An undersized dosing pump cannot hold a residual during peak bather load, and an oversized one overfeeds, wastes chemicals and makes tight ORP control almost impossible. Both mistakes are common because dosing pumps are often selected from a catalog by habit rather than calculated from the pool's actual chlorine demand, injection pressure and chemical strength. For contractors, MEP engineers and facility managers, the calculation is simple once the steps are laid out. This guide shows how to estimate feed rate, convert it into GPH capacity, choose between peristaltic and diaphragm pumps, check injection pressure against system TDH, and interlock the pump so it feeds only when water is flowing.

What Is a Chemical Dosing Pump and What Does It Control?

A chemical dosing pump is a metering pump that injects a controlled volume of liquid chemical, such as sodium hypochlorite or acid, into the circulation line. In a commercial pool, it is usually driven by a controller that reads ORP, free chlorine or pH and adjusts feed to hold a setpoint. Industry practice targets free chlorine of 1.0–3.0 mg/L at pH 7.2–7.8. The dosing pump is the actuator that makes this range achievable under changing bather loads.

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Calculating Chlorine Feed Rate

Chlorine feed rate depends on pool volume and peak chlorine demand. A simple planning rule is that 1 mg/L of chlorine per million gallons requires 8.34 lbs of available chlorine. To calculate, multiply your volume (in millions of gallons) by 8.34, then by your estimated peak daily demand (mg/L). Convert this to product volume based on chemical strength (e.g., 12.5% sodium hypochlorite yields roughly 1 lb of available chlorine per gallon).

Key Rule: Always size to peak demand during open hours, not daily averages, to ensure the pool stays sanitized during busy periods.

Determining GPH Capacity and Headroom

Once you have the peak volume per hour, apply a safety factor of 2–3 times that rate. This ensures the pump runs mid-range (where accuracy is highest) and has headroom for weaker chemical batches or sudden bather spikes. If your calculated rate is extremely low, choose a pump with a wide turndown ratio rather than a large pump that will struggle to maintain accuracy at its minimum output.

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Peristaltic vs. Diaphragm Pumps

• Peristaltic Pumps: Ideal for sodium hypochlorite because they are self-priming and handle off-gassing bubbles well. The pump tube is a wear item needing scheduled replacement.
• Diaphragm Pumps: Handle higher injection pressures efficiently, but may require degassing heads to manage off-gassing chemicals like chlorine.

Injection Pressure and System Interlocks

Dosing pumps must overcome the return-line pressure at the injection point. Ensure the pump's rated pressure exceeds the system's maximum pressure. Safety is non-negotiable: the dosing pump must be interlocked with the main circulation pump using a flow switch or electrical relay to ensure chemical feed stops instantly if water flow ceases, preventing concentrated chemical buildup.

Frequently Asked Questions

Q: How do I size a pump GPH capacity?

A: Calculate peak lbs/day of chlorine needed, convert to GPH, and select a pump with 2-3x headroom.

Q: Why peristaltic for chlorine?

A: They are self-priming and immune to gas bubbles (off-gassing) that can cause air-locks in diaphragm pumps.

Q: Is chemical flow interlock required?

A: Yes, mandatory to stop feed if flow stops, preventing dangerous concentrated chemical pockets in the plumbing.

Q: How do I select by accuracy?

A: Avoid running pumps at the very bottom of their turndown range; size for the mid-range to maintain dose precision.

Conclusion

Sizing a dosing pump requires matching GPH capacity to peak demand with 2-3x safety headroom, considering chemical type, and ensuring flow interlocks are installed for safety. Relying on professional sizing prevents both under-dosing and chemical waste. Request a Technical Consultation →

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