Common pump faults and what they mean

Pump fault finding

Match the symptom to the part

Why does my pump hum but not start?

A pump that hums, gets hot and does not turn has usually lost its start capacitor. The same goes for one that only runs once the cooling fan has been nudged. A seized impeller or a collapsed bearing will hum and sit still in exactly the same way, so the capacitor is the first suspect, not the only one.

Do not spin or poke the fan on a pump that is switched on. A stalled motor can take off the instant it frees, and the fan is behind a shroud on a live 230 volt machine. If you already know it starts that way, that is a symptom to report, not a test to repeat: isolate the pump and let an electrician confirm it.

A single phase motor has no starting torque of its own, so it cannot work out which way to turn. The capacitor puts the current in the start winding out of phase with the run winding, and that difference gives the rotor its kick. Once spinning it keeps spinning, which is why a nudged pump runs happily and will not start next time. Stalled, it draws locked rotor current and heats fast, and if the thermal cutout cannot keep up it cooks the winding.

Repair or replace

Heat, time and starts kill capacitors. A true start capacitor is an electrolytic type that dries out with age, and every start loads it, so a pump switching on and off constantly gets through them fast. Many house pumps instead run a permanently connected run capacitor, which is a film type and fails differently, but heat and hours get both in the end. A failed one sometimes has a domed or split top, but plenty fail looking perfect.

It is almost always a repair: the capacitor is among the cheapest parts on the pump. The replacement must match the microfarad value printed on the old one, and carry at least its voltage rating, because the wrong value means weak starting and a shorter motor life. A capacitor can hold a charge after the pump is unplugged, and it lives behind the terminal cover on a 230 volt motor, so that is an electrician's job.

Why is my pump so noisy?

A pump that has got steadily louder over months, rumbling, grinding or whining, usually has bearings on the way out. What separates it from the rest is where the noise sits: bearings talk from the motor end, and it usually arrives with vibration through the pipework and mounting feet. Gravel rattling at the wet end is more often cavitation, from a restricted or airy suction, and that is a different fix. Heat on its own tells you little, because a pump motor runs warm and always has, but one running hot on a duty it used to run cool on has something wrong.

Hours and heat get there in the end. The most common cause we see on house pumps is water past a failed mechanical seal. Constant starting and stopping is second.

Bearings are a standard part, but the job is pulling the pump down and pressing them off and on. On a small house pump that labour, plus the seal and impeller you find worn on the way through, usually costs more than a new pump. On a bigger multistage or three phase unit the numbers flip and a rebuild makes sense.

Why is water leaking between the motor and the pump?

A drip, a rust streak or a white crust at the joint between the motor and the pump body is nearly always the mechanical seal, though a perished casing o ring or a cracked housing can look the same. Water at a fitting or a union on the discharge side is a plumbing leak and a smaller problem. A leak on the suction side is not: it lets air in, the pump loses prime, and that is how the seal gets killed in the first place.

A mechanical seal is two flat lapped faces, one turning with the shaft and one fixed in the housing, held together by a spring. A thin film of the water you are pumping sits between them and both seals and lubricates. Take it away and the faces run dry, and the damage is done in seconds.

That is why running dry is the number one killer of seals. An empty tank, a closed valve on the suction side, an air leak in the suction line or a bore drawn below the pump will all do it. Sand and grit score the faces.

A seal that weeps a few drops on the first run after standing sometimes beds back in. One that drips steadily will not. It is a wear part and meant to be replaceable, so the sum is seal kit plus labour against a new pump: an easy yes on bigger pumps, often not on a small garden pump. Either way, fix the reason it failed.

Why will my pump not stop, or not start?

The pressure controller decides when your pump runs: a mechanical pressure switch and pressure tank on older systems, an electronic controller such as a Davey Torrium2 or Grundfos PM Plus on newer ones. The electronic ones start the pump when the pressure drops as a tap opens, and stop it once the flow stops.

What you can safely check before you call

1. Isolate it

   Unplug it, or switch it off at the isolator, before you touch anything.

2. Check it has water to pump

   Tank level, suction valve open, foot valve clear. A lot of dead pumps are just dry pumps.

3. Press the reset

   A controller that will not start is often locked out from a dry run.

4. Check the filters

   With the pump off and the housing pressure released, check the cartridges. A blocked one starves the pump and holds pressure down. See the filter change guide or the replacement cartridges.

5. Write down what it does

   Humming or silent. Noise at the motor end or the wet end. Wet or dry underneath. Make and model off the plate. That turns a site visit into a phone call.