Your Jet Pump Lost Its Prime — How to Reprime It Safely (and When to Stop)

The pump is running. No water is coming out. Or it sputters, builds a little pressure, and quits — and the same thing happens every time you try again. If you have an above-ground jet pump, the most likely explanation is the simplest one: the pump has lost its prime, the water that has to be inside the pump and suction pipe before it can lift anything from the well.

Repriming is a homeowner job — fill the pump with water, let the air out, start it, and watch the pressure. It is also a job with two hard rules that come straight from the pump manufacturers: never run the pump dry, and never work on it with the power on. This guide gives you both kinds of jet pump (shallow-well and deep-well), the exact stop rule, and the point where repriming stops being the fix and becomes the symptom. If your pump is a submersible — the sealed unit down in the well — this guide does not apply to you: there is no priming plug to fill. Start with the no-water triage wizard instead.

For how a jet pump actually moves water, and why it needs prime in the first place, see how a well pump works.

Is it really lost prime?

Before you open anything, confirm the symptom. Lost prime looks like this: the motor runs normally, but little or no water reaches the faucets, the pressure gauge barely moves or will not climb to the cut-out pressure, or the pump needs repriming again after sitting idle overnight. Air sputtering from the faucets as the pump struggles is another tell — the pump is moving air, not water.

Two quick checks come first, because neither is a priming problem. If the pump is silent, check the breaker and the power supply before anything else — the no-water triage wizard walks through the safe power checks in order. And if the pump runs but the pressure tank is behaving strangely — rapid cycling with no water use, or a gauge that drops steadily while nothing is running — read the pump and pressure tank troubleshooting guide first; a waterlogged tank or a faulty pressure switch can mimic prime loss, and repriming will not fix either one.

One more distinction matters. A healthy jet pump system holds its prime through a power outage. If every outage costs you your prime, or you are repriming weekly, something is wrong downstream of this procedure — usually a leaking foot valve or an air leak in the suction line. Repriming buys you water today; it does not repair the leak. The root-cause section below tells you how to tell which one you have.

What "prime" actually is

A jet pump does not suck water up the way a straw seems to. It spins an impeller that can only move water — air is too thin for it to build any useful pressure difference, so an impeller spinning in air just spins. Priming means filling the pump casing and the suction pipe with water, so the impeller has liquid to work with and the pump can push water through its venturi ejector and start the suction that lifts more water from the well. Goulds, one of the major jet-pump makers, puts the venting rule bluntly: water cannot enter unless air can escape. Every step below is about getting water in and air out, in that order.

Two parts hold the prime between runs. The foot valve — a one-way valve with a screen, at the bottom of the suction pipe or on the jet assembly down in the well — lets water in and stops it draining back into the well when the pump stops. And the suction pipe itself has to be airtight: Sta-Rite's manual warns that if the suction pipe can suck air, the pump will not be able to pull water from the well at all. A pinhole that never drips a drop of water can still admit air while the pump is under suction. Lose either part — a foot valve that no longer seats, or a joint that breathes — and the water column drains back or thins out, and the pump wakes up full of air.

Which jet pump you have decides the procedure. A shallow-well jet has one pipe to the well, the ejector built into or bolted onto the pump, and a water level within about 25 feet of the pump — that 25-foot figure includes friction loss in the suction piping, not just vertical lift, so it is a lift limit, not a depth limit. A deep-well jet has the ejector down in the well and two pipes (or a packer system using the casing as the second passage), and it needs a pressure-control valve on the discharge to give the jet the back pressure it runs on. If you are not sure which you have, count the pipes leaving the pump toward the well: one is shallow-well, two is deep-well. Your pump's manual wins over this guide whenever they differ — find the model number on the pump nameplate and check its priming section first.

How to reprime a shallow-well jet pump

Have ready: clean water in a bucket or jug, a funnel, the wrench that fits your priming plug, and a towel. Do not use water of unknown quality for priming — clean drinking water only. Work through the steps in order, and keep a faucet open slightly, as the manuals direct, so air has somewhere to go.

