Hayward Sense & Dispense Overdosing Acid or Chlorine: Causes and Fix
Quick Summary
- Acid overdosing almost always traces to the injection point being too close to the flow cell, causing the probe to read low pH during dosing while the pool pH is actually fine.
- Chlorine overdosing is typically a set point or configuration issue — ORP set point too high, or an ORP probe reading low and demanding more chlorine to compensate.
- Any time the system reads a different chemistry value than your independent test kit by more than 10%, recalibrate or clean the probe before assuming a configuration problem.
- Stuck-open solenoid valves on CO2 systems and failed Stenner pump tubes that drip continuously are mechanical causes of overdosing independent of the probe or control board.
Diagnosing Acid Overdosing
An acid overdose from a Sense & Dispense system typically manifests as chronically low pH despite the system appearing to operate normally — no error codes, probe reading normal pH, but pool pH crashes within days of each service visit. The cause is almost always one of four things.
Cause 1: Injection Point Too Close to the Flow Cell
The acid injection point must be located downstream of all pool equipment and — critically — downstream of the flow cell water connections. If acid injects upstream of the flow cell influent connection, the acidic water reaches the probe before it mixes with the pool volume. The probe sees low pH and calls for more acid. The pool pH is actually fine or even normal, but the system keeps dosing in a loop.
To diagnose: take a pH reading from a sample drawn from the pool body (not the flow cell) while the acid system is actively dosing. If pool pH is normal but the flow cell is reading low pH, the injection point is the problem. Relocate the acid injection point to a location that ensures full mixing before water reaches the flow cell sample port.
Cause 2: Probe Calibration Drift
A pH probe that has drifted downward will read pH as lower than it actually is, causing the system to dose acid when none is needed. If your independent test shows pH of 7.4 but the system displays 6.8, the probe has drifted by 0.6 units — well past the 10% threshold. Clean the probe, run the system for two hours, and recalibrate using your independent sample from the flow cell chamber as the reference.
Cause 3: Stuck-Open CO2 Valve (AQL-CHEM2 Only)
On CO2 acid systems, the solenoid valve that admits CO2 into the injection line can fail in the open position. When this happens, CO2 flows continuously regardless of whether the system is calling for pH reduction. The result is continuous pH reduction that the control board cannot detect because the valve failure is mechanical, not electrical.
To test: with the system not commanding any acid dosing (display shows pH within set point), observe whether CO2 is still bubbling into the injection line. If CO2 continues to flow when the system is not calling for pH reduction, replace the solenoid valve: AQL-CHEM2 or AQL-CHEM2-240 depending on voltage.
Cause 4: pH Set Point Configured Too High
If the pH set point is configured at 7.8 or 7.9 and the pool naturally stabilizes at 8.0–8.1 due to high-chlorine production and low CYA, the system will dose acid continuously trying to maintain an ambitious target it cannot realistically hold. Consider whether the set point is realistic for this pool's chemistry profile. A set point of 7.4–7.6 is more appropriate for most salt pools with active SWG operation.
Diagnosing Chlorine Overdosing
Chlorine overdosing presents as chronically high chlorine despite the system appearing to function normally, or as an ORP High / Chlor. Off error that fires and resets repeatedly.
Cause 1: ORP Set Point Too High
The ORP set point determines when the chlorinator shuts off. If set to 750mV or higher on a pool with normal chemistry, the system may run the cell at very high percentages for extended periods to reach the target, resulting in high chlorine accumulation. The default set point is 650mV. Lower it to 600–650mV and see if chlorine levels stabilize.
Cause 2: ORP Probe Reading Low Artificially
A fouled or failed ORP probe reads a lower ORP than actual. The system interprets this as insufficient sanitizer and commands the cell to run harder and longer. Meanwhile, the actual chlorine level climbs. Clean the ORP probe thoroughly and run for two hours before rechecking. If ORP rises to normal after cleaning, the probe was suppressing the reading. If ORP remains lower than expected for the confirmed chlorine level, investigate CYA level (high CYA suppresses ORP) and whether any chlorine reducers have been used.
Cause 3: Chlorinator Percentage Set Too High in Timed % Mode
If the system is configured in "Chlorine Feed – Timed %" mode rather than "Auto Sensing," the cell runs at a fixed percentage of pump runtime regardless of ORP. If that percentage is too high for the pool volume and bather load, chlorine will accumulate. Switch to "Auto Sensing" mode so the ORP probe governs chlorination, or reduce the timed percentage to 40–60% as a starting point and adjust based on actual chlorine results over the next week.
Acid Overdose Health Risk
Acid overdosing that crashes pool pH below 7.0 can cause severe eye, skin, and respiratory irritation for swimmers. If pH is confirmed below 7.0 on an independent test, do not allow anyone in the water until it is corrected. Disable the acid feed system manually and raise pH with soda ash before re-enabling automated dosing.
Verifying System Operation After Correction
After identifying and correcting the overdosing cause, run the system through at least two full dosing cycles before leaving. Verify the following:
- Manually test both flow cell water and pool water. They should read within 10% of each other for pH. ORP should read in the expected range for the confirmed chlorine level.
- Observe whether the dosing system (pump or valve) stops when the system display shows the set point has been reached. If dosing continues after the set point is met, there is a stuck valve, relay, or board output fault.
- If CO2 is the acid feed, observe whether gas flow stops completely when the system is not calling for pH reduction. Even a small constant leak will drive pH down over several days.
- Check that the pH set point is appropriate for this pool — 7.2–7.6 is the standard range, with 7.4 being the ideal target.
Frequently Asked Questions
The pool pH keeps crashing to 6.8 overnight even though the probe reads 7.4 at service time. What is happening?
The most likely explanation is probe calibration drift that corrects during the day but worsens overnight, combined with continuous dosing. A probe that reads 7.4 while actual pool pH is 6.8 is drifted by nearly 0.6 units. Clean the probe, recalibrate using actual pool water sampled from the flow cell, and check that the injection point is not upstream of the flow cell water connections.
Can the system overdose chlorine on a salt pool?
Yes. On a salt pool using ORP control in Auto Sensing mode, the system will run the cell at full percentage continuously if the ORP probe is reading artificially low. Free chlorine can climb to 10+ ppm before the ORP probe reading corrects. Always verify chlorine with a DPD test kit when investigating high ORP readings, regardless of what the display shows.
How can I tell if my CO2 solenoid is stuck open?
Observe the CO2 line or bubbler visible at the injection point when the control display is not calling for pH reduction (pH is within set point, no dosing commanded). If CO2 bubbles are still entering the water, the solenoid is stuck open or leaking. You can also briefly turn off power to the acid dosing relay — if CO2 flow continues after power is removed from the relay, the valve is failed mechanically.
What is a reasonable ORP set point for a typical residential salt pool?
For most residential salt pools with CYA in the 60–80 ppm range and chlorine maintained at 2–3 ppm, an ORP set point of 600–650mV is appropriate. Higher set points (700+ mV) will drive the cell harder and typically produce chlorine levels above 4–5 ppm. Lower set points (500–550mV) allow the chlorine to drop further before dosing begins, which works well on pools with consistently high CYA that suppress ORP naturally.