How to Prime a Pentair Pressure-Side Booster Pump

Why a Booster Pump Needs Priming

A pressure-side booster pump is a separate pump that raises water pressure to drive a pressure-side pool cleaner. Unlike the main filtration pump, it pulls water from the pool through a dedicated line and pushes it under pressure to the cleaner. For the pump to work correctly, the pump housing must be full of water with no air trapped inside. Air in the impeller area creates an air lock, which stops the pump from moving water and can also let the pump run dry, damaging the seal or impeller.

Priming simply means replacing the air in the pump housing with water. Most Pentair booster pumps have a small vent or drain plug on the housing that you open to let air escape. When you fill the pump with water and then close the plug, the pump can start moving water immediately. The Pool & Hot Tub Alliance (PHTA) publishes standards for pool equipment installation and maintenance, and following proper priming steps is part of keeping your equipment safe and reliable. For more on how booster pumps work, see pressure-side booster pump requirements.

If your pool cleaner is moving slowly, hesitating, or not moving at all, a booster pump that has lost its prime is often the cause. This can happen after winter storage, after the pump has been serviced, or when the pool water level drops below the skimmer or dedicated line intake.

What to Check Before Priming

Before you touch the booster pump, make sure the main pool pump and the booster pump are both turned off. Working on a pump that is running or under pressure is dangerous. Also, confirm that the pool water level is high enough. The booster pump takes water from the pool, usually through a dedicated suction line or a tee off the skimmer. If the water level is too low, the pump will draw in air instead of water, which makes priming impossible.

Check the valves on the suction and discharge lines. Both should be open so water can flow freely. If you have a shutoff valve on the booster pump inlet or outlet, open it fully before priming. A closed or partially closed valve restricts flow and can prevent the pump from pulling water. If you are new to pressure-side systems, review our setup guide for pressure-side cleaners.

Inspect the pump strainer basket or any filter screen near the booster pump. A clogged basket restricts water flow and can cause air to be pulled into the pump. Clean the basket if needed before you begin priming.

How to Prime the Booster Pump

Follow these steps to prime a Pentair pressure-side booster pump. The exact location of the vent plug or drain plug varies by model, so look for a small screw or plug on the top or side of the pump housing, usually marked with an arrow or a water drop icon.

Turn off the breaker or switch for the booster pump, and also turn off the main pool pump to avoid any risk of water flow while you work.

Open the vent plug on the pump housing. Loosen it slowly, because some water may be trapped under pressure. Keep a small bucket or towel nearby to catch any drips.

Fill the pump housing with water. You can use a garden hose or pour water into the pump through the vent opening. Continue until water flows out of the vent opening steadily. This shows the housing is full and air has escaped.

Reinstall the vent plug and hand-tighten it. Do not overtighten, as you may strip the threads.

Turn on the main pool pump first, then the booster pump. Let the booster pump run for 10 to 20 seconds. Listen for a steady flow and check that the pump is not making a high-pitched cavitation noise. If the pump still sputters or does not push water, turn it off and repeat the priming process.

Once the booster pump runs quietly and smoothly, your pressure-side cleaner should begin moving. Watch the cleaner for a minute to confirm it is traveling normally.

• Always power off the pump before opening the vent plug.
• Use clean water for filling, never rely on pool water that may contain debris.
• Check the pump's owner manual for the exact priming procedure specific to your model.
• If you cannot get the pump primed, there may be a suction leak or a blocked line.

After Priming: What to Look For

Once the booster pump is primed and running, check the pressure gauge if your system has one. The gauge should show a steady reading that matches the pump's normal operating range, which you can find in the pump's spec sheet. If the gauge fluctuates, air may still be trapped in the line.

Inspect all connections on the suction and discharge side of the booster pump for leaks. A small drip can allow air to enter the pump and cause it to lose prime again. Tighten fittings as needed, but do not overtighten plastic or threaded connections.

Confirm that the pool cleaner is getting enough water pressure to move properly. Pressure-side cleaners rely on the booster pump's flow to drive their internal turbine and sweep hose. If you have a pressure-side pool cleaner, watch for a steady stream of water exiting the cleaner's tail or sweep hose. If the stream is weak, the pump may need more time to purge air, or there could be a blockage elsewhere. Also check the flow regulator to ensure it is set correctly for your cleaner.

Troubleshooting Air Locks and Startup Problems

Even after a thorough priming, some booster pumps develop air locks again after a few minutes of operation. This usually points to a suction-side leak. Check the O-rings on all unions and the pump lid. A worn O-ring can let micro bubbles of air into the pump, which gradually accumulate and disrupt flow.

If the pump loses prime every time you restart it, verify that the suction line from the pool does not have an anti-entrapment device or valve that closes when the pump is off. Some systems use a check valve that may stick closed. Tap it gently to free it, or clean it if debris is present.

Another common cause is a clogged impeller. Small debris like sand or leaves can block the impeller vanes. To clean it, you must disassemble the pump housing, which should be done with the power off and the pump drained. If you are not comfortable doing this, call a pool professional. Always consult the pool cleaner troubleshooting guide for related steps. For a specific issue with the pump not starting, see pool cleaner won't start.

Remember that a booster pump should never run dry for more than a few seconds. Running dry can damage the mechanical seal and the impeller. If you suspect the pump has been run dry, have it inspected by a qualified service technician.

When to Call a Professional

If you have followed the priming steps and the booster pump still will not hold a prime, or if you notice unusual noises, leaks, or vibration, it is time to get professional help. The Pool & Hot Tub Alliance standards recommend that pool equipment be installed and serviced by qualified professionals to ensure safety and proper operation.

A technician can test the pump's electrical supply, inspect the motor, and verify that the suction and discharge lines are correctly sized and routed. They can also replace worn seals or impellers if needed. While a DIY priming attempt is safe with proper precautions, deeper mechanical issues are best left to someone with the right tools and training.

Before calling, check the pump's warranty and service documentation. Pentair products typically require professional installation for warranty coverage; you can read more about Pentair cleaners in our guide to Pentair pool cleaners. Keep records of any maintenance you perform, and follow the manufacturer's recommended service intervals.

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Questions

How often does a Pentair booster pump need priming?

A properly installed and sealed booster pump only needs priming after the pump has been drained, after winter storage, or after the system has been opened for maintenance. If you find yourself priming it repeatedly, there is likely a suction leak or another issue that needs repair. See our troubleshooting guide for help.

Can I prime a booster pump while it is running?

No. Never open the vent plug or attempt to fill a pump while it is running. The pump must be completely turned off and depressurized before you open any part of it. This is for safety and to avoid damaging the pump.

What happens if a booster pump runs dry?

Running dry causes the mechanical seal to overheat and fail, leading to leaks and possible motor damage. The impeller can also be damaged by friction. If you suspect your pump ran dry, turn it off and have it inspected by a professional.

Why does my booster pump lose prime every time I start it?

This usually means air is entering the suction line, either through a loose fitting, a damaged O-ring, a cracked pipe, or a low water level. Check all suction-side connections and the pump lid. If the problem persists, call a technician to test the system.

Is priming a booster pump the same as priming a filter pump?

Similar in concept, but a booster pump is smaller and dedicated to the cleaner. The priming procedure is simpler because the pump is usually mounted above the water line and has a vent plug. Always follow the manufacturer's instructions for your specific model.

Can I use a wet-dry vacuum to help prime a booster pump?

Some technicians use a vacuum to pull water into a pump, but for a pressure-side booster pump it is not necessary. Filling the housing manually with water is the recommended method. Never apply vacuum to the pump while it is running.