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In most cases, you should not try to prime a water pump while it is running. Turn the pump off first, fill the pump casing and suction line as required, close the priming port, and then start the pump.
Running a pump dry while you add water can damage the pump seal, overheat parts, or keep the pump from building suction. Opening a priming plug while the pump is running can also expose you to moving water, pressure, and electrical hazards.
Why Water Pumps Need Priming
A typical surface water pump moves water by creating suction on the inlet side. It works well with water, but it cannot move a large amount of trapped air as effectively.
Priming removes that air and fills the pump with water before startup.
Priming is common with:
- Shallow-well jet pumps
- Garden irrigation pumps
- Transfer pumps
- Some rainwater harvesting pumps
- Pumps drawing from tanks, cisterns, or IBC totes
Submersible pumps are different. They normally sit in the water and usually do not need manual priming.
Why You Should Prime the Pump With the Power Off
Turning the pump off gives you control over the process.
When a pump runs without enough water, several things can go wrong.
The Pump Seal Can Overheat
Many pumps depend on water to cool and lubricate the mechanical seal. Running dry can heat that seal quickly.
A damaged seal may cause water to leak around the pump shaft.
The Pump May Never Catch Prime
If the suction pipe contains a large pocket of air, running the motor does not always pull water into the pump.
The pump may simply spin while moving little or no water.
The Priming Port May Be Under Pressure
Once a pump begins moving water, the pump housing may become pressurized.
Removing a priming plug from a running pump can release water or air suddenly. Always shut off power and relieve pressure before opening a pump housing unless the manufacturer gives a different procedure.
Water and Electricity Are a Bad Combination
Priming often involves pouring water directly into a pump housing. That puts water close to a motor, switch, wiring, or electrical connection.
Keep electrical parts dry and disconnect power before working on the pump.
How to Prime a Water Pump Safely
The exact method depends on the pump, so use the pump manufacturer's instructions when available. For many above-ground centrifugal or jet pumps, the basic process is similar.
1. Turn Off the Pump
Switch off the pump and disconnect electrical power when practical.
Do not rely only on an automatic pressure switch. The pump could restart while you are working on it.
2. Check the Water Supply
Make sure there is enough water available at the source.
For a rainwater system, check the:
- Rain barrel
- IBC tote
- Cistern
- Underground tank
Trying to prime a pump from an empty or nearly empty tank will not solve the problem.
3. Check the Suction Line
The suction line is the pipe or hose between the water source and the pump inlet.
Look for loose fittings, cracked hoses, damaged seals, or connections that may pull in air.
Even a small air leak on the suction side can stop a pump from holding prime.
4. Open the Priming Port
Remove the pump's priming plug or cap only after the pump is off and pressure has been relieved.
Some pumps have a dedicated priming opening. Do not remove random fittings from the pump housing.
5. Fill the Pump With Water
Slowly add clean water until the pump casing is full.
Depending on the installation, you may also need to fill part or all of the suction line.
If the water level drops after you stop pouring, the suction pipe may still be filling. Add more water until the level remains near the priming opening.
6. Replace the Priming Plug
Reinstall and tighten the plug according to the manufacturer's directions.
A loose priming plug can allow air into the system.
7. Start the Pump
Restore power and start the pump.
It may take a short time for the pump to push remaining air out of the discharge line and establish steady flow.
For reasons a water pump will not prime, it is useful to verify control choices for anticipated demand and plumbing.
Do not let the pump continue running dry while you wait. Follow the manufacturer's maximum allowable priming or dry-running time.
What If the Pump Loses Prime Again?
A pump that repeatedly loses prime usually has another problem.
Common causes include:
- Air leaking into the suction pipe
- A leaking fitting or gasket
- A bad foot valve
- A leaking check valve
- Water level dropping below the intake
- A suction hose collapsing
- A blocked intake screen
- Too much vertical lift
- A damaged pump seal
A foot valve is a one-way valve near the bottom of some suction lines. It allows water to enter the pipe but helps stop it from draining back toward the source when the pump shuts off.
