Why a small pump won't prime or loses flow

A pump that runs but moves no water is frustrating. The good news is that the causes are few, and you can check them in order without any special tools.

First, what "prime" means

Priming is filling the pump with liquid so it can start moving water. Wikipedia says all centrifugal pumps need liquid in the casing to prime. If the casing fills with vapour or gas, the impeller becomes gas-bound and can't pump¹. The impeller is the spinning wheel inside. Air doesn't give it anything to grip.

Some pumps cope better than others. A plain centrifugal pump is described as unable to operate when air is present¹. Diaphragm pumps are described as self priming with good suction lift². If you're not sure which you have, the guide to pump types will help.

Before you touch anything

Pumps are connected to power. A pump maker's checklist starts with locking out the power and isolating energy sources before any diagnostic work³. For a 12V pump on a battery, that means isolating the battery first. Follow the maker's manual for your pump. If you need to disconnect or reconnect a battery lead, fuse or cable and you aren't sure how, use a licensed electrician. Don't treat it as routine DIY.

The checks, in order

1. Is the pump full of water? Fill the pump and the suction line, where your manual allows it. A pump maker lists filling the casing and venting high points as a baseline step³. Whether your pump may run dry is a question for your manual. A diaphragm pump is described as having good dry running characteristics, but that's a general statement².

2. Is the strainer clear? A strainer catches debris before it reaches the pump. Wikipedia lists the strainer as the part that removes impurities to avoid choking the pump⁴. A maker's quick sweep also lists "strainer clear"³. Pull it out and rinse it.

3. Is the suction side airtight? This is the check people skip. The checklist asks for clamps, gaskets, fittings and seals to be airtight³. A tiny leak lets air in, and air stops the pump from pulling water. The guide suggests a soap test on the joints and O-rings, then re-gasketing or tightening³.

4. Is the hose the right kind? The same guide says to use a rigid suction hose that won't collapse³. A soft hose can squash flat under suction and cut off the water.

5. Is the suction lift too big? If the pump sits a long way above the water, it may not be able to pull. The checklist's answer is a shorter suction lift, or a higher water level³. What suction lift and head mean explains the terms.

6. Is there a foot valve or check valve in the line? These valves hold water in the pipe. The checklist says to inspect or replace a foot, flapper or check valve if you can't prime³. A suction valve is a non-return valve that lets water flow from the tank to the pump and not back⁴.

Losing flow, not losing prime

Sometimes the pump primes but the flow is weak. The checklist for low flow includes removing restrictions downstream and using the right size for long hose runs³. Look for kinks, a part-closed valve or a blocked outlet. The same guide wants the discharge path open, with no kinks, and the check valve moving freely³.

Low flow isn't harmless for every pump. Wikipedia lists overheating due to low flow among the problems of centrifugal pumps¹. If the water doesn't come, switch off rather than let the pump run.

Mistakes that waste a day

• Changing three things at once. Adjust one thing and test again, as the maker's guide advises³.
• Blaming the pump when the cause is a loose hose clamp.
• Ignoring the strainer because it "looked fine". Take it out and check.
• Running the pump for a long time while it moves no water.

When none of this helps, the manual or the maker's support line is the next stop.