Salt Cell Output Falls Within Days After Acid Clean Decision Tree

Why this matters

A salt cell that scaled, got acid-cleaned, and is back to low output two weeks later is a chemistry problem, not a cell problem. Replacing the cell on a recurring-scale call is the most expensive misdiagnosis in saltwater pool service - a new T-15 or VX55 retails over MSRP for cells the customer thinks they need every spring. The decision tree below isolates whether the recurrence is calcium-driven, pH-driven, low-salt-driven, polarity-driven, or a flow-sensor false read, in a defined order so the next visit corrects the cause, not the symptom.

Symptom presentation

Tech acid-cleaned a cell two to four weeks ago. Cell appears visually clear at the plates. Output reading on the chlorinator is below 50 percent of nameplate or showing a low-salt / inspect-cell warning. Free chlorine in the water has fallen. The customer reports no obvious change in water use, no major rain event, and no shock dose since the previous clean.

Quick checks

1. Test salt level with a calibrated meter or strip. Hayward T-Cell wants 3200 ppm. Pentair IC40 wants 3200 to 3400 ppm. Jandy AquaPure wants 3000 to 3500 ppm. A reading 400 ppm below target throttles output.
2. Test pH and total alkalinity. pH above 7.8 with TA above 100 ppm and calcium above 350 ppm drives recurrent scale onto fresh plates within days.
3. Test water temperature. Most cells throttle output below 60 F (16 C) by spec, not fault.
4. Look at the cell power supply for a polarity-reversal interval setting and confirm it is at 2 to 5 hours, not extended.
5. Verify the flow sensor or pressure switch actually energizes the cell. A failing reed switch reads "no flow" intermittently and the chlorinator displays low output as if the cell were weak.

Isolation tree

Step 1: Inspect the cell plates with a flashlight through the housing. Fresh white scale on plates that were clean two weeks ago confirms recurrent calcium scaling, driven by chemistry. Move to chemistry remediation. Plates that look clear move to Step 2.

Step 2: Check cell voltage and amperage at the connection plug with the cell active. Pentair IntelliChlor reports cell voltage on the display. Hayward AquaRite shows volts and amps on the diagnostic screen. A voltage at the high end of spec with amps at the low end means the cell is losing surface area (worn coating, end of life), not scale. A voltage at the low end with normal amps means the power supply is failing.

Step 3: If voltage and amperage are nominal but output is reported low, suspect the flow sensor. Disconnect the flow sensor connector and shake the inline body. A reed switch with a stuck float reads inconsistently. With the pump running, the sensor should latch closed and show continuity on the inline connector pins.

Step 4: If chemistry, voltage, and flow sensor all check out, suspect stabilizer drift. Cyanuric acid below 30 ppm in direct sun strips free chlorine faster than the cell can replenish, and the customer reads the falling FC as "the cell failed." Test stabilizer; a healthy salt pool runs 60 to 80 ppm CYA per APSP guidance.

Confirming diagnosis

Calcium scale recurrence confirms when the LSI (Langelier Saturation Index) calculated from the customer's current values is above plus 0.3. Run the LSI with pH, TA adjusted for CYA, calcium, temperature, and TDS. A high LSI predicts scale on the cathode plate within days regardless of how clean you got the plates. Power-supply failure confirms when a known-good cell on the same power supply also reads low output. Worn cell confirms when the cell hours on the display exceed 8,000 to 10,000 (T-Cell-15) or the IC40 has more than 10,000 hours.

Remediation

For recurrent calcium scale: drop pH to 7.4, drop TA to 70 to 80 ppm, and confirm calcium hardness is in range (200 to 400 ppm for plaster, 150 to 250 for vinyl). If calcium is above 600 ppm, the long-term fix is a partial drain and refill or RO water service. Schedule the customer for a balance recheck in 30 days. For worn cells: replace the cell with the same model; cross-brand cells void the chlorinator warranty and often misreport salt. For failing power supplies: replace the chlorinator. For low CYA: add stabilizer in 10 ppm increments until target is reached.

Document the cell hours and the chemistry baseline at every cell-clean visit. A recurring-scale customer needs a chemistry plan, not a cleaning schedule.

References

1. ANSI/APSP-11 American National Standard for Water Quality in Public Pools and Spas, Section 4 on saturation index management.
2. Hayward T-Cell Owner's Manual, sections on cleaning frequency, voltage diagnostic readings, and cell life expectancy.
3. Pentair IntelliChlor Installation and User's Guide, including the cell-life hours counter and salt-range table.
4. Jandy AquaPure 1400 Owner's Manual, covering output diagnostics and the polarity-reversal interval.
5. APSP Service Tech Manual, Chapter 9 on chlorine generator chemistry, LSI calculation, and recurrent-scale remediation.