Repairs & how-to
How to prime a pump
Priming is the most important habit with any pump: filling the pump body and the suction line with water so it has something to grab when it starts. Run a pump dry, even for a few seconds, and you can ruin the seal. It takes a minute.
What priming is, and why it matters
A pump moves water by pushing on water that’s already there. If the pump body or the suction line is full of air, the impeller just spins in air, moves nothing, and overheats the seal. Priming fills that space with water first. Self-priming pumps can clear some air on their own, but they still need water in the pump body to start.
What you need
- A way to get water into the pump, usually a priming plug or port on top.
- A bucket of clean water or a hose.
- A rag, and a spanner if the plug needs one.
Step by step
- Switch off and isolate the pump at the power point.
- Open the priming plug or port on top of the pump body.
- Slowly fill the pump body (and, on a suction setup, the suction line) with clean water until it overflows and no more air bubbles up.
- Refit the priming plug firmly.
- Open a tap or the discharge so air can escape as it starts.
- Switch on. Water should establish within a minute; if it doesn’t, switch off and re-prime, there’s still air in the line.
If it won’t hold prime
A pump that keeps losing prime is drawing air in on the suction side: a loose fitting, a perished seal, or a leaking foot or non-return valve letting water drain back when it stops. Tighten the suction fittings and check the foot valve (see our check-valve guide).
Safety: Isolate the power before opening the pump. Never run a pump dry to see if it catches, that’s exactly what damages it.
Common questions
Do I need to prime every time?
No. Once primed and running, a pump with a good foot or non-return valve holds its prime between uses. You only re-prime after it has been drained, serviced, or has lost prime through an air leak.
What does self-priming mean?
A self-priming pump can clear air from the suction line on its own within limits, but it still needs water in the pump body to start. It isn’t a licence to run it dry.