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Purging a water pump usually means removing trapped air from the pump and connected piping so water can flow normally. It is often needed after a pump has been drained, repaired, disconnected, or installed.
The exact method depends on the pump. A surface-mounted centrifugal or irrigation pump may need to be filled with water before it can purge air. A submersible pump is already surrounded by water, so you normally purge air from the piping instead of filling the pump.
Do not keep running a pump that is not moving water. Many water pumps can be damaged by dry running.
How to Purge Air From a Water Pump
For a typical surface-mounted centrifugal, transfer, or irrigation pump, use this general process.
1. Turn Off the Pump
Disconnect electrical power before opening a pump plug, vent, fitting, or pipe.
If the system has a pressure tank, pressure switch, or pressurized plumbing, relieve the pressure before loosening anything.
Never remove a plug from a pressurized pump.
2. Check the Water Supply
Make sure the pump actually has water available.
Check that:
- The storage tank, rain barrel, cistern, or other source contains enough water.
- The inlet valve is open.
- The suction hose or pipe is not blocked.
- A foot valve or check valve, if used, is installed in the correct direction.
- Suction connections are tight enough that they cannot pull in air.
A tiny suction-side air leak can keep a surface pump from working correctly even when no water leaks from the fitting.
3. Fill the Pump if It Requires Priming
Many centrifugal pumps cannot remove a large pocket of air from an empty pump housing.
Remove the designated priming plug only while the pump is off and depressurized. Fill the pump housing with clean water until it remains full, then reinstall the plug.
Priming means filling the pump and suction path with enough water for the pump to begin moving water. Purging removes the remaining trapped air.
Manufacturer instructions should take priority because pump designs differ. Some pumps have a separate vent plug, while others are designed to vent themselves. Pentair, for example, describes a centrifugal pump as primed when air has been removed from the suction line and pump housing and replaced by water.
4. Give the Air Somewhere to Escape
Open a safe discharge point before starting the pump.
Depending on the system, this could be:
- A hose outlet
- An irrigation zone
- A nearby faucet
- A flush valve
- A manufacturer-provided air vent
Do not trap air between closed valves.
Route the discharge somewhere that escaping water will not damage equipment, wash out soil, or create an electrical hazard.
5. Start the Pump
Restore power and start the pump.
Watch the discharge. You may first see:
- Bursts of air
- Spitting water
- Uneven flow
- Bubbles
- Short pressure changes
As the air leaves, the water stream should become steadier.
Do not run a pump dry while waiting for it to purge. Pump manufacturers warn that dry operation can damage components such as mechanical seals. They also warn against operating some pumps with the discharge completely closed because pressure and heat can build inside the pump.
6. Let Water Run Until the Flow Is Steady
Continue purging until you have a solid flow without repeated bursts of air.
You may also notice that:
- Pump noise becomes more even.
- Pressure becomes steadier.
- Flow at the outlet stops surging.
A few small bubbles immediately after startup are not always a problem. Continued air after several minutes usually means there is still air entering or trapped somewhere in the system.
7. Check for Leaks and Normal Operation
After purging, inspect the system while it runs.
Check:
- Suction fittings
- Hose clamps
- Threaded adapters
- Pump plugs
- Valves
- Filters
- Tank connections
- Pressure gauge readings, if fitted
Stop the pump if it repeatedly loses water, cannot build normal flow, makes unusual noises, or runs without moving water.
Purging Versus Priming a Water Pump
The terms are sometimes used as if they mean the same thing, but there is a useful difference.
Priming prepares a pump to start pumping water.
Purging removes unwanted air from the pump or piping.
For example, an above-ground irrigation pump may need to be filled with water first. Once started, it pushes the remaining air through the discharge. The filling step is priming. Removing the remaining air is purging.
A pump can also lose its prime again if air enters the suction line.
How to Purge a Pump Connected to a Rainwater Tank
A rainwater system often has the pump above or beside a barrel, IBC tote, or cistern.
Before purging, make sure there is enough water above the tank outlet.
Then check the whole path:
Tank → outlet fitting → valve → filter → suction pipe → pump → discharge
An empty filter housing, loose bulkhead connection, blocked strainer, or leaking suction fitting can introduce air.
If the pump sits above the water level, it also has to lift water up the suction pipe. This vertical distance is part of the pump's suction lift. A pump that works when mounted beside a full tank may have trouble when placed much higher.
Prime the pump according to its manual, open a discharge outlet, and run it until the air clears.
If the pump repeatedly needs purging after sitting unused, look for a suction leak or a check valve that is allowing water to drain backward.
How Do You Purge a Submersible Water Pump?
A submersible pump normally does not need to be manually filled because the pump operates underwater.
Instead, trapped air is usually removed from the discharge piping.
Regarding practical steps for bleeding air out of a water pump, verify pump controls matched to demand and connected plumbing.
