Pentair FNS Plus: Pressure Drops on the Gauge
A pressure gauge that drops well below its normal operating range — or falls to near zero with the pump running — signals that the pump is not generating its normal output. This is the opposite of a clogged filter problem, and diagnosing it correctly matters: a low pressure reading does not mean the filter is clean. It means water is not moving through the system properly. The FNS Plus manual's troubleshooting guidance directs you to the pump, not the filter grids, when pressure drops unexpectedly.
Understanding Pressure Gauge Readings
The pressure gauge on the FNS Plus monitors pressure on the outlet side of the filter. When the pump is off, the gauge should read zero — the manual specifies this as a requirement for a properly functioning gauge; if it does not return to zero when the system is shut off, the gauge itself needs replacement (P/N 53003201). When running, pressure builds from the pump's output and the resistance created by the filter media.
| Gauge Reading | Interpretation |
|---|---|
| At or near normal baseline | System operating normally |
| 10–12 psi above baseline | Time to backwash — grids loaded |
| Below baseline | Pump not generating normal output — investigate pump and suction |
| Near zero while pump runs | No meaningful water flow — suction blocked, pump not primed, or impeller stalled |
Step 1: Shut Off the Pump and Check for Obstruction
The FNS Plus manual's first step for a pressure drop is to shut off the pump. With the pump off:
- Turn off power to the pump at the circuit breaker or timer
- Turn off any automatic controls that could restart the pump unexpectedly
- Attempt to rotate the pump motor shaft manually with your fingers
The shaft is accessible from the back of the motor. If the shaft turns freely with light finger pressure, the motor is not mechanically seized and the problem is likely a clogged impeller or a suction obstruction. If the shaft does not turn, or turns with extreme resistance, the motor may have failed mechanically — refer to the pump manual and consider motor service or replacement.
Step 2: Inspect the Pump Strainer Basket and Skimmer Basket
Before opening the pump, check the two most common flow restrictions:
- Pump strainer (hair and lint pot) basket: If the basket is fully packed with debris, no water can reach the impeller; the pump runs dry and generates no pressure. Open the strainer pot (after shutting off the pump and ensuring the system is depressurized), remove the basket, and clean it thoroughly.
- Skimmer basket: A blocked skimmer starves the pump from the suction side. Remove and clean all skimmer baskets.
- Pool water level: Verify the water level is high enough that the skimmer is not drawing air. A skimmer drawing air produces the same low pressure symptom as a blocked basket.
If both baskets are clear and the water level is adequate, the impeller is the next point of investigation.
Step 3: Check the Impeller for Clogs
Per the FNS Plus manual, if the pump motor shaft turns freely with fingers but pressure is still low, disassemble the pump and check the impeller for obstructions. Hair, string, small leaves, and debris can wedge in the impeller vanes and dramatically reduce or eliminate water movement without stopping the motor.
- Shut off power at the circuit breaker
- Open the Manual Air Relief Valve on the filter and allow pressure to fully release
- Drain the pump strainer pot
- Disassemble the pump per the pump manufacturer's instructions to access the impeller
- Remove all debris from the impeller vanes — use needle-nose pliers or a small hook tool to extract hair and string wound around the impeller hub
- Reassemble the pump, fill the strainer pot with water, and restart
Step 4: Check for a Suction Line Obstruction
If the impeller is clear but the shaft would not rotate freely in Step 1, or if low pressure persists after cleaning the impeller, the manual points to an obstruction in the line between the pool and the pump. This includes:
- A valve that is partially or fully closed on the suction side
- A collapsed flexible hose section
- A large debris blockage in a pipe run (leaves packed at a fitting or elbow)
With the pump off, verify that all suction-side valves are fully open. If a valve was inadvertently closed during service, reopening it will restore flow immediately.
Step 5: Verify the Pressure Gauge Itself
The FNS Plus manual specifies that the pressure gauge should be replaced if it does not return to zero when the system is off, if it does not indicate pressure when the system is running, or if the gauge face is cracked, fogged, or damaged. A failed gauge can mislead diagnosis entirely — what appears to be a system pressure drop may simply be a gauge that has stopped working.
Replacement pressure gauge P/N: 53003201