Causes of Loss of Water Pump Prime

Diagnosing the cause of a dry pump directs the repair

If your water pump is in the building and the pump keeps losing its prime, 

a shallow well jet pump well line could have a bad foot valve (in the well) and so be losing prime.

• Watch out: If the well pump is running continuously and not delivering water,
  
  shut it off immediately to avoid pump damage.

A leak in the well line can also lead to loss of prime.

If priming the well water pump using one of our methods shown below seems to fix the problem but soon the well pump loses prime again, your plumber will want to check for a bad foot valve in the well or a leak in the well piping between the well and the building.

Common causes of repeated loss of well pump prime & thus loss of water pressure

• Air leaks in the well piping, or where well piping connects through the well casing cap (on some wells).
  
  In our photo shown above the red arrows point to possible leak points at the injection pipe at the cap of an older well casing top. Air leaks here permit water to drain out of the well piping, losing prime.
• Air leaks at an above-ground jet pump itself,
  
  in the pump casing or impeller assembly can cause loss of well prime.
• Bad check valve
  
   at the well pump or bad intermediate check valves on deep wells using multiple check valves
• Bad foot valve:
  
  If you keep losing prime at the water pump where a two-line jet pump is installed, it's possible that a check valve at the pump or more likely at the foot valve in the bottom of the well needs to be replaced. But there could be other causes of repeated loss of pump prime.
• Leaks at the well pump itself
  
  such as a missing gasket in the pump or a loose priming plug or loose air vent plug can cause the pump to lose prime; the risk of this prime loss increases if the pump is left OFF for longer periods, and leaks are also a cause of intermittent pump cycling on when no water is being run.
  
  Examples are at PRIME LOSS DUE TO PUMP LEAKS
• Leak in well piping:
  
  a leak in the well piping between the foot valve and the pump - anywhere along that route, or even at the above-ground well pump itself or at the water pressure tank can cause loss of pump prime.
  
  For example, a leak in the well piping inside the well can permit water in the piping and well pump to siphon backwards out of the well pump (and even the water pressure tank) down into the well when the pump as stopped.
  
  If we prime a pump and it seems to work fine but then loses prime again after sitting un-used over night or for a longer period, we'd ask our plumber to check for a foot valve problem or a leak in the well piping.
• Lost electrical power:
  
  Loss of electrical power to a water or well pump does not directly cause loss of pump prime, but if there is a pre-existing leaky check valve, well pipe, or air leak into the pump, during a longer power outage indeed the pump will lose prime.
  
  Details are at PUMP PRIME LOSS DURING POWER OUTAGE
• Pump impeller damage or debris?
  
  A damaged or debris-clogged pump impeller can cause the well pump to stop moving water, but it's not a direct cause of lost well prime.
  
  However a combination of loss of pump lift due to a bad impeller along with a leaky foot valve might show up as intermittent pump running when no water was being run and a condition that gets worse as the impeller fails until finally you have no water.

Does Loss of Electrical Power Cause Loss of Pump Prime?

Bottom line: Not directly but if there's a check valve or well pipe leak you lose prime during the power outage.

Often a long-existing but small leak at a check valve or in well piping or even in the pump body itself may go undiscovered as long as the pump has electrical power. As water leaks out of the pump and piping the water pressure drops and periodically the pump simply turns on and pumps water pressure back up well before the pump has "gone dry".

But when there is a power outage that lasts long enough that same leak will lead to complete loss of pump prime.

The clue that this condition exists in a building is the observation that the pump seems to run periodically when we think that no one is using water.

Detailed diagnosis and repair for this problem are at See WATER PUMP INTERMITTENT CYCLING where we discuss common sources of unanticipated operation of the well pump.

Can a well pump lose prime through a loose priming plug?

Our reader asked:

Would a leaking or non-tightened prime plug make a 2 line jet pump lose prime overnight? On 2022-10-25 by Joe -

Reply by InspectApedia (Editor) - yes, especially if there's a bad check valve too

@Joe,

That could happen particularly if there is also a leaky check valve or foot valve - the priming plug allows air into the system.

Besides reading the article above on tihs page see these helpful FAQs pages

PUMP PRIMING PLUG Q&A

SIGNS that the WATER PUMP HAS LOST ITS PRIME FAQs - the pump chamber is dry and pump won't deliver water

PUMP PRIME LOST DURING POWER OUTAGE Q&A

PUMP PRIME LOST, TEST for LEAKS Q&A