After installation or plumbing work:
- Make sure the pump is fully submerged as required by its manufacturer.
- Open a discharge point.
- Start the pump.
- Allow water to flow until air stops coming from the line.
- Check the piping for leaks and normal flow.
Some submersible pumps have built-in venting features. Grundfos, for example, documents pumps that automatically allow trapped air to escape.
Do not assume every submersible pump works the same way. Follow its installation manual.
What If the Pump Will Not Purge?
If you keep getting air or cannot establish water flow, stop the pump rather than letting it run continuously.
Common causes include:
The Pump Was Not Fully Primed
There may still be a large air pocket inside a surface pump or suction pipe.
Turn the power off, relieve pressure, and repeat the manufacturer's priming procedure.
There Is an Air Leak on the Suction Side
This is one of the most common causes.
Check threaded fittings, hose connections, gaskets, unions, filter housings, and tank fittings between the water source and pump.
A suction leak can pull air inward without leaking much water outward.
The Water Level Is Too Low
The pump may be drawing air because the tank outlet or suction intake is no longer completely covered.
This can happen with rain barrels and shallow tanks as their water level falls.
The Suction Line Is Blocked
Leaves, sediment, algae, or other debris can reduce water flow through:
- Tank strainers
- Foot valves
- Filters
- Hoses
- Pipe fittings
Turn off the system before inspecting or opening these parts.
The Check Valve Is Not Holding
A check valve allows water to move in one direction.
If it leaks backward, the suction line may empty whenever the pump stops. The pump then has to remove the air again at the next startup.
The Suction Lift Is Too Great
A surface pump can only lift water a limited distance, and real systems lose additional performance through pipe length, bends, filters, valves, and restrictions.
Check the pump manufacturer's allowed suction conditions rather than assuming a larger pump will solve the problem.
Purging Air From a Pressure Water System
If the pump supplies a pressure tank or household-style plumbing system, air may remain in branch lines after pump work.
Once the pump itself is operating normally, open water outlets one at a time and allow them to run until the flow becomes steady.
Be careful with hot-water plumbing. Air, hot water, and pressure can create burn hazards.
If a pressure tank, pressure switch, electrical control, or well system is behaving abnormally, a qualified pump or well technician may be the safer choice.
Does Purging Clean or Disinfect a Water System?
No.
Running water through a pump can flush loose air, sediment, or old water from the piping, but it does not prove that the water is safe to drink.
This matters after repairs to a drinking-water well or potable plumbing. Potable means suitable for drinking.
If a drinking-water system may have been contaminated, flushing alone is not a substitute for proper inspection, treatment, disinfection when appropriate, and water testing. EPA recommends using certified laboratories for private-well drinking-water testing and contacting local health authorities when contamination is suspected.
Rainwater intended for drinking also needs to be treated as a complete water-quality system. Purging the pump does not make roof runoff potable.
When to Stop Troubleshooting
Stop running the pump and get qualified help if:
- The pump repeatedly runs dry.
- Electrical parts are wet or damaged.
- A breaker repeatedly trips.
- The pump becomes unusually hot.
- Water leaks from the motor or electrical housing.
- A pressurized fitting cannot be safely isolated.
- The pump cannot maintain prime even after obvious air leaks are corrected.
- A well pump must be removed or serviced.
- Drinking-water plumbing may have been contaminated.
Pump housings, pressure tanks, and plumbing can store pressure even after the pump stops. Electrical work around water adds another hazard.
Frequently Asked Questions
How long does it take to purge a water pump?
A small system may clear its air quickly once the pump has a good water supply. Longer pipes can take more time. If the pump continues moving mostly air or no water, stop it and check the prime, suction line, valves, and water level rather than letting it run dry.
Should the discharge valve be open when purging a pump?
The trapped air normally needs an open path out of the system. Follow the pump manufacturer's startup procedure because some pumps call for a particular valve position during priming and startup. Never assume a pump should run for an extended period against a completely closed discharge.
Why does my water pump keep getting air in it?
Common causes are a leaking suction connection, low water level, bad check valve, loose filter housing, damaged suction hose, or a suction intake that occasionally draws air.
Can I purge a pump without priming it?
Only if the pump is designed to operate that way. Many surface centrifugal pumps need water in the pump housing before startup. Submersible and some self-venting pumps work differently.
Why is my pump spitting water and air?
A little air can be normal just after plumbing has been drained. Continued spitting usually means trapped air is still clearing or new air is entering the suction side.
Do I need to purge my rainwater pump after cleaning the tank?
You may need to if the pump or suction line was drained during cleaning. Make sure the tank has enough water, reopen the correct valves, prime the pump if required, and allow air to escape through the discharge before returning the system to normal use.
Does purging a water pump make the water safe to drink?
No. Purging removes air and may flush some loose material from the piping. It does not disinfect the water or confirm that it is safe for drinking.
