If a pump is delivering less flow than expected, which condition is most likely the cause?

• A. Discharge head is too low
• B. Discharge head is too high
• C. Check valve open
• D. Pump is primed

Answer: (B)

Flow from a pump is determined by the resistance the system presents to the flow, i.e., the discharge head. When the discharge head is higher, the pump must push water against more opposition, so the amount of water it can deliver at the same speed drops. This is why delivering less flow than expected is most likely due to the discharge head being too high—the pump is operating against a heavier load and sits at a point on its curve with lower flow.

If the discharge head were too low, you’d expect more flow, not less. If a check valve is open, it usually reduces resistance rather than increases it, so it wouldn’t explain reduced flow. A primed pump is ready to move water; if it weren’t primed, flow would be minimal or none, which is a different situation.