Well Pump Pressure Tank Replacement
Why this matters
A well-water customer who reports "the pump won't stop running" or "the pump kicks on every time I run a faucet for 2 seconds" usually has a failed pressure-tank bladder, not a failed pump. Replacing the pressure tank correctly extends the pump life by years (because the pump cycles less); replacing it incorrectly burns out the pump within a season. The pressure switch, the tank, and the pump form a system, and the technician who arrives understanding all three closes the job in one visit. Tank replacement is also one of the highest-margin well-water service calls: the part is wholesale and the labor is 2 to 4 hours.
When to use
Replace the pressure tank when:
- Pump short-cycles (kicks on and off more than 8 times per minute under steady-flow demand)
- Tank "thumps" when pump kicks on (water hammer from no air cushion)
- Tank precharge pressure cannot be re-established (bladder failed or punctured)
- Water leaks from the air valve when pressed (waterlogged bladder)
- Tank is over 8 to 12 years old and other symptoms of bladder failure are appearing
- Tank shell is rusted or visibly leaking
- Customer reports pressure drop during use even though pump runs continuously
Do NOT replace when:
- Pump is the problem (verify with amp draw, voltage drop test, and well-yield test)
- Pressure switch is the problem (verify with multimeter at the switch terminals)
- Foot valve or check valve is leaking back (verify with the well-house pressure-decay test)
Tools and materials
- Replacement pressure tank, properly sized (see sizing section)
- New tank tee with pressure switch, gauge, drain, and relief valve ports
- New pressure switch (typically square-D Pumptrol, FSG2, FSG-22)
- New tank-mount pressure gauge (0 to 100 psi)
- Tank elbow fitting (1 inch FIP to 1 inch sweat or 1 inch FIP to 1 inch PEX, depending on system)
- Teflon tape (PTFE) and pipe dope (RectorSeal #5 or equivalent)
- Pipe wrenches (14 inch minimum)
- Channel locks
- Voltage tester
- Bicycle pump or air compressor with quality pressure gauge (or dedicated tank pressure gauge)
- 5-gallon bucket and shop rags
- Pressure gauge with male-thread connection for static-pressure test
Safety
The pump motor is 240 VAC typically, fed from a dedicated breaker. Verify the breaker is off and confirm dead voltage at the pressure switch before touching wiring.
A pressure tank holding water at 50 psi has stored mechanical energy. If the tank shell is compromised or the air valve is depressed under pressure, the discharge of water plus the rebound of the tank can injure. ALWAYS depressurize the tank fully (open a downstream faucet, watch the gauge drop to zero) before disconnecting any fittings. Never depress the air valve on a pressurized tank with your face within 12 inches of the valve.
Some well pumps have a stored capacitor (motor-side or in the control box for 3-wire pumps) that holds charge after disconnect. Wait 60 seconds after breaker off before touching pump-side terminals.
Sizing the new tank
Pressure tank sizing is driven by the goal of pump run-time per cycle. A pump that runs less than 1 minute per cycle wears out the motor start windings quickly. A pump that runs over 5 minutes per cycle is oversized for the tank.
The "draw-down" is the usable water volume between pressure-switch on and off setpoints. For typical 30/50 psi switch settings, draw-down is approximately:
| Tank size (gallons total) | Draw-down at 30/50 psi (gallons) |
|---|---|
| 20 | 4 to 5 |
| 32 | 7 to 9 |
| 44 | 11 to 12 |
| 86 | 22 to 24 |
| 119 | 30 to 33 |
The rule of thumb: 1 gallon of draw-down per gpm of pump output, minimum. A 10 gpm pump should pair with a tank that delivers at least 10 gallons of draw-down. A 5 gpm pump can run with a 5-gallon draw-down tank.
For customer with frequent peak demand (large family, irrigation), oversize the draw-down. A 12 gpm pump on a 119-gallon tank delivers 30+ gallons of draw-down, supporting a 3-minute toilet refill or a 1.5-minute shower without the pump cycling.
Pre-charge pressure
The air pre-charge on the tank's air-valve side determines when the bladder begins to flex. The rule:
Pre-charge pressure = pressure-switch cut-in pressure minus 2 psi
Common settings:
| Switch setting | Cut-in pressure | Cut-out pressure | Tank pre-charge |
|---|---|---|---|
| 20/40 | 20 psi | 40 psi | 18 psi |
| 30/50 | 30 psi | 50 psi | 28 psi |
| 40/60 | 40 psi | 60 psi | 38 psi |
A tank pre-charge above cut-in pressure produces no draw-down (the bladder is fully expanded against the diaphragm). A tank pre-charge well below cut-in produces a tank that water-hammers when the pump kicks on (the bladder collapses against the tank wall on every cycle). Pre-charge is measured with the tank empty of water (pressure-switch open, downstream drained, water side at zero pressure).
Procedure
Step 1: Diagnose and confirm
Verify pressure tank is the failure mode:
- Pump short-cycles under steady demand
- Tank air-valve releases water when depressed (waterlogged) or releases nothing (no bladder, no air)
- Tap the tank top-to-bottom; the top should sound hollow (air-side), the bottom should sound dull (water-side). A fully waterlogged tank sounds dull top to bottom.
If the tank's air-valve releases air at the correct pre-charge pressure and there is no water from the valve, the tank is healthy and the problem is elsewhere.
Step 2: Power down and isolate
Turn off the pump breaker. Verify dead at the pressure switch terminals with a multimeter.
Close the supply valve from the well (if there is one). Open the lowest faucet downstream of the tank to drain pressure. Watch the gauge drop to zero.
Step 3: Drain the tank
Open the tank's drain port (typically on the tank tee or at the lowest point). Drain into a bucket or to a floor drain. A 32-gallon tank holds 4 to 9 gallons of water; an 86-gallon tank holds 20+ gallons.
Step 4: Disconnect the old tank
Disconnect the pressure switch wiring (note the wire colors and terminal positions before pulling). Unscrew the tank tee from the tank. Unscrew the tank from its base. Tank fittings are typically 1 inch FIP to 1 inch MIP; use two pipe wrenches.
Step 5: Test the new tank pre-charge before installing
Set the new tank on the floor (or carefully position on the base if heavy). Check the air pre-charge with a tire-gauge-style pressure gauge on the air valve. Manufacturers ship tanks at typical pre-charge (30 to 38 psi); confirm it matches your pressure switch setting.
If pre-charge is wrong, adjust:
- Too high: depress the air valve briefly to release air
- Too low: add air with a bicycle pump or low-pressure compressor
References
- NSF/ANSI 61 (Drinking Water System Components).
- ASTM F480 (Plastic-bodied tanks).
- Manufacturer literature: Amtrol Well-X-Trol, Flexcon WellMate, Goulds AquaPro.
- Square-D Pumptrol pressure switch installation guide.
- Local well-water code (varies by state; many jurisdictions require licensed well-pump installer for any work below the pitless adapter).
- Manuall internal: Plumbing Well Pump Troubleshooting, Plumbing Well Water Systems.