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What Happens If the Pump Is Not Primed?
If a pump that needs priming is started without enough water inside it, the pump may move little or no water. It can also overheat and damage seals or other parts if it keeps running dry.
This is most common with surface-mounted centrifugal pumps used with rainwater tanks, cisterns, irrigation systems, and shallow wells. These pumps normally need water inside the pump housing and suction line before they can create enough suction to lift water.
A submersible pump works differently. Because it sits underwater, it usually does not need manual priming as long as the pump remains submerged.
What Does Priming a Pump Mean?
Priming means filling the pump housing, and often the suction pipe, with water before starting the pump.
A centrifugal pump moves water by spinning an impeller. The impeller is a wheel-like part inside the pump. It works well with water but usually cannot remove a large amount of air from an empty suction pipe.
If the pump contains mostly air, it may spin without creating enough suction to pull water from the tank.
Some pumps are called self-priming pumps. These can remove some air from the suction line after their pump casing has been filled. "Self-priming" usually does not mean the pump can safely start completely dry.
What You May Notice When a Pump Is Not Primed
An unprimed pump often gives several clear signs.
You may notice:
- The motor runs, but no water comes out.
- Water flow starts and then stops.
- The pump sounds louder than normal.
- Water pressure stays very low.
- The pump runs continuously without reaching its shutoff pressure.
- The pump becomes unusually warm.
- The pump repeatedly loses its prime after being shut off.
Do not keep running the pump hoping that water will eventually appear. If the pump requires water for cooling or lubrication, continued dry running can cause damage.
Why an Unprimed Pump Cannot Move Water Properly
A typical centrifugal surface pump needs water around its impeller to create the pressure difference that moves water.
Air is much less dense than water. When the impeller spins in air, it usually cannot create enough suction to lift water through an empty suction pipe.
The problem becomes harder when the pump is located well above the water level.
The vertical distance the pump must lift water before the water reaches the pump is sometimes called suction lift. Pump specifications place limits on how much suction lift a pump can handle.
Even a properly primed pump may struggle if:
- The pump is too far above the water.
- The suction pipe is too small.
- The suction pipe is very long.
- There are too many restrictive fittings.
- The intake is blocked.
- Air is leaking into the suction line.
Priming cannot correct a poorly designed suction system.
Running an Unprimed Pump Can Damage It
Starting an unprimed pump for a brief moment does not always destroy it. The risk depends on the pump design.
However, running a pump dry for an extended period can damage several components.
Mechanical Seals Can Overheat
Many centrifugal pumps use the pumped water to help cool and lubricate the mechanical seal around the pump shaft.
Without water, friction can cause the seal faces to become hot.
A damaged seal may later leak even after the pump has been properly primed.
Plastic Pump Parts Can Become Too Hot
Some pumps contain plastic impellers, housings, diffusers, or other internal parts.
Water normally carries heat away from these components. Without water flow, temperatures inside the pump can rise.
Heat can deform or weaken some parts.
The Motor May Run for Too Long
In an automatic pressure system, the pump normally shuts off when it reaches a set pressure.
An unprimed pump may never build that pressure. The motor may continue running until someone turns it off or a protection device stops it.
Some pumps have thermal protection or dry-run protection. These features can reduce the chance of damage, but they should not be treated as a substitute for correct installation and priming.
Why Does a Pump Lose Its Prime?
If you prime a pump successfully but it keeps losing its prime, there is usually another problem.
A Foot Valve or Check Valve Is Leaking
A foot valve is a one-way valve normally installed near the bottom of a suction pipe.
It lets water move toward the pump but helps stop the water from draining back into the tank when the pump shuts off.
A check valve performs a similar one-way function.
If the valve leaks, water can slowly drain out of the suction pipe. The next time the pump starts, it may contain too much air to operate correctly.
Air Is Entering the Suction Pipe
A suction-side leak can be difficult to spot because it may pull air inward rather than leak water outward.
Possible leak points include:
- Threaded fittings
- Unions
- Hose connections
- Pump inlet fittings
- Cracked suction hose
- Loose clamps
- Damaged seals
Even a small air leak can make priming difficult.
The Water Level Is Too Low
If the water level falls below the intake, the pump will pull air.
This can happen in a rainwater tank during dry weather or when the intake is installed too high.
Look closely at air-related water-pump problems to evaluate flow and lifting requirements at the outlet.
A floating intake can also become exposed if the tank level drops too far.
The Suction Line Has a High Spot
Air can become trapped in poorly routed suction piping.
Whenever practical, the suction line should be arranged according to the pump manufacturer's installation instructions so that large air pockets cannot remain trapped.
The Intake Is Blocked
Leaves, sediment, algae, or other debris can restrict the pump intake.
A blocked intake may make the pump behave as though it has lost its prime because very little water reaches the impeller.
How to Prime a Water Pump
The exact procedure depends on the pump, so follow its manual when available.
For many surface centrifugal pumps, the basic process is:
- Turn off electrical power to the pump. Do not open fittings or plugs while the pump is running.
