Why does my pool pump keep losing prime?

Pool pump losing prime: what it means

A pump is primed when its housing and suction lines are full of water. The impeller moves water, not air, so when air collects in the pump it spins without pulling water through the system. You may see a half-empty pump basket, bubbles or foam in the strainer pot, air spitting from the returns, a low reading on the filter gauge, or a pump that runs for a while and then loses flow.

Losing prime is different from a pump that won’t start at all. If the motor is silent or only hums, see why a pool pump won’t turn on.

Common reasons a pool pump loses prime

• Low water level: when the water drops below the middle of the skimmer opening, the skimmer can gulp air, especially on windy days or at high pump speeds. If the level keeps falling, see why a pool loses water.
• Full or clogged baskets: a packed skimmer basket, pump basket or cleaner bag starves the pump of water and can pull in air.
• Pump lid and O-ring: a cracked lid, a dry or worn lid O-ring, or a lid that isn’t seated squarely lets air in every time the pump runs.
• Suction-side fittings and valves: loose unions, cracked fittings, valve stems and drain plugs on the suction side tend to pull air in rather than drip water, so they often look dry.
• Underground suction leaks: a cracked suction line between the pool and the equipment can draw air while the pump runs and lose water while it’s off. Our guide to underground pool plumbing leaks explains how those are found.
• Closed or unbalanced valves: a suction valve turned too far, or skimmer and main drain valves left off balance after cleaning, can restrict flow.
• Clogged impeller or line: leaves, pine needles and small debris can block the impeller or a suction line and cut the water the pump can pull.
• Pump above the water level: pumps set well above the pool work harder to hold prime, and a worn check valve can let water drain back when the pump stops.
• Variable-speed settings: a variable-speed pump running at a very low speed may not pull enough water to keep prime, especially at startup or with a long suction run. Most have a priming setting for startup.

Simple checks you can do safely

1. Turn the pump off at its normal control before opening anything.
2. Check the water level and bring it up to about the middle of the skimmer opening if it’s low.
3. Empty the skimmer and pump baskets, and check the cleaner bag or canister.
4. Look at the pump lid and its O-ring. Clean the O-ring and its groove, and replace an O-ring that is flat, cracked or stretched. Use only a lubricant made for pool O-rings.
5. Make sure the suction valves are open in their normal positions.
6. Fill the pump’s strainer pot with water, close the lid hand-tight and squarely, and open the filter’s air relief valve.
7. Start the pump and watch the basket. When a steady stream of water comes out of the air relief valve, close it.

If the pump doesn’t catch prime within a few minutes, turn it off. A pump running without water can overheat and damage the pump housing, lid and nearby plastic fittings.

When a pump that loses prime needs a technician

Call us if the pump keeps losing prime after these checks, if you see bubbles in the pump basket even with a good water level and clean baskets, if the pump runs hot, or if the pool is losing water at the same time. A technician can check unions and valves under pressure, pressure-test the suction lines and look for cracked fittings that are hard to spot from outside.

Air in the system also wears a pump over time. If the pump is older or parts are hard to find, our pool pump repair and replacement page explains how we compare a repair with a new pump.

How to keep your pump primed

• Keep the water level near the middle of the skimmer opening, especially before running the pump at high speed.
• Empty the baskets regularly, and more often after storms and heavy leaf drop.
• Clean and lubricate the lid O-ring when you clean the pump basket.
• Return valves to their normal positions after cleaning or vacuuming, and ask us to mark them if you’re not sure which is which.
• With weekly pool service, the water level, baskets and pump operation are checked at every visit.

Related pool pump questions

• Why is my pool pump noisy or losing flow?
• Why won’t my pool pump turn on?
• Pool pump repair in Jupiter, FL
• More pool problems and answers

Let’s find the right next step

Get Wet Pools serves Jupiter, North Palm Beach, Tequesta and nearby communities. Tell us what the pump is doing and when it started, and confirm service availability with the office.