Low chlorine on a salt pool: is it the cell, the salt, or something else?

1Look at the cell

• Switch the chlorinator and pump off, then pull the cell.
• White, chalky scale? That's the usual cause of low output. It builds faster with high calcium hardness or pH, and self-cleaning cells still scale in hard water. Some manufacturers warn that granular chlorine additives can clog the electrodes.
• Water must cover the flow switch or gas sensor when the pump is running.
• Air pocket at the top of the housing? On cells with a sensor in the housing, it can bring up a no-flow fault. Fix the cause, usually low flow or air drawn in on the suction side; don't just bleed the air.

Scaled? See Cleaning a cell safely, and check the balance with the LSI calculator so it doesn't scale straight back up.

2Test the salt, don't trust the light

• A "low salt" warning can also mean a scaled cell, cold water, an ageing cell or, on some units, low mains voltage. Test with a calibrated meter or a LaMotte Insta-Test salt strip, or a sample to the pool shop, before you add salt.
• Salt target: check your model's manual. Most standard units run at about 3,000–6,000 mg/L (ppm); low-salt models and some brands run lower. Running below the minimum damages the cell, and some manufacturers say it voids the warranty.
• Salt is lost through backwashing, splash-out, rain overflow and leaks, not used up or evaporated. Add it to the pool, not the skimmer box.

3Flow, run time and output setting

• Blocked baskets, a closed valve or a filter that needs a backwash all cut flow and output.
• Variable-speed pump? On a low setting it can drop below the cell's minimum flow (60–80 L/min on some models). Check your manual's figure against the pump speed during chlorination hours.
• Check the timer and output setting: typically about 8 hours a day in summer, less in winter.

Never disable, bridge out or tamper with the flow switch or gas sensor. It shuts the cell down when flow stops or hydrogen gas builds up.

Chemistry that makes a good cell look bad

Cleaning a cell safely

• Use a commercial cell cleaner, or hydrochloric acid (AquaGUARD Hydrochloric Acid Balance 5) at your model's ratio (most manuals say about 1 part acid to 10 parts water; some stronger). Never guess the mix, never use it undiluted. Too strong or too often strips the plate coating and shortens cell life.
• Add acid to water, never water to acid. Wear eye protection and acid-resistant gloves; work with good ventilation.
• Immerse the plates only, not the terminals. Lift the cell out when the fizzing stops, typically within about 10 minutes, and never past the manual's maximum (15 minutes on some models).
• No metal tools or scourers. Rinse, and dry the connections before you refit.

Keep acid away from chlorine. WorkSafe WA lists cell cleaners and other acids as products liquid chlorine must not contact. Safe Work Australia advises keeping acids and chlorine (up to 250 L or kg) at least 3 metres apart or behind a solid, chemical-proof barrier, in the ute or van too.

Disposal in WA. Never into a stormwater drain: in WA, discharging acid below pH 4 into the environment (stormwater included) in the course of business is an offence. Follow the SDS and your local water authority.

When it isn't the chlorinator

• Demand is higher than output. Hot weather, strong sun and heavy bather loads use chlorine faster. Check the chlorinator is sized for the pool; some manufacturers rate the same model for a smaller pool in a hot climate.
• The pool is already at zero. Shock with liquid chlorine (AquaGUARD AquaFresh 1250) first (follow the label), then let the chlorinator hold it. Boost mode is slower and harder on the cell. Gone green? See green pool recovery.
• Scale keeps coming back. See calcium scale and the LSI.

Final step

Cell clean, salt confirmed, flow and run time right, water warm, and output still low? The cell is probably near the end of its life. Have it tested before you replace it.