If that valve leaks, you may have to prime the pump every time you use it.
Check the Suction Lift
A surface pump can only lift water a limited vertical distance.
The vertical distance from the water surface to the pump inlet is often called suction lift.
Do not confuse this with head height, which describes the height and resistance the pump must overcome as it moves water through the full system.
If your pump is mounted too far above the water level in a cistern or tank, priming may be difficult even when the pump itself is working normally.
Also remember that the water level changes as the tank empties. Measure lift from the lowest water level you expect to use, not only when the tank is full.
Check for Air Leaks Before Repriming Again and Again
Air leaks on the suction side are especially troublesome because they may not leak visible water.
Instead, the pump can pull air inward when it runs.
Check threaded fittings, hose connections, unions, valves, and seals between the water source and pump.
A suction hose also needs to be suitable for suction service. A thin garden hose can flatten under vacuum and restrict water flow.
What About Self-Priming Pumps?
A self-priming pump can remove some air from its suction line once its pump casing has been filled correctly.
"Self-priming" does not always mean you can install a completely dry pump and simply switch it on.
Many self-priming pumps still require an initial water fill before their first use or after the pump has been drained.
Check the manual for your specific pump.
Can You Add Water While the Pump Is Running?
Avoid doing this unless the pump manufacturer specifically provides a safe procedure for it.
Pouring water into an open priming port while the pump runs creates several problems:
- The opening may become pressurized.
- Water may spray from the pump.
- The pump may continue running dry.
- Water can reach electrical parts.
- Air may still enter through the open port.
Shutting the pump off usually makes priming safer and more effective.
Rainwater Pump Systems Need Dry-Run Protection
Rainwater tanks sometimes run empty between storms. A pump connected to one should be protected from running without water.
Depending on the system, dry-run protection may use:
- A tank-level switch
- A float switch
- An electronic pump controller
- A pressure-and-flow controller
- Built-in dry-run protection
These controls do not replace proper pump sizing or installation, but they can reduce the chance of leaving a pump running after the tank empties.
When to Stop Troubleshooting
Stop using the pump if you notice:
- Burning smells
- Smoke
- Repeated breaker trips
- Severe vibration
- Water leaking into electrical parts
- A badly damaged power cord
- A motor that hums but does not turn
- A pump that becomes unusually hot
Disconnect power and have the pump or electrical system checked by a qualified professional.
Pressure systems and permanently wired pumps can involve dangerous voltage and stored water pressure. Electrical repairs should not be attempted unless you are qualified to do them.
Frequently Asked Questions
Can running a pump dry damage it?
Yes. Many water pumps rely on water for cooling and lubrication around the seal and other parts. Dry running can cause overheating and premature damage.
How long should a pump run before it becomes primed?
There is no single safe time for every pump. Some pumps establish flow quickly, while others take longer because of the suction-line length and layout. Follow the manufacturer's instructions and do not allow a pump to keep running dry while waiting for water.
Do I need to fill the suction pipe when priming a pump?
Sometimes. Filling the pump housing may be enough for some self-priming installations. Other systems work better when the suction pipe is also filled. The pump manual should explain the required procedure.
Why does my water pump lose prime overnight?
Water may be draining backward through a leaking foot valve or check valve. Air may also be entering through a loose suction-side connection.
Does a submersible pump need priming?
Usually not. A properly installed submersible pump sits below the water surface, so its pump section is already surrounded by water. It still needs adequate water around it and should not be allowed to run dry.
Can I prime a pump from a rain barrel or IBC tote?
Yes, if the pump is designed for the installation and the water level, suction lift, pipe size, and connections are within its operating limits. Make sure the intake remains underwater and that suction-side fittings do not pull in air.
Does a self-priming pump ever need manual priming?
Yes. Many self-priming pumps need to be filled with water before the first startup or after they have been drained for storage, repair, or freeze protection.
