What priming means and why your jet pump needs it
Priming means filling the pump casing and intake line with water so the pump can create the suction needed to pull water up from your well. A jet pump cannot start pulling water from below ground if air is in the lines instead — it will just spin without moving anything. When you first install a jet pump, after you drain the system for repairs, or if the pump loses prime during operation, you have to refill those spaces with water before the pump will work again.
You will know your jet pump has lost prime if you turn it on and hear it running but no water comes out of your taps, or if water comes out very slowly and then stops. The pump itself is fine; it just needs water in the chamber to function.
Key Takeaways
- Priming means filling the pump casing and intake line with water so the pump can create suction to pull water from the well.
- Most jet pumps have a priming plug or priming port on top of the casing where you pour water in until it overflows slightly.
- You should prime the pump before starting it for the first time, after draining the system, or if it loses prime during use.
- If the pump will not hold prime after you fill it and restart it, the problem is usually a leak in the intake line or a failed check valve, and you will need to inspect those parts.
Locate the priming plug on your pump casing
The priming plug is a small bolt or cap, usually brass or plastic, screwed into the top of the pump housing. On most residential jet pumps, it sits on the upper side of the casing, often near where the discharge line (the pipe carrying water out) connects. Check your pump's manual or look for a small opening labeled "prime" or "fill".
If you cannot find it by looking, trace the pipes connected to your pump. The intake line runs down into the well; the discharge line runs to your house. The priming port is almost always on the casing itself, between those two connections or slightly to one side. Some older or larger pumps have the port on the side rather than the top, so check all faces of the casing if the top is blank.
Gather water and a container to pour it in
You will need a bucket, pitcher, or funnel and a source of clean water. A bucket works fine — fill it from a tap in your house if you have water pressure, or from a well or spring if you do not. You need roughly one to three gallons depending on the size of your pump and how much of the intake line is empty. A standard residential jet pump usually takes about one gallon to prime fully.
If you have a funnel, place it in the priming port opening to make pouring easier and reduce spillage. If you do not have a funnel, you can pour slowly and carefully, or fashion one from a plastic bottle with the bottom cut off.
Remove the priming plug and pour water into the casing
Use an adjustable wrench or the correct socket size to unscrew the priming plug. Turn it counterclockwise until it comes free. Set it aside somewhere you will not lose it — you will need to screw it back in.
Pour water slowly into the opening until water begins to overflow slightly from the hole. This means the casing and the line leading down into the well are now full of water. Do not overfill; once you see water coming out, stop pouring. Wipe away any spilled water with a rag so you can see clearly.
Screw the priming plug back in by hand first, then tighten it with your wrench. Do not over-tighten — snug is enough. If you crack the plug or strip the threads, you will have a leak.
Start the pump and listen for normal operation
Turn on the pump using the power switch or breaker. A primed pump should start smoothly and begin moving water within a few seconds. You should hear a steady hum and feel vibration, not a high-pitched whine or grinding sound. Within 10 to 20 seconds, water should flow from your taps.
Let the pump run for a minute or two to confirm water is flowing steadily at normal pressure. If water comes out weakly or sputters, the system may still have air in it — this is normal after priming and usually clears on its own as the pump runs. If water does not appear at all after 30 seconds, turn the pump off and check for leaks around the priming port or intake line connections.
What to do if the pump loses prime again after you restart it
If you prime the pump, it runs for a while, and then loses prime the next time you turn it on, something is letting air back into the system. The most common cause is a leak in the intake line — the pipe running from the pump down into the well. Even a small hole or loose fitting will suck air when the pump is off, emptying the casing again.
Check the entire intake line for visible cracks, holes, or loose connections. Pay special attention to joints and fittings. If you find a leak, you will need to replace that section of pipe or tighten the fitting. The other common cause is a failed check valve — a one-way valve that should prevent water from flowing backward out of the pump when it shuts off. If the check valve is stuck open, water drains back down the intake line and air enters the casing. A failed check valve usually needs replacement.
If you cannot find a visible leak and the check valve appears intact, the problem may be inside the pump itself — a worn impeller or internal seal. At that point, you should call a well pump technician to inspect the pump.
Priming after you drain the system for repairs
If you have drained your well system to replace a pipe, repair a fitting, or service the pump, you will need to prime again before restarting. Follow the same steps: locate the priming port, pour water in until it overflows, screw the plug back in, and start the pump. The only difference is that you may need more water if you drained a long intake line — have two or three gallons ready.
After you refill the casing, turn the pump on and let it run for several minutes. Air pockets may still be trapped in the intake line, and the pump will push them out as it runs. Water may sputter or flow unevenly for the first minute or two; this is normal. If water does not appear after five minutes of running, turn the pump off and check for leaks in the lines you just worked on.
Frequently Asked Questions
How often do I need to prime my jet pump?
You should only need to prime when you first install the pump, after you drain the system for repairs, or if the pump loses prime during normal use. A healthy pump that holds prime should not need repriming. If you find yourself priming every few weeks, there is a leak or valve problem that needs attention.
Can I use any water to prime the pump?
Yes, any clean water works — tap water, well water, or spring water are all fine. Avoid dirty or muddy water, as particles can clog the pump or damage the impeller. If you are priming a new pump before it has ever drawn from the well, using tap water is easiest.
What if water leaks out around the priming plug when I pour?
A small amount of seeping is normal if the plug threads are not perfectly tight. Tighten the plug slightly with your wrench. If water continues to leak after you tighten it, the threads may be damaged or the plug seal may be worn. You may need a new priming plug or a rubber washer under the existing one.
Do I need to prime if I just turned the pump off for a few hours?
No. Priming is only needed if the pump has lost water from the casing and intake line. A pump that was running normally and then shut off will still be primed when you turn it back on, even after several hours or days. It only loses prime if there is a leak or if you drain the system intentionally.
Should I call a professional if the pump will not prime?
If you have poured water into the priming port and the pump still will not move water after running for a minute, or if water leaks out of the casing or intake line, contact a well pump technician. The problem is likely inside the pump or in a part of the system you cannot see or reach safely.