There are few more frustrating sounds around a Cape Town pool than a pump that whirs away but moves no water. If your pump keeps losing prime, it means air has crept into the system and the impeller is spinning against nothing but a pocket of air instead of a solid column of water. The good news is that a pump losing prime is almost always a fixable problem, and most causes come down to a handful of usual suspects. Here is how to diagnose and solve it.
What “losing prime” actually means
A pool pump is designed to stay full of water. When it is primed, the pump basket and volute are flooded, the impeller grips the water, and it pushes flow through the filter and back to the pool. “Losing prime” happens when air replaces some of that water. The pump then cavitates, the water level in the strainer basket drops, and circulation grinds to a halt. Left unchecked, a dry-running pump can overheat and damage the seal or motor.
Common reasons a pool pump keeps losing prime
In our experience servicing pools across the Cape Peninsula, air is getting in on the suction side. The pressure side (after the pump) simply leaks water; it is the suction side that draws air. Check these first:
- Low pool water level. If the water drops below the skimmer mouth, the skimmer sucks air. In summer, evaporation and the South-Easter can lower levels faster than you expect.
- A loose or dry pump lid O-ring. The clear pump lid must seal tightly. A perished, dirty or dry O-ring lets air whistle in. Smear it lightly with silicone lubricant, never petroleum jelly.
- A cracked pump basket or lid. UV and years of Cape sun make plastic brittle. A hairline crack is enough to break the seal.
- Loose unions or fittings. The threaded unions either side of the pump can loosen over time or lose their gasket.
- A blocked or air-locked suction line. Leaves, debris or a partly closed valve on the skimmer or weir line restricts flow and encourages air.
- A leaking suction-side valve. Multiport and gate valves have internal seals that fail with age.
The weir and skimmer weir door
Many Cape Town pools use a built-in weir. If the weir door (the floating flap) is jammed or missing, the skimmer can gulp air whenever the water surface ripples. Make sure the weir door swings freely and the basket underneath is not overflowing with leaves.
How to fix a pump that loses prime
Work through these steps in order, and switch the pump off before opening anything:
- Top up the pool until the water sits at least halfway up the skimmer opening.
- Open the pump lid, clean out the basket, and inspect it for cracks.
- Remove the lid O-ring, clean it, check for flat spots or splits, and lubricate it with silicone before refitting.
- Fill the pump basket with water from a hose to re-prime it, then close the lid firmly.
- Hand-tighten the unions on either side of the pump; do not over-torque and crack the fittings.
- Restart the pump and watch the strainer basket. It should fill and hold a steady, bubble-free flow within a minute or two.
A quick way to find a stubborn suction leak is to run the pump and slowly pour a little water over each joint and the lid seal. If the stream of bubbles in the basket briefly stops, you have found the leak.
When it is more than the pump
If you have replaced the O-ring, topped up the water and tightened every fitting but the pump still loses prime, the problem may lie in the pool shell or plumbing rather than the pump itself. Cracks in an old marbelite surface or leaks around the skimmer throat can let the water level drop and draw air. In those cases a proper marbelite pool resurfacing can seal the shell for good and protect your equipment. A switch to a durable fibreglass pool lining does the same. A leaking underground pipe is another culprit worth investigating before you replace an expensive pump.
A pump that loses prime is usually telling you something simple: air is getting in, or water is getting out. Methodically ruling out each cause will get most Cape Town pools circulating again. If you would rather have it sorted properly the first time, the team at Aquatic Pools can trace the leak, service your equipment and keep your water crystal clear. Feel free to get in touch with our Cape Town team for a hand.
