Why a pool pump will not prime

What does priming actually mean on a pool pump?

A pool pump is not a vacuum machine. It is a centrifugal pump, which means it moves water by throwing it outward with a spinning impeller, and an impeller can only throw what it is already touching. Priming is the act of getting water into the pump housing and up to the impeller so it has something to work with.

Once primed, the pump maintains its own suction and the whole loop runs as a closed column of water from the skimmer, through the pump, through the filter and back to the returns. Break that column anywhere on the suction side and the pump starts spinning air, which it cannot move.

This is why the fix is almost never inside the motor. The motor is doing its job. Something between the pool and the impeller is letting air into a space that is supposed to hold only water.

Where is the air getting in?

Start at the water. If the pool level has dropped below roughly the middle of the skimmer mouth, the skimmer gulps air on every wave and the pump loses prime within minutes. In July this happens quietly, because evaporation takes an inch a day and nobody notices until the pump starts surging.

Then the pump lid and its o ring. That o ring is the single most common cause of a no prime call in this trade. It dries out, flattens, twists in its groove, or gets pinched when somebody puts the lid back on in a hurry. A dry o ring seals well enough when wet and leaks air the moment the pot drains.

Then the drain plugs on the bottom of the pump pot, which get loosened for a freeze and never fully snugged again. Then the valves, because a multiport or a three way left half turned will starve the pump without looking obviously wrong. Then the union nuts, which back off slowly as the pad shifts.

Last is the buried plumbing. A crack in a suction line under the deck pulls air rather than leaking water, because that pipe is under vacuum while the pump runs. That is why a suction side leak shows no puddle and still stops the pump from priming.

What can you check yourself in ten minutes?

Water level, first and always. Get it back up to the middle of the skimmer opening and try again. This one costs nothing and solves a real share of no prime calls between June and September.

The lid o ring next. Turn the pump off at the breaker, take the lid off, pull the o ring out, wipe it, look for cracks and flat spots, and put it back with a thin film of a silicone lubricant made for pool o rings. Never use petroleum grease, which swells the rubber and ruins it.

Then fill the pump pot with a bucket of water, put the lid back on straight, and start it. Watch the pot. If it fills and holds, you are done. If you can see a swirling funnel of air in there while it runs, the leak is upstream of the pump and you are looking at the skimmer, a valve or the buried line.

What you should not do is keep running a pump that will not catch. A dry pump has no water to cool the shaft seal, and a shaft seal that overheats takes the seal, sometimes the impeller, and occasionally the housing with it.

Why does a pump prime in the morning and lose it by afternoon?

Because the leak is small and the conditions change. Heat expands plastic, sun softens an old lid, and afternoon evaporation drops the water line another fraction of an inch. A marginal seal that holds at seven in the morning gives up at four in the afternoon.

A variable speed pump adds its own version of this. Many pads are programmed to prime at high speed and then drop to a low cruise speed, and a small air leak that the high speed can overcome will unprime the pump the moment it slows down. If your pump runs fine for the first few minutes of every schedule and then surges, look at the speed change before you look at the pump.

The third pattern is a suction side cleaner. A pressure or suction cleaner hose that has developed a split, or a leaf canister lid that no longer seals, introduces air only while that line is in use.

When is a no prime call actually a pump failure?

When the impeller is packed. Acorn caps, a wad of leaves that got past a cracked basket, or a chunk of a broken skimmer weir can jam the impeller vanes and leave a pump that spins, sounds normal and moves almost nothing. That is a wet end repair, not a replacement.

When the shaft seal has already gone. If the pump has been run dry, water gets past the seal into the motor, and now you have a pump that will not prime and a motor that is on a countdown. Any drip from the joint between motor and housing is worth acting on immediately.

And when the housing itself is cracked. A freeze split in the pump pot lets air in above the water line and water out below it, and no amount of o ring work fixes that. On a housing crack the pump gets replaced, because a patched pressure vessel fails without warning.