1. Power off. Switch the breaker for the pump off. Confirm the pump cannot start — no hum, no click from the pressure switch.
2. Relieve the pressure. Open a faucet and let the system drain until water stops flowing, then leave that faucet open slightly. Goulds warns not to open it too far: the pump can expel too much water and need repriming again.
3. Find the priming opening. On most shallow-well jets it is the priming plug on top of the pump casing. On some models you fill through the pipe plug or the pressure-gauge bushing in the discharge tee instead, and on others through a priming tee fitted in the suction piping. Your manual names the right one. If you cannot identify a priming opening with confidence, stop here — a well contractor can add a priming tee rather than have you open the wrong fitting under pressure.
4. Vent while you fill. If your pump has a small vent plug (often a 1/8-inch plug between the suction and discharge ports), remove it so air can escape as water enters. Pour water in slowly through the priming opening until the casing is full and water stands at the top of the opening without air bubbles. For a long suction pipe you may need to fill, wait for the water to settle down the pipe, and fill again — Goulds notes you may need to add water to the casing several times.
5. Watch the level before you start. If the water level at the opening keeps dropping while you pour, water is draining back toward the well — a leaking foot valve or check valve, or a suction leak. Do not just keep pouring. Note it, and go to the root-cause section below; a system that will not hold fill water will not hold prime either.
6. Replace the plugs. Put the vent plug back, then the priming plug, snug and sealed. A loose priming plug is itself an air leak. On a shallow-well installation the control valve stays open throughout — do not close it.
7. Power on and watch the clock. Restore power. The pump should deliver water within two or three minutes. Watch the pressure gauge: it should climb steadily toward your system's cut-out pressure. If there is no water and no pressure after two or three minutes, stop the pump at the breaker, refill, and try again — Sta-Rite allows two or three rounds to clear trapped air. Do not let the pump run hot while you hope; another manufacturer's manual caps the attempt at five minutes, and that is a ceiling, not a target.
8. Let it flush, then let it shut off. Once water is flowing steadily, let the system run for several minutes to clear air and dislodged sediment from the pipes. Then close the faucet and let the pump build pressure until it reaches cut-out and shuts off by itself. That shut-off is the prime holding.

How to reprime a deep-well jet pump

The deep-well procedure fills more pipe — the pump plus both the suction and pressure (drive-water) pipes — and it uses the control valve deliberately. The control valve is the pressure-control or regulator valve on the pump's discharge: it throttles the flow just enough to keep back pressure on the down-hole jet, which is what makes the jet work. Do not confuse closing it for priming with running against a closed discharge — a faucet stays open throughout, so water always has somewhere to go.

1. Power off and relieve pressure, exactly as in the shallow-well steps above.
2. Open the control valve as far as possible, then remove the priming plug and fill the pump, all piping between the pump and the well, and the piping in the well until everything is full. If a priming tee is fitted in the suction piping, remove its plug and fill through it as well. On horizontal convertible jets, Goulds directs priming through a tee or the control valve assembly at the pump discharge; on vertical pumps, through the pressure-gauge port. Again, your manual names the opening.
3. Replace all fill plugs and close the control valve completely.
4. Power on and watch the gauge. Pressure should build rapidly as the pump primes — on Sta-Rite's 30/50 example settings, toward about 50 psi. If the gauge shows no pressure after two or three minutes, stop the pump, reopen the control valve, refill, and repeat. Close the control valve again before each restart.
5. Open the control valve slowly. Once pressure has built and stabilized, turn the valve open a little at a time and let the pressure drop until the gauge needle just starts to flutter and the pump gets noisy. Then close the valve slightly — just enough to stop the flutter. That setting is the pump's most efficient operating point for your well. Opening the valve too far will cause the water to stop flowing; Wayne's manual says to leave the valve opened as much as possible without losing pressure, with a steady flow.
6. Verify with full cycles, as in the next section, before you call it done.

Do not leave the control valve at an arbitrary setting you found by feel and forget it. Its correct setting depends on the pump model, the jet assembly, and the depth to the jet — Goulds publishes a control-valve setting chart by model and depth in the manual. If someone has been turning the valve, or the flutter point keeps moving, have a well contractor set it to the chart value rather than guessing.

