Well Pressure Tank Replacement and Setup
Purpose
A pressure tank is the consumable in a well system, and it is replaced badly far more often than it is replaced late. Two things decide the outcome and both are set before the truck leaves: the drawdown the household actually needs at the switch setting you intend to run, and a precharge set on an empty, isolated tank. Size it off the gallon number printed on the label and it will short cycle from the day it goes in, and the shop that fitted it gets blamed for the pump that burns out behind it.
Scope
Covers selection, replacement and setup of a bladder or diaphragm pressure tank on a private well or booster system, residential and light commercial, from the tank tee to the pressure switch.
Does not cover deciding whether the tank is the fault, which the well pump service call SOP owns through its precharge and drawdown readings; this procedure starts once that verdict exists. Does not cover pulling a submersible pump, well yield, or a galvanized air-over-water tank, which has no bladder and is set with an air volume control instead.
Roles and responsibilities
| Role | What they own | The handoff |
|---|---|---|
| Dispatcher | The diagnostic verdict on the ticket and whether the house can lose water for the swap | Puts the prior visit's drawdown finding on the new ticket; a tank quoted without one is a guess |
| Technician | Sizing arithmetic, isolation, the precharge on the empty tank, the measured drawdown | Returns required drawdown, the model's published drawdown at the setting run, precharge as set, and measured drawdown; a swap without those four is not signed off |
| Lead or owner | Any case where no tank that fits meets required drawdown | Owns the call between two tanks in parallel, a wider switch differential, or a location change |
Procedure
Confirm the tank is the fault and write down the switch setting you will run. Take the drawdown and precharge findings from the diagnostic visit, then record the cut-in and cut-out this system will run after the swap. Acceptance: a prior measured drawdown against the published figure, plus a stated cut-in and cut-out in psi. Wrong looks like replacing a tank because it feels light. Stop rule: no drawdown finding on file means the well pump service call SOP runs first, because a collapsed bladder and a failing pump produce the same customer sentence. Hazard: this step is a gauge reading and a records check, so no enclosure is opened and no hazard is created at it.
Size the tank from required drawdown, never from the label. Required drawdown in gallons is the pump's delivery in gpm times the minimum run time in minutes, which is one minute as a shop default unless the motor manufacturer publishes a longer minimum run or a maximum starts per hour that demands more. Acceptance: a computed required drawdown in gallons, and a tank model whose published drawdown at the exact cut-in and cut-out from step 1 meets or exceeds it. Wrong looks like matching the nominal gallon number on the old shell, which is total volume and is roughly three to four times the drawdown at a common residential setting. Stop rule: if nothing that fits the space meets required drawdown, it goes to the lead as a parallel-tank or wider-differential decision rather than being fitted short. Hazard: none, this is arithmetic done at the truck; the harm from getting it wrong lands months later on the motor.
De-energize, prove dead, isolate, and take the tank to zero. Open the pump breaker, verify absence of voltage at the pressure switch terminals with a tester proved on a known live source immediately before and after, then close the isolation valve and open a drain until the gauge sits at zero. Acceptance: no voltage on a proved instrument, the system gauge reading zero with the drain running clear rather than trickling. Wrong looks like trusting a labeled breaker in a panel rearranged over thirty years. Stop rule: a gauge that will not fall to zero means a check valve is holding pressure on the tank side, so isolate further before anything is unthreaded. Hazard: this is electrical utilization work, excluded from 29 CFR 1910.147 at (a)(1)(ii)(C), so the practice is 29 CFR 1910.333(b)(2) by a qualified person under 1910.332, live-dead-live per NFPA 70E-2021, 120.5, and 1926.417 on construction; the tank is also a pressure vessel, so it is relieved through a drain, never by cracking a fitting.
