What Priming a Deep Well Pump Means and Why You Need to Do It
Priming a deep well pump means filling the pump casing and intake line with water before you start the pump. A deep well pump cannot pull water up from below 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 within minutes.
You will need to prime your pump after you first install it, after you drain the system for maintenance, or if the pump has sat unused for more than a few days in cold weather. The process takes 15 to 30 minutes and requires only a bucket of water and a wrench.
Key Takeaways
- Priming fills the pump casing and intake line with water so the pump can create suction to pull water from the well.
- You pour water into the priming plug (a small opening on top of the pump) until water comes out the discharge line or until the pump is visibly full.
- The pump must be off and the system depressurized before you open the priming plug to avoid injury from pressurized water.
- If water drains back down the intake line after priming, you may need a check valve installed, or the intake line may have a leak.
- Deep well pumps lose prime in freezing weather or after sitting idle, so you may need to re-prime seasonally or after extended downtime.
Locate the Priming Plug and Prepare the Pump
The priming plug is a small bolt or cap on top of the pump casing, usually about the size of a dime or quarter. On most submersible pumps, it sits near the motor end. On jet pumps (which sit above ground), it is on the side of the casing. Consult your pump manual if you cannot find it—the location varies by manufacturer.
Before you open the priming plug, turn off the pump at the breaker or switch and wait five minutes for any pressure in the system to release. Open a faucet or valve downstream to bleed off remaining pressure. If you open the priming plug while the system is pressurized, water will spray out with force and can cause injury.
Place a bucket or pan under the priming plug to catch water that may drain out when you open it. Use an adjustable wrench or the correct socket size to loosen the plug—turn it counterclockwise. Once it is loose enough to turn by hand, unscrew it the rest of the way and set it aside where you will not lose it.
Fill the Pump Casing with Water
Fill a bucket with water from a hose or faucet. Pour the water slowly into the priming plug opening, watching the casing fill. You will see the water level rise inside the pump. Keep pouring until one of two things happens: water flows out of the discharge line (the pipe that carries water to your house), or the casing is completely full and water reaches the opening of the priming plug.
If water flows out the discharge line, that is the sign the pump is fully primed. Stop pouring and move to the next step. If the casing fills to the brim but no water comes out the discharge line, the intake line may be blocked or the check valve may be stuck. Do not force more water in—stop and check for blockages before proceeding.
Pouring water in slowly prevents air pockets from forming inside the casing. If you pour too fast, air can get trapped and the pump will not prime properly. Take your time—this step usually takes five to ten minutes.
Reinstall the Priming Plug and Start the Pump
Once the pump is full, screw the priming plug back in by hand until it is snug. Do not overtighten it—hand-tight is enough. Use the wrench only if you need to make a quarter turn more to seal it completely. An over-tightened plug can crack the casing or strip the threads.
Turn the pump back on at the breaker or switch. Listen for the motor to start and watch for water to flow from your faucets or the discharge line. The pump should begin delivering water within 30 seconds. If it does not, turn it off when ready and let it cool for five minutes before trying again.
If the pump runs but no water comes out after one minute, turn it off and let it rest. The pump may have lost prime due to an air leak in the intake line. You will need to re-prime and then investigate where air is entering the system.
Troubleshoot If the Pump Loses Prime Repeatedly
If you have to re-prime the pump more than once a season, something is allowing air into the intake line. The most common cause is a leak in the intake pipe between the well and the pump. Check all visible joints and connections for dripping water or wet soil around the well head. Even a pinhole leak will let air in and cause the pump to lose prime.
A second common cause is a missing or faulty check valve. This one-way valve sits in the intake line and prevents water from draining backward into the well when the pump stops. Without it, the intake line empties every time you turn the pump off, and you have to re-prime each time. If your pump loses prime within hours of running, ask a well contractor to inspect the check valve.
In freezing climates, deep well pumps lose prime in winter because water in the intake line freezes and expands, breaking the seal. If this happens to you, you may need to drain the system before the first freeze and re-prime in spring, or install a heat trace cable along the intake line.
When to Call a Well Contractor
If the pump will not hold prime after you have re-primed it twice, or if water leaks from the priming plug area when the pump is running, stop using the pump and contact a well contractor. Continuing to run a pump that cannot hold prime will damage the motor.
If you cannot locate the priming plug, or if opening it releases pressurized water that you cannot control, do not force it. A contractor can identify the plug and prime the pump safely. The cost of a service call is less than the cost of replacing a burned-out motor.
Frequently Asked Questions
Can I use any water to prime the pump, or does it have to be well water?
You can use tap water, hose water, or any clean water. The water you use for priming will be pushed back down into the well once the pump starts, so it does not matter what source you use. Avoid using water with sediment or dirt, as this can clog the pump intake.
How much water do I need to prime the pump?
Most deep well pumps need between one and five gallons to prime, depending on the size of the casing and how deep the intake line is. You will know you have used enough when water flows out the discharge line or the casing is visibly full. If you have poured in more than five gallons and nothing is happening, stop and check for blockages.
What if water drains out of the priming plug after I screw it back in?
A small amount of dripping is normal for the first few seconds. If water continues to drip steadily, the plug may not be sealed tightly. Tighten it a quarter turn with the wrench. If it still drips, the threads may be damaged and you may need a new plug or a slightly larger bolt that fits the same hole.
Do I need to prime my pump every time I turn it on?
No. Once a pump is primed and running regularly, it should hold prime indefinitely. You only need to re-prime after the system has been drained, after the pump has sat unused for several days in cold weather, or if you notice the pump is not delivering water even though it is running.
Is it safe to prime the pump while it is running?
No. Always turn off the pump and wait for pressure to release before opening the priming plug. Opening it while the pump is running can cause pressurized water to spray out and cause injury. It can also damage the pump or allow air to enter the system.