How to tell it worked

A successful reprime announces itself plainly: steady, air-free water at the faucets, pressure climbing to your system's cut-out setting, and the pump shutting off on its own. Then verify it the way the manuals do: open a faucet or two, run enough water out for the pressure to fall to cut-in so the pump starts, close the faucets, and let the pump build back to cut-out and stop. Run through one or two complete cycles. If the pump starts at cut-in, stops at cut-out, and holds pressure with nothing running, the prime is holding and the system is back in normal operation. (The 30/50 psi figures in the manuals are example settings — yours may differ. What matters is that the pump reaches your cut-out and stops.)

If the pump holds pressure for an hour and then needs repriming after sitting overnight, or the pressure gauge bleeds down slowly with no water being used, that is the classic leaking-foot-valve pattern: water is draining back into the well a little at a time. One exception is worth knowing about. On a packer-jet system — the single-pipe deep-well setup that seals against the well casing with leather cup seals — Sta-Rite notes that the seal is not perfect, so pressure in a dormant system leaks off over time and the pump cycles periodically to maintain it. Occasional cycling on a packer system is expected; repeated full loss of prime is not, and still means a fault.

If it won't prime, or won't stay primed

Repriming twice with no result — or a prime that holds for a day and dies — means the cause is still in the system. Work through the possibilities below without ranking them: no source we found puts a frequency on these, and guessing wrong wastes the service call. Everything in this section that involves opening the well, pulling pipe, or touching wiring belongs to a well contractor.

Two installation details are worth checking with your contractor while they are there, because both cause repeat prime loss and neither is visible from the pump room. First, the foot valve has to sit off the bottom of the well — manufacturers differ on the exact clearance (Xylem says no closer than 2 feet from the bottom, to avoid sand and mud; Goulds says at least 5 feet off the bottom of a clean well), so your manual's number wins, and a foot valve buried in sand or mud is a known cause of lost prime. Second, the suction pipe's route matters: a long horizontal run should pitch upward toward the pump (Xylem specifies about an inch per 10 feet) with no sags, so air cannot pocket in the line. Neither detail is something to fix by pulling pipe yourself.

When repriming won't help

Be honest about the stop point. Repriming will not help — and repeated dry running will damage the pump — when any of these is true:

• The well is dry or drawn down. If the water level has fallen below the foot valve or the jet assembly, the pump pulls air no matter how well you prime it. Rest the well, reduce demand, and have a contractor measure the water level and the well's recovery before you spend anything on the pump.
• The foot valve or check valve is stuck shut, plugged, or broken. A screen caked with sand or iron, a valve jammed by debris, or a cracked valve body all need the pipe pulled and the part replaced.
• The ejector or impeller is plugged. Sand, scale, or debris in the venturi nozzle stops the jet from working. Cleaning it means opening the pump or pulling the jet assembly — contractor work.
• The suction line is cracked or a joint has failed underground. No amount of fill water fixes a pipe that breathes air under suction. Locating it means pressure or vacuum testing the line, and usually digging or pulling.
• The pipes are frozen. A frozen suction or discharge line will not prime, and forcing the pump against it risks the closed-discharge hazard. Thawing and freeze protection are a separate job — see the well and septic winterization guide.
• You cannot find a priming opening, or the pump has no manual and no nameplate. Do not improvise an opening. A contractor can identify the pump, find its priming port, or fit a priming tee.

When you call, tell the contractor what saves them a trip's worth of diagnosis: jet pump or submersible, shallow-well or deep-well, what the gauge did during your reprime attempts, whether the fill water held or drained away, and how quickly the prime dies. "It loses prime overnight, the fill water holds, and the gauge climbs to cut-out then bleeds down" points at the foot valve. "The fill water never holds" points at a valve or suction leak. "It never builds pressure at all after three refills" points at water level, a plug, or the jet itself. A contractor who hears the pattern before arriving brings the right parts.