Pump Losing Prime? Here's What to Check First

The pump starts.

It moves water.

Then flow disappears.

Or maybe it works after someone manually primes it, only to lose prime again after shutdown.

If you have a pump losing prime, repeatedly adding water is not a long-term solution.

Something is allowing the pump or suction system to lose the conditions it needs to operate.

Why Is Your Pump Losing Prime?

Many centrifugal pumps require the pump casing and suction arrangement to contain liquid before they can operate properly.

Xylem's centrifugal pump troubleshooting guidance identifies suction-side leaks, excessive suction lift and accumulated air among potential causes of loss of prime.

Here are some of the first areas to investigate.

1. Suction-Side Air Leaks

This should be high on the list.

A small leak at a fitting, flange, gasket or connection may allow air into the suction line.

Because the line can be under negative pressure, you may not see water leaking out.

The system simply draws air in.

Check connections carefully.

2. Foot Valve or Check Valve Problems

In systems that rely on a valve to keep the suction line full when the pump stops, a valve that no longer seals properly can allow water to drain away.

The next time the pump starts, the suction line may contain air instead of water.

If the system consistently loses prime after sitting idle, investigate whether it is holding water between operating cycles.

3. Water Level Is Too Low

Has the source level changed?

A tank, pond, reservoir or well may have provided adequate conditions earlier in the season but dropped as summer progressed.

If the suction inlet is no longer properly submerged, the pump may begin drawing air.

Check the actual operating water level rather than assuming it has not changed.

4. Blocked Suction Line or Strainer

A clogged strainer or restricted suction line can make it difficult for liquid to reach the pump.

Xylem troubleshooting documentation specifically identifies blocked suction lines and suction-side leakage among conditions associated with poor pump performance.

Clean and inspect the inlet before assuming the pump needs repair.

5. Excessive Suction Lift

There are limits to how effectively a pump can lift liquid from a source below it.

If the difference in elevation has changed or the original arrangement is marginal, the pump may struggle to maintain prime.

This is especially important when water levels fall through the summer.

Does the Pump Lose Prime While Running or After Shutdown?

That distinction matters.

If the pump losing prime problem occurs while it is operating, look closely at:

• Water level

• Suction leaks

• Inlet restrictions

• Air entrainment

If it loses prime only after sitting idle, look closely at whether water is draining back through the suction system.

The timing gives you a clue.

Don't Keep Running a Pump Without Proper Prime

A centrifugal pump that is intended to operate full of liquid should not simply be allowed to run dry while someone waits for water to appear.

Dry operation can create heat and damage seals or other components depending on the equipment.

Correct the cause before repeated failed starts create a larger repair.

Find Out Where the Air Is Coming From

A pump losing prime is usually telling you something about the suction system.

Air is entering.

Water is draining away.

The source level has changed.

Or the inlet is restricted.

Find that condition first.

Academy Pump & Motor provides pump sales, troubleshooting, service and repair throughout Calgary and surrounding areas.

Pump losing prime? Call Academy Pump & Motor at 403-437-7888 or visit academypump.ca.