A pump that has "lost its prime" has lost the water inside it and is sucking in air. Without water it can't push anything, and it also heats up: the mechanical seal, the part that stops water escaping along the motor shaft, gets damaged if it runs dry. So the first thing to do is switch it off.
Priming the pump, in order
How to tell it has lost prime
- The pre-filter bowl (the clear lid) is half empty or full of bubbles.
- No water, or very little, comes out of the return inlets.
- The filter pressure gauge reads zero or much lower than usual.
- The motor is noisier than usual or feels very hot.
Why it loses prime
- Low water level: if the water drops below the middle of the skimmer mouth, the skimmer swallows air. It's the most common cause in summer, because of evaporation.
- Pre-filter lid not properly closed, or its O-ring dry, deformed or dirty.
- Blocked baskets: skimmer or pre-filter baskets full of leaves don't let the water through.
- Suction valves closed (skimmers, main drain or vacuum line) or nearly closed.
- Air getting into the suction pipe: a fitting, a joint or a valve that doesn't seal properly.
- Pump above the water level: when it stops, the water in the pipe runs back into the pool if the check valve doesn't close properly, and the next day the pump starts empty.
How to prime it, step by step
- Switch the pump off at the control panel.
- Check the pool level: it must reach at least halfway up the skimmer mouths. If not, top up first.
- Set the multiport valve to "filter" and open the suction valves (skimmers and main drain) and the return valve.
- Open the pre-filter lid, take out the basket and clean it.
- Fill the pre-filter with water using a hose, right to the top, slowly, so the start of the pipe fills too.
- Check the lid O-ring: clean it, make sure it isn't cracked or flattened and apply a little silicone lubricant. Close the lid firmly.
- Switch the pump on and open the filter air bleed (the screw on top) until water comes out without air.
- Wait one or two minutes: the pre-filter should fill and the gauge should rise. If not, switch off and repeat from step 5.
If it keeps losing prime
Then air is getting in somewhere on the suction side, before the pump. In order, check:
- The pre-filter lid seal: if it's old, replace it. It's the most common cause and the cheapest.
- Bubbles at the return inlets with the pump running: a clear sign that air is getting in.
- Fittings and joints on the suction pipe next to the pump: with the pump running, pour a little water over a joint; if the sound changes or the bubbles stop for a moment, you've found the leak.
- The check valve, if the pump sits above the water level.
If it's still the same after all this, there may be a leak in a buried pipe or a problem with the pump itself. That's when it's worth having a technician look at it. If you're also losing water, see our guide to finding leaks.
Frequently asked questions
How long can the pump run without water?
As little as possible. If it hasn't picked up water within a couple of minutes, switch it off and prime it again. Running dry overheats the pump and can damage the mechanical seal.
Why does the pump lose prime every morning?
Usually because it sits above the water level and, when it stops overnight, the water in the pipe runs back into the pool. It's normally down to a check valve that doesn't close properly or an air leak on the suction side.
What lubricant should I use on the lid seal?
A silicone lubricant made for seals. Avoid petroleum-based oils or greases, which can swell or damage some rubbers.