Pool Pump Humming But Not Pumping? Here Is Why
A humming pump is getting power but not turning. Before anything else, go to the breaker and switch the pump circuit off — a pump that runs dry can melt its own housing in under an hour, which is how a cheap repair becomes an expensive one.
We take this call several times a week through a North Texas summer. The good news is that a humming pump is usually the cheapest kind of pump failure, because the motor itself is still alive. Here is how to work out which of the three causes you have.
What the humming actually means
An induction motor needs a shove to start turning. Once it is spinning it keeps itself going, but from a dead stop it needs starting torque. A hum means the windings are energised and trying, and something is stopping the rotation.
That something is one of three things: the part that provides the shove has failed, the impeller is jammed, or the shaft has seized. In that order of likelihood.
Cause 1: a failed start capacitor
The most common by a wide margin, and the cheapest to fix. The capacitor is a small cylinder under a cover on the back or top of the motor, and it stores the jolt of energy that gets the motor moving.
How to spot it
A dead capacitor often looks dead. The top is domed or split instead of flat, or there is oily residue around it. If the motor hums and the shaft still spins freely by hand, the capacitor is the prime suspect.
Why they fail here specifically
Heat. Capacitors are rated to a temperature range, and an equipment pad in Arlington in August runs well past it — especially a pad in full afternoon sun with no airflow. We replace more capacitors in July and August than in the rest of the year combined.
One caution: a capacitor holds a charge after the power is off, enough to hurt you. If you are not confident discharging one safely, stop here and let a technician do it.
Cause 2: a jammed impeller
The impeller is the vaned disc that actually moves the water. It sits behind the basket, and anything small enough to slip past a cracked or badly seated basket can wedge in it — pebbles, acorns, a plastic fragment, pieces of a disintegrating skimmer weir.
How to spot it
With the power off, open the pump pot and look at the basket. A cracked basket, or one that does not seat properly, is a strong hint that something got through. If you can reach the impeller, try turning it by hand. Gritty, stiff or locked solid means debris.
Clear the obstruction and the pump usually starts normally again. Replace the basket while you are in there, or you will be doing this again next month.
Cause 3: a seized motor shaft
The least common of the three and the most expensive. Bearings corrode and bind, usually on a pump that sat unused over winter, or one that has been running with a leaking shaft seal letting water into the motor.
How to spot it
Take the cover off the back of the motor and you will find the end of the shaft, often with a screwdriver slot in it. Try to turn it. A healthy shaft turns with light resistance. A seized one will not move at all, or moves with a grinding you can feel through the handle.
If the shaft is seized, the motor is the repair rather than the pump — and on a unit over about ten years old, replacement is usually the better value.
A two-minute test to tell them apart
With the breaker off, take the back cover off the motor and spin the shaft by hand. What happens next tells you most of what you need to know.
Shaft spins freely, pump still hums
Capacitor. Nothing mechanical is binding, so the motor simply has no starting torque.
Shaft is stiff or will not turn
Either a jammed impeller or seized bearings. Open the pot and check the basket and impeller first, because that is the cheap one of the two.
Shaft spins and the motor runs, but no water moves
That is a priming problem rather than a humming problem — an air leak on the suction side, a low water level, or a blocked line.
What it costs to put right in Arlington
A capacitor replacement is the common outcome and the cheapest of the three. Clearing a jammed impeller is usually absorbed into the service call. A motor replacement is a bigger job, and on an older single-speed pump it is often the moment to move to a variable-speed unit instead, because the energy saving pays back a real share of the difference.
Our service call is $149, it covers the full diagnosis, and it comes off the repair price if you go ahead. Every repair carries a one-year warranty. See pool pump repair for the detail, or equipment repair if it turns out to be the filter or the plumbing rather than the pump itself.
Keeping it from happening again
Shade the equipment pad if you can, even a simple cover, because heat is what kills capacitors here. Replace a cracked pump basket straight away rather than at the end of the season. And if you close the pool over winter, turn the motor over by hand once a month — thirty seconds of effort that prevents the most expensive of these three.
Pool Pros — Family-owned pool service in Arlington, TX since 2005. Call 682-399-2593 for a free estimate.