Most pump faults come from a short list of causes. Work through the symptom systematically, cheapest and most likely checks first, and you will find the great majority without special tools.
Before anything else: isolate the power before touching any electrical component or opening the pump. If the fault involves burning smells, scorch marks, water at electrical connections, or repeated tripping of an RCD or breaker, stop and call a licensed electrician.
The pump will not start
| Check | Action |
|---|---|
| Power at the outlet | Test the socket with something else. Check the breaker and RCD at the board |
| Thermal overload tripped | The motor may have cut out on heat. Let it cool 30 minutes and retry |
| Pressure switch or controller | Is it calling for the pump? System pressure may be above cut-in |
| Seized impeller | Isolate power, then turn the shaft by hand. If it will not move, the pump is jammed |
| Capacitor failure | A motor that hums but will not turn usually has a failed start capacitor |
| Loose connection | Isolate, then inspect terminals for looseness or corrosion |
Motor hums but does not turn. Almost always either a failed start capacitor or a seized impeller. Isolate the power and try turning the shaft. If it turns freely, suspect the capacitor — an electrician’s job. If it will not turn, the pump needs opening and clearing.
Trips the breaker immediately. An electrical fault — winding failure, damaged cable, or water ingress. Do not keep resetting it. Call an electrician.
The pump runs but no water comes out
The most common complaint, and nearly always a suction-side problem.
| Cause | Check |
|---|---|
| Lost prime | Refill the pump casing and the whole suction line, then restart |
| Air leak on the suction side | Brush soapy water over the joints while running, look for bubbles drawn in |
| Foot valve stuck or leaking | Lift and inspect. Debris in the seat prevents it holding prime |
| Suction lift too high | More than ~7–8 m vertical will not prime. Lower the pump |
| Blocked strainer or inlet | Clear debris |
| Water source empty | Check tank or dam level |
| Air pocket in the suction line | A high point mid-run traps air. The line must rise continuously to the pump |
| Wrong rotation | After electrical work, a three-phase motor may be wired backwards |
Do not keep running a dry pump while diagnosing. Every minute damages the mechanical seal. Two minutes without water is enough — switch off and investigate.
The suction line requirements in Installing a Domestic Pressure Pump cover the design faults behind most of these.
Low pressure or weak flow
| Cause | Check |
|---|---|
| Partially blocked strainer or inlet | Clean it |
| Worn impeller | Flow falls gradually over years. Compare against commissioning figures |
| Undersized pipe or hose | Check delivery size against the flow — see the sizing tables |
| Leak in the system | Walk the line. Underground leaks show as damp patches or unexplained running |
| Pressure switch set low | Have the cut-out setting checked |
| Partially closed valve | Check every valve in the run |
| Pressure tank waterlogged | Press the air valve — water instead of air means a failed diaphragm |
| Too many outlets at once | Demand may simply exceed the pump’s capacity at that head |
| Air in the system | Bleed at the highest outlet |
Pressure fine at first then fades. Usually a source problem — the tank or dam drawing down, or a foot valve slowly losing prime.
Pressure good near the pump, poor at distance. Friction loss. The pipe or hose is undersized for the run, or too long. See Understanding Flow Rate, Head and Pump Curves.
The pump cycles on and off rapidly
Called short-cycling. It is the fastest way to destroy a motor, because starting is what stresses it.
| Cause | Fix |
|---|---|
| Pressure tank pre-charge wrong | Set to 2 bar with the tank drained of water |
| Pressure tank diaphragm failed | Air valve gives water instead of air — replace the tank |
| Tank too small for the pump | Drawdown is insufficient. Add a second tank in parallel |
| Small leak in the system | A dripping tap or weeping joint bleeds pressure and restarts the pump |
| Pressure switch differential too narrow | Have the gap between cut-in and cut-out widened |
| Faulty non-return valve | Water flows back through the pump after it stops |
Start with the pre-charge. It is the cause more often than everything else combined, it costs nothing to check, and it is described step by step in Pressure Tanks Explained.
A controller such as the RMPC-10 or RMPC-30 handles start/stop on flow rather than pressure and avoids tank-related cycling entirely — though it will start the pump for every draw. The trade-offs are in Pressure Tank or Pump Controller?.
The pump runs constantly and will not stop
| Cause | Check |
|---|---|
| Cannot reach cut-out pressure | Worn impeller, or demand exceeds capacity |
| Leak in the system | Pressure never builds. Look for a running toilet, open valve or burst line |
| Pressure switch stuck | Contacts welded closed — replace |
| Controller not sensing flow stop | Debris in the flow sensor, or the unit is mounted wrongly |
| Lost prime | Motor runs, no water moves, pressure never rises |
A pump that runs continuously will overheat. If you cannot find the cause quickly, switch it off.
Noisy or vibrating
Rattling, like gravel through the pump — cavitation. Vapour bubbles forming in the suction and collapsing violently in the impeller. It erodes metal quickly and must not be left. Causes:
- Suction line too small or too long
- Suction lift too high
- Blocked strainer or foot valve
- Pump running far past its design point
- Water too warm
Whining or grinding. Bearing wear. It will get worse. Plan a service.
Knocking on start-up or shutdown — water hammer. The moving column of water stopping abruptly. Fit or check a small pressure tank such as the RM-R2 or RM-R8, which are designed for exactly this, and use slow-closing valves.
Vibration through the mounts. Loose bolts, an unlevel or soft base, or a damaged impeller. A pump on soft ground vibrates itself apart and stresses the pipework.
Loud but otherwise normal. Check the pump is not mounted on a resonant surface. A rubber mat under the base often makes a surprising difference.
The thermal overload keeps tripping
The motor is protecting itself from heat. Something is making it work too hard.
| Cause | Check |
|---|---|
| Blocked ventilation | Clear debris and vegetation around the motor |
| Ambient temperature too high | Shade and ventilate the pump |
| Running dry | No water means no cooling. Check the source and the prime |
| Voltage too low | Long or undersized cable runs. Have voltage measured at the pump under load |
| Pump running past its curve | Too much flow at too little head overloads the motor |
| Bearing or seal failure | Increased friction, increased current |
| Dead-heading | Running against a closed outlet heats the water in the casing rapidly |
Repeated tripping is a symptom, not a nuisance. Find the cause rather than resetting it repeatedly.
Water is dirty or has air in it
Air in the water — spitting taps. Air leak on the suction side, water level dropped to where the foot valve is drawing air, or the pressure tank diaphragm has failed and is passing air into the water.
Sandy or silty water. From a bore, the pump may be set too close to the bottom, or the bore screen may be failing. From a dam, the foot valve is too near the bed — raise it and fit a strainer.
Discoloured water after a period of no use. Often just settled sediment disturbed on restart. Run to waste until it clears. If it does not clear, investigate the source.
The pump has stopped and everything looks fine
Check, in order:
- Has run-dry protection activated? Many controllers latch off and need a manual reset.
- Has the thermal overload tripped? Wait 30 minutes for it to cool.
- Is the RCD or breaker tripped at the board?
- Is the tank empty?
- Is the pressure switch or controller getting power?
Faults needing a professional
- Any burning smell, scorch mark or melted insulation
- Water present at electrical connections
- Repeated RCD tripping
- Motor winding or capacitor replacement
- Any work on fixed wiring — licensed electricians only, under AS/NZS 3000
- Bore pump extraction
- Internal pump repairs on a unit still under warranty — opening it may void cover
Prevention
Most of the faults above are avoidable. The Water Pump Maintenance Schedule sets out the checks that catch them early, and the two that matter most are simple: keep the pressure tank pre-charge correct, and never let the pump run dry.
