What priming means and why shallow well pumps need it

Priming means filling the pump casing and intake line with water before you start the pump. A shallow well pump cannot pull water up from the ground on its own — it needs water already inside the system to create the suction that draws more water up. Without priming, the pump will run dry, overheat, and fail.

Shallow well pumps sit above ground (unlike deep well submersible pumps that sit in the water itself). The intake line runs down into the well, sometimes 25 feet or more. When the pump has been off for weeks, or after you have drained the system for maintenance, air fills that entire line. The pump cannot suck air hard enough to pull water up — it needs water in the line first to create a seal and start the siphon.

You will know priming is needed if the pump runs but no water comes out, or if it shuts off after a few seconds. This is different from a pump that will not start at all, which usually points to an electrical problem.

Key Takeaways

  • Priming fills the pump casing and intake line with water so the pump can create suction and pull water from the well.
  • The most common method is pouring water into the priming plug (a threaded opening on top of the pump) until water runs out the discharge line.
  • You must close the priming plug and the check valve (if your system has one) before starting the pump, or water will drain back into the well.
  • If priming does not work after two or three attempts, the problem is usually a leak in the intake line or a failed check valve, and you will need to inspect the line or call a pump technician.

Locate the priming plug and check valve on your pump

Before you prime, you need to find two parts: the priming plug and the check valve. The priming plug is a threaded cap on top of the pump casing, usually 1/2 inch or 3/4 inch in diameter. It may be labeled "prime" or straightforward look like a bolt sticking up. If you cannot find it, check your pump manual — the location varies by manufacturer.

The check valve is a one-way valve installed in the intake line (the pipe running down into the well). It sits either at the bottom of the intake line, inside the well, or just above ground near the pump. Its job is to hold water in the line after the pump stops, so you do not have to re-prime every time. If your system has no check valve, water will drain back into the well as soon as the pump shuts off, and you will need to prime again each time.

If you cannot locate the check valve or are unsure whether your system has one, look at the intake line where it enters the pump. If there is a small bulge or a union fitting (two pieces screwed together) in that section, the check valve is likely there. Your pump manual will confirm.

Gather water and tools for priming

You will need a bucket or pitcher to pour water, and a wrench or adjustable pliers to open and close the priming plug. Have at least 2 to 3 gallons of water ready — use clean water from a hose or a tap, not well water (which may contain sediment that clogs the pump).

Wear work gloves and safety glasses. Priming involves opening pressurized parts of the system, and water can spray when you remove the plug. Make sure the pump is switched off at the breaker before you start. Do not attempt to prime while the pump is running.

Prime the pump by filling the casing

Turn off the pump at the electrical breaker and wait 30 seconds for any residual pressure to drop. Use your wrench to slowly unscrew the priming plug on top of the pump casing. Turn it counterclockwise — do not force it if it is stuck, as you can strip the threads.

Once the plug is loose, water or air may dribble out. Have a bucket ready. Remove the plug completely and set it aside where you will not lose it. You should now see an opening into the pump casing.

Pour water slowly into the opening until the casing is full and water begins to overflow. This usually takes 1 to 2 gallons. Stop pouring once water runs out steadily — do not overfill. The casing is now primed.

Screw the priming plug back in by hand first, then tighten it with your wrench. Do not over-tighten — snug is enough. If your system has a check valve above ground (near the pump), make sure it is also closed and not leaking.

Start the pump and watch for water flow

Switch the pump on at the breaker. Listen for the motor to start and watch the discharge line (the pipe carrying water out of the pump). Water should begin flowing within 10 to 30 seconds. If it does, the prime has held and the pump is working normally.

If no water comes out after 30 seconds, turn the pump off when ready. Let it rest for 1 minute, then repeat the priming process. Sometimes a second or third prime is needed if air pockets remain in the line. Do not run the pump dry for more than a minute — this will damage the pump seal.

If water still does not flow after two or three priming attempts, the problem is likely a leak in the intake line, a failed check valve, or a blockage in the well. At this point, you should stop and contact a pump technician, as further attempts risk overheating the pump.

Check for leaks in the intake line

Once water is flowing, inspect the entire intake line from the pump down to where it enters the ground. Look for water dripping, spraying, or pooling around any joints or along the pipe itself. Even a small leak will allow air to enter the line, which breaks the prime and causes the pump to lose suction.

If you see a leak at a threaded joint (where two pieces of pipe screw together), try tightening the fitting with a wrench. Turn clockwise and stop as soon as the leak slows — over-tightening can crack the fitting. If the leak is in the middle of a solid pipe section, that section will need to be replaced.

If you find no visible leaks but the pump keeps losing prime (water stops flowing after a few minutes), the leak may be underground or the check valve may be failing. A pump technician can pressure-test the line to find the leak or replace the check valve.

Maintain your pump to avoid repeated priming

Once your pump is primed and running, you should not need to prime it again unless the system is drained for maintenance or a leak develops. To keep the prime stable, make sure the check valve is working — it should prevent water from draining back into the well when the pump stops.

If you notice the pump losing prime frequently (needing to be re-primed every few days or weeks), this is a sign of a leak or check valve failure. Do not ignore it. A pump running dry will overheat and fail, and replacing a pump is far more expensive than fixing a leak or valve.

Keep the priming plug accessible and do not paint over it or cover it with insulation. You may need to prime again if you drain the system for repairs, and a buried plug is hard to find in an emergency.

Frequently Asked Questions

Can I use well water to prime the pump?

You can, but clean tap water is better. Well water may contain sediment, sand, or minerals that can clog the pump impeller or damage the seal. If you must use well water, let it sit in a bucket for a few minutes so sediment settles to the bottom, then pour from the top.

What if the priming plug is stuck and will not turn?

explore penetrating oil (like WD-40) around the plug and wait 15 minutes for it to soak. Then try again with your wrench, turning slowly and gently. If it still will not budge, stop — forcing it can strip the threads or crack the pump casing. A pump technician has tools to remove stuck plugs without damage.

How do I know if my check valve is broken?

If the pump loses prime within minutes of stopping (water drains back into the well), the check valve is likely stuck open. You can also listen at the intake line — a working check valve makes a soft click when it closes as the pump shuts down. If you hear nothing, the valve may have failed and will need replacement.

Can I prime the pump while it is running?

No. Always turn off the pump at the breaker before opening the priming plug. Opening it while the pump is running can cause water to spray out under pressure and may damage the pump or injure you.

What if water comes out of the priming plug instead of going in?

This means the casing is already full of water and the pump is primed. Screw the plug back in and start the pump. If water sprays out when you open the plug, the system is under pressure — turn off the pump, wait 30 seconds, and try again.