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How to prime a centrifugal pump

Updated: 22 October 2026

How to prime a centrifugal pump starts when the motor runs and the hose stays empty. Usual causes are an empty casing, air in the suction, a collapsed inlet hose, or too much lift. This page is the fill order, not a ranking. The listings sit on the [centrifugal pump comparison](/hub/garden-watering/centrifugal-pump). Who should buy which pump is on the [best centrifugal pump](/hub/garden-watering/centrifugal-pump/sub/centrifugal-pump-best) page. The butt layout is on [centrifugal pump for a water butt](/hub/garden-watering/centrifugal-pump/sub/centrifugal-pump-rainwater-tank), and duty sizing is the [centrifugal pump buying guide](/hub/garden-watering/centrifugal-pump/sub/centrifugal-pump-buying-guide). Peter F., editor for DIY & tools, updated this on 22 October 2026. We did not time a prime on these ASINs. A qualifying purchase can earn a commission at no extra cost to you.

How to prime a centrifugal pump starts when the motor runs and the hose stays empty. Usual causes are an empty casing, air in the suction, a collapsed inlet hose, or too much lift.

Our top picks

KATSU Tools KATSU 151116

Best overall

KATSU Tools KATSU 151116
8.8/10about £69.99
View offer
KATSU Tools KATSU 151112

Best value

KATSU Tools KATSU 151112
8.0/10about £49.99
View offer
Clarke Clarke ECP15A1

Alternative

Clarke Clarke ECP15A1
7.0/10about £125.92
View offer
On this page
  1. 1.Why it will not lift on an empty casing
  2. 2.Fill, foot valve, and the first start
  3. 3.If it primed once and then stopped
  4. 4.When to isolate it

Why it will not lift on an empty casing

A standard centrifugal impeller moves liquid. It does not seal a path that can evacuate air. Without liquid in the housing, the pump cannot clear the suction column. Even a self-priming pump still needs liquid in the casing and is not a dry-run tool. Both KATSU models, the 151112 and the 151116, say they are not self-priming. A UK review on the 151116 says the impeller chamber has to be primed before water moves. The Clarke and Pedrollo extracts do not claim self-priming, so a blank answer means you must fill it. Stated suction is 8 m on the KATSU pair and 7 m on the Pedrollo rows. Those are maxima. Near that ceiling, a running motor and a dry outlet means air, not a longer run.

Fill, foot valve, and the first start

Isolate power before you open plugs. Close any drain cock. Fill the pump casing through the priming port until water stays at the top and no large air pocket remains. Fill the suction hose as far as you can, or arrange a flooded suction so the inlet stays underwater. Fit a foot valve or non-return at the strainer if the water sits below the pump. A foot valve can hold prime after a stop, with a slam risk if it closes hard. Keep the strainer submerged and clear of silt. Use reinforced suction hose: ordinary soft delivery hose can flatten under vacuum and starve the pump even when the casing looks full. Seal every suction joint. A weeping inlet thread pulls air and looks like a weak pump. Restore power only when the casing is full and the drain is closed. If nothing arrives in the first minute, stop and re-check the fill and the joints. For a butt, place the pump beside the tank inside the suction figure.
KATSU Tools KATSU 151116

Testsieger

KATSU Tools KATSU 151116
8.8/10about £69.99
View offer

If it primed once and then stopped

A pump that worked yesterday and will not restart often lost its water column overnight. The foot valve leaked, a suction joint drew air, or someone opened a drain. Refill the casing and the suction before you blame the impeller. Check that the strainer is still submerged when the butt is low. A pump on a stand above a nearly empty tank pulls air. Separate lost prime from a motor protection trip. Reviews on the KATSU 151112 describe a thermal cut-out after about 5 to 20 minutes. That is heat protection, not a cue to keep resetting an empty casing. If the pump hummed, ran hot, or tripped, isolate it and cool it. Re-priming a hot dry pump is how seals and windings fail. If prime holds only while you pour into the port, the suction side is still leaking.

When to isolate it

Stop and isolate if the motor is hot, humming without flow, repeatedly tripping, or smelling of burnt insulation. A few seconds of dry running can damage a clean-water seal. Minutes of dry running are not a priming technique. Call a competent person for a new outdoor circuit, a missing 30 mA RCD, a burned motor, or a leaking mechanical seal. UK outdoor portable equipment wants RCD protection, and no kept file here proves a BS 1363 plug is fitted. Do not buy a taller suction claim to fix a pump that sits above the water on soft hose. Move it lower, shorten the suction, fit a foot valve, and fill the casing. Priming will not save a clean-water impeller in dirty water. If the job is mains boosting, follow water-fittings rules instead. When the casing stays full and the suction is tight, the empty hose usually ends. When it does not, the fault has moved into the electrics or a damaged pump.

Common questions

Why does the motor run and the hose stay empty?
Usually the casing or suction line still holds air, a joint leaks on the inlet, the soft hose has collapsed, or the lift is beyond the real suction. Fill and seal before you run it longer.
Do I fill only the pump or the whole suction hose?
Fill the casing fully and fill or flood the suction as far as you can. An air pocket in the inlet hose will undo a full casing.
What does a foot valve change?
It can hold water in the suction after you stop, so the next start keeps prime. It does not remove the need to fill the first time, and a slamming valve can shock the line.
Can a soft hose on the inlet stop priming?
Yes. Delivery hose often collapses under suction. Use hose rated for vacuum on the inlet side.
How long may I run it with no water?
As little as possible. Treat a dry run as a fault. Stop, cool, and refill. Thermal cut-out on the 151112 after minutes is a stop example, not a target.
What if it worked once and will not restart?
Refill and check the foot valve and suction joints for drain-back or air leaks before you replace the pump.
Are the Clarke and Pedrollo listings self-priming?
The extracts in this set do not claim it. The KATSU pair explicitly say they are not. Assume you must prime unless a listing clearly says otherwise.

Related pages

Centrifugal pump vs peripheral pump

Centrifugal pump

Best centrifugal pump UK: pressure, volume or skip

Centrifugal pump buying guide: flow, head and prime

Centrifugal pump for a water butt next to the tank

Sources

Public UK sources for abstraction, water fittings, hose taps and garden electrics. Maker and pump-trade pages for duty, priming and the CP family head. Listing prices, stars and review counts are the amazon.co.uk lines saved on 21 October 2026. This is not a lab test.

  1. GOV.UK — check if you need a licence to abstract water (retrieved 21.10.2026)
  2. Water Supply (Water Fittings) Regulations 1999 (retrieved 21.10.2026)
  3. Water Regs UK — hose union taps (retrieved 21.10.2026)
  4. Electrical Safety First — garden electrics (retrieved 21.10.2026)
  5. IET Wiring Matters — BS 7671 regulation 411.3.3 (retrieved 21.10.2026)
  6. Pedrollo — single-impeller centrifugal pumps (retrieved 21.10.2026)
  7. UK Flow Centre — which pump do I need (retrieved 21.10.2026)
  8. Michael Smith Engineers — self-priming pumps (retrieved 21.10.2026)