To prime a deep well pump, shut off power, locate the priming plug on the pump housing, fill the suction line and pump casing with water until it overflows the priming port, replace the plug, then restore power and open a nearby faucet to bleed air. The process takes 5-15 minutes for most jet pump setups. Submersible pumps are permanently submerged and do not require priming.
Key Takeaways
– Deep well jet pumps lose prime when the water level drops below the foot valve or when a check valve fails – not simply from being turned off.
– Fill the pump housing until water overflows the priming port; partial fills leave air pockets that prevent the pump from drawing water.
– A foot valve seated more than 10-15 feet below the static water level reduces re-priming frequency significantly.
– If the pump loses prime within 24 hours of repriming, a leaking check valve or cracked suction line is the likely cause – not operator error.
– Submersible pumps never need priming; only jet pump configurations (single-pipe shallow well and two-pipe deep well) require it.
Which Well Pumps Actually Need Priming
Not every well pump requires priming. Understanding the difference prevents unnecessary work.
Pump Type | Priming Required? | Why |
Deep well jet pump (two-pipe) | Yes | Above-ground motor must pull water up; needs water to create suction |
Shallow well jet pump (single-pipe) | Yes | Same above-ground suction principle |
Submersible pump | No | Motor sits below water level; self-priming by design |
If you have a submersible deep well pump, skip this process entirely. If you have a deep well jet pump system with two pipes running into the well, this guide applies directly to your setup.
Before You Start: Inspection Checklist
Priming a pump that has a crack, failed valve, or loose fitting wastes time – the prime will not hold. Run through this checklist before adding any water.
– Pump housing: Look for visible cracks, hairline fractures, or corrosion on the casing. A cracked housing cannot hold pressure.
– Suction line connections: Check every fitting from the pump down to the well casing. Loose joints allow air infiltration that breaks the vacuum.
– Check valve: The check valve on the suction line keeps water from draining back into the well when the pump is off. Press the valve flap manually – it should seat firmly with no wobble.
– Foot valve: Located at the bottom of the drop pipe inside the well. If the foot valve leaks, water drains back and prime is lost within hours. A failed foot valve is the single most common cause of repeated lost prime.
– Pressure tank: Inspect the tank for waterlogging or a failed bladder. A waterlogged tank causes short-cycling that can disrupt prime over time.
Do not proceed if you find cracks in the pump housing or a visibly damaged suction line. Replace those components first.
Step-by-Step: How to Prime a Deep Well Jet Pump
Step 1 – Cut Power
Switch off the circuit breaker for the pump. Never work on a pump with power live. Confirm the pump is off before touching any fittings.
Step 2 – Locate the Priming Plug
Find the priming plug on top of the pump housing. It is typically a 1/4-inch or 3/8-inch threaded plug. Remove it with a wrench and set it aside.
Step 3 – Fill the Pump Housing
Pour clean water slowly into the priming port. Continue filling until water overflows back out of the port – this confirms the suction line and pump casing are fully saturated with no air pockets remaining. For a two-pipe deep well jet pump, this can require 1-2 gallons depending on pipe depth.
Step 4 – Replace the Priming Plug
Thread the plug back in firmly. Do not overtighten – hand-tight plus a quarter turn is sufficient for most cast-iron housings.
Step 5 – Restore Power and Bleed Air
Turn the breaker back on. Open the nearest faucet and let it run. The pump should build pressure within 30-60 seconds.
If water flows steadily within 2 minutes, priming is complete. If the pump runs but no water appears after 2 minutes, shut it off immediately to prevent dry-run damage and repeat from Step 2.
Step 6 – Verify Pressure
Check the pressure gauge on the pressure tank. A properly primed system should reach cut-out pressure (typically 40-60 PSI depending on your pressure switch setting) within one pump cycle.
For well-water projects, verify casing diameter, static water level, flow demand, and cable length against Mislier submersible well pump options before ordering equipment.
Why a Deep Well Pump Loses Prime
Lost prime is a symptom, not a root cause. The four most common causes:
Failed foot valve – The foot valve at the bottom of the drop pipe holds water in the suction line when the pump is off. When it fails, water drains back into the well and the pump runs dry on the next start. Replace the foot valve if prime is lost consistently within 12-24 hours of repriming.
Air leak in the suction line – Any crack, loose fitting, or degraded pipe joint between the pump and the well casing allows air to enter the suction side. Even a small air leak is enough to break the vacuum. Inspect every joint under pressure using soapy water – bubbles indicate the leak point.
Dropping water table – If the static water level in the well drops below the foot valve during drought or heavy use, the pump pulls air instead of water. This is a well-capacity issue, not a pump failure. A well driller can assess whether the well needs deepening or the drop pipe needs to be lowered.
Decision Table: Diagnosing Lost Prime
Symptom | Most Likely Cause | First Action |
Loses prime within hours of repriming | Failed foot valve | Pull drop pipe, inspect and replace foot valve |
Loses prime after pump cycles off overnight | Leaking check valve | Test check valve, replace if flap does not seat |
Pump runs but no pressure builds | Air leak in suction line | Inspect all fittings with soapy water under pressure |
Prime holds but pressure is low | Waterlogged pressure tank | Check tank pre-charge pressure; recharge or replace bladder |
Well runs dry during heavy use | Low water table | Contact a licensed well driller to assess well yield |
Common Mistakes During Priming
Partial fill – Stopping before water overflows the priming port leaves an air pocket in the suction line. The pump will run but cannot draw water. Always fill until overflow.
Running the pump dry – If the pump does not pull water within 2 minutes of starting, shut it off. Running a jet pump dry for more than 2-3 minutes can damage the impeller and seal. Dry-run damage is not covered under most pump warranties.
Skipping the inspection – Priming a pump with a cracked housing or failed foot valve produces a temporary fix at best. The prime will not hold, and repeated dry-running accelerates wear. The inspection step in this guide is not optional.
FAQs
Does a well pump lose prime every time it’s turned off?
No. A properly functioning pump with an intact foot valve and check valve holds prime indefinitely when off. If your pump loses prime after a normal shutdown, the foot valve or a check valve is failing – not the pump itself.
How long should it take a deep well jet pump to prime?
Pressure should build within 30-60 seconds of startup after a correct priming procedure. If the pump runs for more than 2 minutes without building pressure, shut it off and re-examine the suction line and foot valve before trying again.
Can you prime a deep well pump without removing the priming plug?
Some installers use a garden hose connected to a nearby faucet to backfill water through the system, but this method is unreliable for two-pipe deep well configurations because it may not fully saturate the suction line. Using the priming plug port is the correct method for deep well jet pumps.
What happens if you don’t prime a jet pump before running it?
Running a jet pump without prime causes dry-run operation. The impeller spins without water lubrication, generating heat that damages the seal and impeller within minutes. Repeated dry-running leads to premature pump failure.
Is a self-priming pump the same as a submersible pump?
No. "Self-priming" refers to jet pumps designed to re-establish their own prime after a brief air ingestion – they still require an initial manual prime on first installation or after a complete drain. Submersible pumps require no priming at all because the motor operates below the water surface.