What Priming Means and Why Your Pump Needs It

Priming a well pump means filling the pump casing and intake line with water before you start it. A pump cannot move air—it moves only water. If your pump runs dry, it will suck air instead of water, and no water will reach your house. Priming fills those empty spaces with water so the pump can do its job the moment you turn it on.

You need to prime after you drain the system for repairs, replace a pump, or if the pump has sat unused for weeks. A pressure tank stores water under pressure, so it helps keep your system primed between uses. Without priming, you risk running the pump dry, which damages the impeller and seals inside.

Key Takeaways

  • Priming fills the pump casing and intake line with water so the pump can push water instead of air when it starts.
  • You prime by pouring water into the priming plug (usually a threaded hole on top of the pump) until water comes out the discharge line.
  • The pressure tank itself does not need priming separately—it fills as the pump runs and water enters the system.
  • After priming, turn on the pump and watch the pressure gauge; it should rise to your system's cut-in pressure (usually 30 to 40 psi) within a few minutes.
  • If the pump will not hold pressure after priming, the intake line may have a leak that sucks air into the system.

Locate the Priming Plug on Your Pump

The priming plug is a small threaded hole, usually on the top or side of the pump casing. On most residential jet pumps and centrifugal pumps, it sits directly above the impeller chamber. Check your pump's manual or look for a plug about the size of a pencil eraser with a hex head (a wrench fits it).

Some pumps have a priming port instead—a small valve with a handle. If your pump has a valve, you will open it by hand rather than remove a plug. Either way, this is where water enters the pump to fill the empty space inside.

Before you start, turn off the pump at the breaker or switch. Do not prime a running pump. Also locate the discharge line—the pipe that carries water out of the pump toward your house. You will watch this line to know when priming is complete.

Fill the Pump Casing with Water

Use a bucket and a funnel to pour water into the priming plug. A standard 5-gallon bucket works well. Insert the funnel into the priming plug opening and pour slowly so water fills the casing without splashing back out. You may need a helper to hold the funnel steady.

Keep pouring until water begins to flow out of the discharge line. This means the pump casing and the intake line are now full of water. Stop pouring once water comes out steadily—you do not need to overfill. If water does not appear in the discharge line after one or two buckets, the intake line may be blocked or have a leak that is pulling air in.

Replace the priming plug or close the priming valve once water stops flowing from the discharge line. Hand-tighten the plug; do not use excessive force, as the casing is often cast iron and can crack.

Start the Pump and Check the Pressure Gauge

Turn the pump on at the breaker or switch. Listen for the motor to run smoothly. Within 30 seconds to a few minutes, you should hear water moving through the pipes. Watch the pressure gauge on the tank—it should begin climbing from zero.

The gauge should rise to your system's cut-in pressure, which is the point where the pump stops running. Most residential systems cut in at 30 to 40 psi, though some are set higher. Once the gauge reaches that pressure, the pump will shut off automatically. This is normal and means priming worked.

If the pump runs but the pressure does not rise, turn it off when ready. The pump is running dry, which will damage it. This usually means the intake line has a leak that is pulling air, or the well itself is too low. Do not run the pump dry for more than a few seconds.

Troubleshoot if the Pump Will Not Prime

If water does not flow from the discharge line after you pour two buckets through the priming plug, the intake line is likely blocked or has a leak. A blocked line might have sediment or a collapsed section. A leak pulls air into the line faster than water can fill it, so the pump never gets fully primed.

Check the intake line for visible cracks, loose fittings, or rust holes. If the line runs from the pump down into the well, you may not see the problem without pulling the line out. Tighten any loose fittings at the pump and at the check valve (if your system has one). A check valve sits at the bottom of the intake line and should have a small opening—if it is clogged, water cannot enter.

If you find a leak, drain the system, repair or replace the section, and prime again. If the intake line is blocked, you may need to pull it from the well and flush it or replace it. This is a job for a well service technician if you are not comfortable working with the well itself.

How the Pressure Tank Stays Primed

Once the pump is primed and running, the pressure tank fills automatically as water enters the system. The tank stores water under pressure, so it keeps the system primed between pump cycles. When you turn on a faucet, water flows from the tank first. Once the pressure drops to the cut-in point, the pump starts again and refills both the tank and the line.

This cycle means you rarely need to prime again unless you drain the system for repairs or the intake line develops a leak. If you notice the pump running more often than usual or the pressure dropping quickly, the tank may have lost its air charge or the pump may be leaking. A pressure tank has an air valve (usually a small stem like a tire valve) at the top—if it leaks, the tank cannot hold pressure and the system will lose its prime.

Common Mistakes to Avoid

Do not pour water into the discharge line by mistake. The discharge line carries water away from the pump; pouring into it will not prime the pump and may damage check valves or pressure switches downstream. Always use the priming plug or priming valve on the pump itself.

Do not run the pump for more than a few seconds if it is not building pressure. Dry running burns out the seals and impeller within seconds. If the pump will not prime after two attempts, stop and find the leak or blockage before trying again.

Do not over-tighten the priming plug when you replace it. Cast iron casings crack easily. Tighten by hand until snug, then give it a quarter turn with a wrench. If water leaks from the plug, tighten it slightly more, but stop if you feel strong resistance.

Frequently Asked Questions

How much water do I need to prime the pump?

Most pumps prime with one to two 5-gallon buckets, depending on the casing size. You will know you have added enough when water flows from the discharge line. If nothing comes out after two buckets, the intake line is blocked or leaking and needs repair before you can prime successfully.

Can I prime the pump if the well is very deep?

Yes, but deep wells (over 25 feet) may need a foot valve or check valve at the bottom of the intake line to hold the prime. Without one, water drains back down the line when the pump stops, and you have to prime again each time. If your pump loses prime frequently, ask a well technician whether a foot valve would help.

What if water leaks from the priming plug after I replace it?

The plug may not be tight enough, or the threads may be damaged. Turn off the pump, remove the plug, and inspect the threads for cracks or debris. Clean the threads and the plug with a dry cloth, then replace it and tighten firmly but not forcefully. If it still leaks, the casing threads may be stripped and the plug may need a larger size or a threaded insert.

Do I need to prime after the pump has been running for a while?

No. Once the pump is primed and the pressure tank is full, the system stays primed as long as there are no leaks in the intake line. You only need to prime again if you drain the system, replace the pump, or if the intake line develops a leak that pulls air into the system.

Why does my pump keep losing prime between uses?

A leak in the intake line is the most common cause. Even a small hole or loose fitting will pull air into the line when the pump is off, breaking the prime. Check all fittings and the line itself for cracks. A failing check valve at the bottom of the intake line can also cause this—water drains back down and air enters the line.