ppm is legal under the 80 ppm cap but is a self-inflicted wound. Dichlor is roughly half cyanuric acid by weight: every 10 ppm of chlorine added as dichlor adds on the order of 9 ppm CYA. Three weeks of "simplified dosing" is the source.
• ORP 615 mV is the honest number. ORP responds to oxidizer activity (essentially HOCl), not to the total chlorine you measure with DPD. A free chlorine of 2.4 ppm in a high-CYA pool and an ORP of 615 mV are perfectly consistent with each other.
• CC 0.9 ppm is the actual complaint generator. Chloramines - the products of chlorine reacting with sweat, urine, and sunscreen - are the "chlorine smell" and the eye and respiratory irritant. Swimmers are not reacting to too much chlorine; they are reacting to chloramine and to a pool with too little effective chlorine.
• pH 7.4 / TA 95 are in range, so pH is not the explanation and you should not go chasing it.

Compliance status: CC 0.9 ppm exceeds the 0.4 ppm maximum and ORP is below 650 mV. That is two separate code failures, not one aesthetic problem.

Could one reading have told you? No. Free chlorine alone reads acceptable. You needed FC plus CC plus CYA plus ORP to see that a pool testing at 2.4 ppm is under-disinfected and over-stabilized at the same time.

Stay open? No. CC 0.9 ppm against a 0.4 ppm maximum and an ORP below the 650 mV requirement are both code violations with a documented bather-health symptom (eye irritation) attached. Close or restrict to no-swim, post the reason, and log the decision and time.

Plan, in order:

1. Stop the cause first. Pull the dichlor feeder. Every tablet you dissolve from here forward adds more CYA and makes the problem harder to reverse. This is a five-minute action that changes the math for everything that follows.
2. Dilute before you shock. Breakpoint chlorination in a pool with CYA 68 ppm is expensive and partially wasted - much of the chlorine you add gets bound before it can oxidize anything. Drain and refill to get CYA down to roughly 30 ppm first (calculation in the next task).
3. Breakpoint chlorinate with an unstabilized product - sodium hypochlorite (liquid) or calcium hypochlorite. Target roughly 10× the combined chlorine, so on the order of 9–10 ppm free chlorine, at night, with no bathers, pumps running, feeder off. Using dichlor here would reintroduce the exact problem you are solving.
4. Hold and verify. Test FC and CC hourly through the night. CC should fall; if it rebounds, you still have a nitrogen load (bather waste, biofilm, a dead animal in a skimmer, a dirty equalizer line) and the source has to be found, not shocked again.
5. Reopen only when you can document CC ≤ 0.4 ppm and ORP ≥ 650 mV with FC inside the code range at normal bather load - not just at 6 a.m. before anyone swims.
6. Control the load going forward. Pre-swim showers, enforced bather-load limits, and ventilation/air exchange - indoor pools concentrate the volatile chloramines that a dehumidification system is supposed to remove. If the air handling system is short-cycling or the outdoor air damper is stuck, no amount of chemistry fixes the smell.
7. Change the feed method. Daily liquid hypochlorite for sanitation; if you want stabilizer, add it deliberately and measure it. Never let a convenience product set your CYA level.

Test-method caution: if you use potassium monopersulfate as a supplemental oxidizer, remember it registers as combined chlorine on many DPD test kits. Wait for it to clear before you declare victory or reopen.

Dilution math (drain-and-refill in one pass):

Fraction of CYA to remove = (68 − 30) / 68 = 0.559

Volume to drain = 0.559 × 75,000 gal ≈ 41,900 gal, call it 42,000 gal

Check: 75,000 − 42,000 = 33,000 gal retained. 68 × (33,000 / 75,000) ≈ 29.9 ppm CYA. ✓

Time at 150 gpm: 41,900 ÷ 150 = 279 minutes ≈ 4.7 hours just to drain, before any refill time. If your refill is 200 gpm, add another ~3.5 hours. That is roughly an eight-hour night - schedule it, because if you start at midnight thinking it takes "a couple of hours," you will run out of time and refill a pool that is only half-diluted.

The shortcut that breaks the math: continuous drain-and-fill. If you crack the drain valve and open the fill line at the same time, you are draining water that has already been partly diluted, so the pool approaches 30 ppm asymptotically and never gets there efficiently - you can move two or three times the calculated volume and still land at 40–45 ppm. Drain first, then fill; if you must refill while draining, put the fill at the opposite end from the drain and expect to use substantially more water than the calculation predicts.

Two more real-world corrections:

• Backwash water counts toward dilution only for the water actually removed - track it, don't guess.
• Check your makeup water for stabilizer if you are on a shared or reclaimed supply; if it carries CYA, your target moves. Test the source once.