Pump losing prime? Find where the air gets in

1Water level, skimmer and baskets

• Water level: low water, or a weir stuck in the up position, lets the skimmer draw air. Some manuals say keep the water at least halfway up the skimmer box.
• Baskets: empty the skimmer and pump baskets.
• Cleaner: disconnect any suction cleaner while you diagnose.
• Valves: open the isolation valves and set diverter valves to normal running. Pump makers warn that running against shut valves can damage it.

Still starved? Check for a blockage in the suction line or at the impeller entry.

2The pump lid and O-ring

Power off and release pressure at the filter’s air-relief valve. Pump below water level? Close the isolation valves first. Run dry? Read Safety before you touch the lid.

Clean the O-ring and its seat, and replace the O-ring if it’s cracked, flattened or damaged. Seat the basket properly or the lid won’t seal. Tighten the lid by hand, never with a tool. Lubricate O-rings with silicone-based lubricant, never petroleum jelly: it breaks down the rubber.

3Unions, fittings and valves on the suction side

Check every union, O-ring, threaded fitting, glued joint and valve between the pool and the pump, plus the strainer drain plugs and valve-stem O-rings (a common leak point).

• Barrel unions: some are hand-tightened and seal on their O-rings, with no sealant or glue at the pump. Check your model’s manual. No thread tape on O-ring unions.
• Threaded fittings: some manuals say to re-seal leaking threaded suction fittings with thread tape.
• Glue: some PVC jointing compounds damage ABS unions. Check compatibility first.

Priming it properly

1. Fill the strainer housing with water and refit the lid.
2. Open the filter’s air-relief valve. Close it once a steady stream of water comes out.
3. Start the pump, standing clear of the filter. Not primed within the time in its manual (some allow about 1–2 minutes; some variable-speed pumps take longer)? Switch off, refill and retry. Some manuals say stop after three failed attempts and look for leaks.

Bubbles at the returns are normal while priming. It’s primed when they stop and the basket is full. Bubbles that keep coming point to a leak.

Long suction lines take longer to prime, and high points in the pipe trap air. Pump more than about 500 mm above water level? Some manufacturers say it generally needs a foot valve or suction-side check valve to hold prime. Check compatibility with any suction vacuum release system (SVRS) first.

Variable-speed pump won’t prime? Check against its manual that the priming cycle isn’t shortened or off and maximum speed isn’t capped too low.

Safety

• Don’t open a pump pot that has run dry or failed to prime. It can hold scalding water under vapour pressure. Open the valves, wait until it’s cool to touch, then open it with extreme caution.
• Release pressure before opening the lid, at the filter’s air-relief valve.
• Trapped air can blow a filter lid off, and pump makers warn this can cause death or serious injury. Bleed all the air out before running, and stand clear of the pump and filter when starting (some manuals say at least 3 m).
• Check valves can interfere with some SVRS products and create an entrapment risk. Read the SVRS manual first.
• Never bridge or bypass a chlorinator’s flow sensor. A cell running with no flow can build up flammable gas, and tampering with the flow switch can cause injury or damage the cell.
• Electrical: the pump must be supplied through a 30 mA RCD and wired to AS/NZS 3000 by a licensed electrician.

Never run it dry

Pool water cools the mechanical seal, so running dry can quickly damage it, and that damage isn’t covered by the manufacturers’ warranties. Water under the pump usually points to a leaking shaft seal: rule out drain plugs, unions and a cracked housing, then call a service technician.

When it isn’t the pump

Final step

Once it’s primed, watch the lid.