Chemistry All In Range But Water Still Uncomfortable: Reading In-Spec Decision Tree

Why this matters

A test kit that returns every value inside the published range is not the same as water that feels right to a swimmer. Eye burn, skin tightness, hair stripping, a chemical smell at the water line, or persistent dullness can all occur while FC, pH, TA, CH, CYA, and salt all read inside PHTA/APSP-11 guidance. The trap is closing the ticket because the numbers pass while the customer experiences a real problem. This decision tree walks the secondary parameters that drive perceived water quality even when the headline panel reads in-spec.

Symptom presentation

The complaint is usually subjective and arrives by text or callback: stinging eyes after a swim, itchy skin overnight, a strong pool smell at the water line, hair feeling waxy or stripped, or cloudy water that polishes only briefly after a clarifier dose. Test strips and reagent kits show FC inside the CYA-adjusted target band, pH between 7.4 and 7.8, TA 60 to 100 ppm, CH 200 to 400 ppm, CYA at the documented target, and salt within the cell tolerance band. Nothing on the headline panel justifies a service ticket, yet the customer reports the water is not comfortable.

Quick checks

Re-run the FAS-DPD test for combined chlorine (CC). The headline DPD test reports total chlorine; if CC is greater than 0.4 ppm the water carries chloramines that cause smell and eye burn even when FC reads on target. Verify CYA was measured this visit and not pulled from a prior log; high CYA suppresses FC effectiveness so the FC reading overstates the sanitizer that is actually working. Pull a TDS reading; a pool that has not been partially drained in 18 months can carry 2,500 ppm or more of dissolved solids that dull water and irritate skin without moving any of the headline numbers.

Smell the water at the surface. A sharp chlorine note at the water line is the signature of CC, not of overdose. Look at the water from the deck edge with the sun behind you; a faint gray or green cast with no algae present often indicates copper or iron loading from a heater or city source. Touch the tile at the water line for a slick film, which can point to residual surfactant from a customer-added product.

Isolation tree

Branch A: combined chlorine. If CC is 0.4 ppm or higher with FC on target, the customer is swimming in chloramine. Confirm by holding a sample to your nose; a strong pool smell from a clear glass is diagnostic. Path to remediation: breakpoint chlorination (raise FC to 10 times CC, hold until CC drops below 0.2 ppm) or non-chlorine shock (potassium monopersulfate) if the customer cannot accept downtime.

Branch B: CYA suppression. If CYA is above 80 ppm with FC at the low end of the CYA-adjusted target, available sanitizer is below the effective threshold even though FC numerically passes. The PHTA recommended ratio is FC at 7.5 percent of CYA as a working minimum. Path: drain and refill to bring CYA into the 30 to 50 ppm band for a non-SWG pool or 60 to 80 ppm for a salt pool.

Branch C: pH at the high end with high TA. pH reading 7.8 with TA above 120 ppm and CH above 300 ppm can drive calcite precipitation that scratches skin and clouds water without moving turbidity into the visibly cloudy band. Confirm by running the Langelier Saturation Index; LSI above plus 0.5 indicates scale-forming water. Path: reduce TA with muriatic acid in stages, hold pH at 7.4 to 7.6, monitor LSI.

Branch D: TDS elevation. If TDS is above 1,500 ppm over fill water, water feels dull and stripped even with all other parameters in range. Path: partial drain and refill, typically 25 to 50 percent depending on starting TDS and fill water quality.

Branch E: metals. If water shows a tint with no algae and a copper or iron strip reads positive, metals are present. Customer-added algaecide with copper, a corroded copper heat exchanger, or city fill water can be sources. Path: sequestrant dose, then identify the source before the next refill.

Confirming diagnosis

Run a FAS-DPD on a fresh sample drawn at elbow depth from the deep end, away from returns. Record FC and CC to 0.2 ppm resolution. Test CYA with a turbidity tube in direct sun and confirm the dot disappears at the documented mark; CYA test strips drift high and should not be used to confirm a sanitizer suppression call. Pull TDS with a calibrated meter rinsed in distilled water between samples. Compute LSI with the customer-confirmed water temperature, not the ambient air reading. Photograph the water line tile and the sample glass for the customer record.

Remediation

For a CC call, run breakpoint at the customer's normal pH or slightly lower (7.2 to 7.4) to speed the reaction; communicate the swim-hold window before FC drops below the CYA-adjusted maximum. For CYA suppression, schedule the drain and refill on a day with no rain forecast and verify the fill water for CYA, CH, and metals before topping. For high-LSI scaling, sequence the acid dose over multiple visits to avoid driving TA below 60 ppm, which destabilizes pH. For TDS, document the starting reading and the post-refill target so the next service has a baseline.

Document the secondary parameter that drove the comfort complaint (CC, CYA ratio, LSI, TDS, metals) in the service note so the next tech does not re-run the headline panel and close the ticket again.

References

• PHTA/APSP/ICC-11, Water Quality in Public Pools and Spas
• CDC Model Aquatic Health Code, Section 5.7 Pool Water Chemistry
• NSF/ANSI 50, Equipment and Chemicals for Swimming Pools, Spas, Hot Tubs, and Other Recreational Water Facilities