- Make sure the water source contains enough water. The intake must remain below the water surface.
- Check the suction line and valves. Make sure valves are open and the suction pipe is connected securely.
- Remove the pump's priming plug or open its priming port.
- Fill the pump housing with clean water suitable for the system's intended use.
- Continue filling until the pump casing is full. Some installations also require the suction line to be filled.
- Replace and tighten the priming plug as directed by the manufacturer.
- Restore power and start the pump.
- Watch for steady flow and normal pressure.
If the pump does not begin moving water within the time allowed by its manufacturer, shut it off rather than letting it continue to run dry.
Repeatedly adding water without finding the reason the prime is being lost usually does not solve the underlying problem.
What If the Pump Still Will Not Prime?
If the pump casing is full but the pump still will not draw water, check the system rather than continuing to run the motor.
Common causes include:
- A leaking foot valve
- A failed check valve
- An air leak in the suction line
- A blocked intake
- A closed valve
- A damaged suction hose
- Excessive suction lift
- An incorrect pump rotation on some motor installations
- A damaged impeller
- A low tank water level
- A suction pipe that is too restrictive
The flow rate is the amount of water a pump moves over time, such as gallons per minute. If the pipe, filter, or intake cannot supply enough flow, even a correctly primed pump may perform poorly.
Priming a Pump Connected to a Rainwater Tank
Rainwater systems often use one of two arrangements.
A surface pump may sit beside the tank and pull water through a suction pipe. This setup commonly requires attention to priming and suction-side air leaks.
A pump can also be installed below the tank outlet so that gravity keeps water at the pump inlet. This is often called a flooded suction arrangement. It can make priming easier because the tank naturally supplies water to the pump, although the pump and plumbing must still be installed within their operating limits.
Another option is a submersible pump placed inside the tank. These pumps normally remain surrounded by water and generally do not require the same manual priming process.
Regardless of pump type, do not allow the pump to draw the rainwater tank completely empty unless the system is specifically designed to protect against dry running.
Can a Self-Priming Pump Run Dry?
Not necessarily.
A self-priming pump is designed to clear some air from its suction line. It normally uses water retained inside its pump casing during the priming process.
If the casing is completely dry, the pump may still be unable to prime. Running it dry can also damage seals or other parts.
Check the manufacturer's instructions for:
- Initial priming requirements
- Maximum priming time
- Maximum suction lift
- Dry-run limitations
- Required check valves
- Recommended suction pipe size
The words "self-priming" should not be taken to mean "safe to run without water."
How to Prevent Priming Problems
A well-installed suction system should normally stay primed between pump cycles.
Helpful measures include:
- Keep the pump within its allowed suction lift.
- Use correctly sized suction pipe.
- Keep suction piping as short and direct as practical.
- Seal suction-side fittings carefully.
- Keep the intake submerged.
- Maintain foot valves and check valves.
- Clean intake screens when needed.
- Protect the pump from running when the tank is empty.
- Inspect hoses for cracks or loose fittings.
- Follow the pump maker's priming instructions after maintenance or winter storage.
In freezing climates, pumps and exposed plumbing may also need to be drained or otherwise protected according to the equipment manufacturer's instructions. A cracked fitting or pump housing can introduce air when the system is put back into service.
When to Stop Troubleshooting
Turn the pump off if it becomes very hot, makes unusual mechanical noises, leaks from the shaft area, trips electrical protection, or still cannot move water after correct priming.
Electrical faults, damaged pump motors, pressure-control problems, and complicated household water systems are good reasons to use a qualified pump technician or electrician.
For a rainwater system, it is usually better to correct the reason a pump is losing prime than to make repeated priming part of normal operation.
Frequently Asked Questions
Will a water pump work if it is not primed?
A centrifugal surface pump that requires priming usually will not move water properly when it contains mostly air. The motor may run, but little or no water will come from the outlet.
Can running a pump without priming damage it?
Yes. Running some pumps dry can overheat mechanical seals and other internal parts. The amount of dry running a pump can tolerate depends on its design, so follow the manufacturer's limits.
How do I know if my pump has lost its prime?
Common signs include a running motor with no water flow, very low pressure, unusual pump noise, or a pump that cannot reach its normal shutoff pressure.
Why does my pump lose prime overnight?
A leaking foot valve or check valve is a common cause. Air may also be entering through a loose fitting, damaged hose, or another suction-side leak.
Do submersible pumps need priming?
Most submersible pumps do not need manual priming because they operate underwater. They still need enough water around them and should not be allowed to run dry unless specifically designed for that condition.
Does a self-priming pump need water added to it?
Usually, yes, especially during initial startup or after the pump casing has been drained. Self-priming generally means the pump can clear air from the suction line once its casing contains the required amount of water.
Why is my pump full of water but still not pumping?
Possible causes include an air leak, blocked intake, failed check valve, excessive suction lift, closed valve, damaged impeller, or low water level. Shut the pump off if it cannot establish normal flow rather than allowing it to run dry.
