Chlorine Locked vs Low Decision Tree

Why this matters

A pool that holds zero free chlorine when the chlorinator is running, the salt cell is healthy, or you just dosed a gallon of liquid is the most common chemistry callback in the trade. The two underlying conditions look identical at the test strip and are remediated in opposite directions. Locked chlorine (high cyanuric acid, mostly OCl-, hidden by CYA over-protection) is a partial-drain-and-refill problem. Low chlorine (genuine demand exceeding supply) is a dose-or-shock problem. Treating a locked pool with more chlorine throws money at a wall - it cannot become available. Treating a high-demand pool with a drain wastes water and the bloom keeps eating chlorine. The tree below splits the two in 15 minutes.

Symptom presentation

Pattern A: free chlorine reads near zero on DPD, total chlorine reads similar to free, cyanuric acid above 80 ppm. Pattern B: free chlorine reads near zero, total chlorine reads high (combined chloramines), water has a strong "pool" smell. Pattern C: chlorine reads briefly after a dose but falls back to zero within hours despite cell or feeder running. Pattern D: chlorine reads adequate by DPD but algae or cloudiness present (ineffective chlorine, pH off). Pattern E: chlorine reads zero, CYA reads zero - new fill or fresh pool.

Quick checks - 12 minute pass

1. Full panel: free chlorine, total chlorine, pH, total alkalinity, calcium hardness, cyanuric acid, salt. Strip pre-screen is fine for FC and pH but every chlorine-lock diagnosis needs a real CYA turbidity test - the only way to confirm CYA at 100 plus.
2. Did the customer recently shock or add a trichlor tablet? Trichlor and dichlor each add CYA - a winter of pucks can push CYA from 30 to 150 ppm by April.
3. Bather load and weather. A pool that hosted a 12-person Saturday in 95-degree heat consumed chlorine at 3-5x normal rate.
4. Phosphate level if available. 500-plus ppb phosphate feeds algae that consume chlorine faster than the chlorinator can produce.
5. Salt cell or feeder runtime and output percentage. Cell at 50 percent output for 4 hours produces less chlorine than the same cell at 100 percent for 8 hours.

Isolation tree

Step 1 - The CYA test

Run the CYA turbidity test (mix sample with reagent, fill the black dot tube, observe at what fill point the dot disappears). The mapping:

• 0-30 ppm: low protection, chlorine burning off in sunlight. Add stabilizer in measured doses.
• 30-50 ppm: ideal residential salt pool.
• 50-80 ppm: high but functional. Watch demand.
• 80-100 ppm: borderline locked. Production cannot keep up, residual will fall.
• 100-plus ppm: locked. Free chlorine target must rise to keep effective hypochlorous acid; practical answer is partial drain.

If CYA is 100-plus, the chlorine is not missing - it is bound to cyanurate ions and unavailable as the active hypochlorous form. No dose of chlorine fixes this. Drain.

Step 2 - Calculate drain percentage

Partial drain math:

• Current CYA: 150 ppm. Target: 40 ppm.
• Replacement fraction: 1 minus (40 divided by 150) = 0.73, or 73 percent of pool volume.

For a 20K gallon pool, that is 14,600 gallons. Many jurisdictions cap pool drains for water-restriction reasons, and the customer pays for refill - have the conversation.

Alternative for severe water restriction: reverse osmosis water treatment by a specialty truck. Filters out CYA without draining. Cost is meaningful, route only when drain is barred.

Step 3 - Combined chloramine path (Pattern B)

If free chlorine is low and total chlorine is high, combined chloramines are the problem. The pool needs a breakpoint shock - 10x the combined chloramine concentration in free chlorine. Calculate:

• Total chlorine 3.0, free chlorine 0.5, combined chloramine 2.5 ppm.
• Breakpoint target = 10 x 2.5 = 25 ppm of free chlorine.

Use liquid chlorine or calcium hypochlorite to hit breakpoint, circulate 8 hours, retest. CYA above 50 raises the chlorine demand to hit breakpoint - this is why a locked pool also looks like a chloramine pool, and why drain-then-shock is the order, never shock-then-drain.

Step 4 - High-demand path (Pattern C)

Chlorine falls fast despite production. Routes:

• Visible green tint or hazy water - algae bloom. Triple-shock and brush. Algaecide as a follow-up.
• Phosphates high - phosphate remover (lanthanum chloride), filter for 24 hours, then re-evaluate chlorine demand.
• Sunlight unprotected (CYA at zero) - add 30 ppm CYA, dose chlorine, retest tomorrow.
• New fill (Pattern E) - water has no chlorine, no CYA. Add CYA to 30 ppm, then chlorinate to 3 ppm.
• Heavy bather load yesterday or last weekend - normal demand spike, increase production or dose.

Step 5 - pH cross-check (Pattern D)

If chlorine reads adequate but is ineffective, pH is the cause. Above pH 8.0, hypochlorous acid fraction falls below 25 percent. Lower pH to 7.4-7.6 with muriatic acid, retest sanitizer effectiveness.

Confirming the diagnosis

CYA turbidity test, not strip. DPD-1 and DPD-3 (or FAS-DPD titration) for free vs total chlorine. Calculate combined chloramine. Document the CYA reading - this is the artifact that justifies a drain to a skeptical customer.

Remediation

• Locked (CYA 100-plus): partial drain to target CYA, refill, rebalance.
• Combined chloramines: breakpoint shock.
• Algae bloom: triple-shock plus brush plus algaecide plus filter run.
• Phosphate-driven demand: phosphate remover, then chlorinate.
• High pH: muriatic acid to 7.4-7.6.
• New fill: stabilize then chlorinate.

Set up the next call by switching the customer away from trichlor pucks if they are CYA-stacking. Salt cell with no auxiliary trichlor, or liquid chlorine with CYA dosed annually, is the maintenance path that does not paint into the locked corner every spring.

References

• ANSI/APSP/ICC-11 Water Quality in Public Pools and Spas, free chlorine and CYA limits.
• CDC Healthy Swimming Program, cyanuric acid and chlorine effectiveness.
• Pool & Hot Tub Alliance Service Tech Manual, breakpoint chlorination chapter.
• National Plasterers Council Technical Manual, CYA management.
• AquaChek and Taylor Technologies reagent test references for CYA turbidity method.