Break the tank loose and get the real weight before anyone lifts. Break the union at the tank tee, tip the tank to drain what the shell still holds, and stage the lift with two people or a dolly. Acceptance: the tank tipped until it stops giving water, and a stated lift plan before it leaves the floor. Wrong looks like assuming a drained tank is empty, when a ruptured bladder leaves the water on the shell side where no drain reaches it. Stop rule: a tank you cannot tip in place gets pumped or drilled at the lead's direction, not carried full. Hazard: water weighs about 8.34 pounds per gallon, so 20 gallons still inside is roughly 167 pounds arriving unannounced and off-axis, which is a dropped tank on a foot and a wrenched back; keep hands out from under it and clear the route first.
Set the new tank so the tee carries no tank weight and no pipe strain. Stand it on a pad or stand, plumb it, and support the tank tee independently so the piping is not holding the tank up. Acceptance: tank plumb by eye against a wall or level, the tee strapped or blocked so it does not move under hand pressure, and a union fitted at the tank so the next tech can isolate it. Wrong looks like a full tank hanging off a threaded tee, which fatigues the fitting until it weeps at the thread a year later. Stop rule: no space to support the tee properly means the location changes rather than the support being skipped. Hazard: a tank tips easily while unsecured, so it stays hand-steadied until it is standing on its own base and nobody works with a foot underneath it.
Charge the empty, isolated tank to cut-in minus 2 psi with a chuck gauge on the valve. With the tank still isolated and at zero water pressure, add air in short bursts, then remove the fill line and read the charge with a chuck gauge on the schrader itself. Acceptance: a chuck gauge on the valve, fill line disconnected, reading cut-in minus 2 psi from step 1, and holding that reading over a full minute. Wrong looks like reading the compressor regulator instead of the valve, which reads line pressure rather than tank pressure and leaves the tank overcharged. Stop rule: a charge that bleeds off in a minute is a leaking schrader core, so replace the core and re-read before accepting the tank. Hazard: a compressor drives a small vessel past its stamped maximum working pressure in seconds, so charge in short bursts against a gauge, keep your face out of line with the valve, and never depress the schrader on a tank holding water because it sprays at system pressure.
Fill, purge the air out of the piping, restore power, and confirm the switch settings. Open the isolation valve slowly, open a high fixture until it runs solid without spitting, then close covers and energize. Acceptance: a fixture running solid with no air, all covers on before the breaker closes, and observed cut-in and cut-out matching step 1 within the switch's own repeatability. Wrong looks like restoring power with the switch cover off to watch the contacts. Stop rule: an observed cut-out more than a few psi off the intended setting is adjusted at the switch before drawdown is measured, since drawdown is only meaningful at a known setting. Hazard: this is the step that puts energy and pressure back with the customer in the house, so covers go on before the breaker closes, nobody stands over the tank tee while it repressurizes, and the first fixture is opened slowly so a slug of air does not blow a screen out.
Measure drawdown into a container and time a cycle before you leave. Build to cut-out, open a fixture into a measured container, and catch water until the gauge falls to cut-in, timing the pump's run on the next start. Acceptance: measured drawdown within 20 percent of the model's published figure at this setting, and a timed run of at least the minimum run time used in step 2. Wrong looks like judging drawdown by how long a shower lasts. Stop rule: drawdown materially below published on a tank you just charged sends you back to step 6 to re-verify the charge on a re-drained tank, not to the pump. Hazard: this step runs with the system live and pressurized, so the container is filled at a fixture rather than by cracking a fitting, and the fixture is closed before the container is moved.
The record this produces
Nine fields filed against the property: pump delivery in gpm used for sizing; minimum run time used; required drawdown in gallons; tank make, model and published drawdown at the setting run; cut-in and cut-out as set; precharge as set, with the gauge type; measured drawdown in gallons; the ratio of measured to published; and the timed run per start.
The office reads the install date and the model when a warranty question arrives, which is the only time anyone will want it. The next tech reads precharge as set against what they measure years later: a tank that has lost charge twice in three years is at the end of its life rather than a puzzle. The lead reads measured against published drawdown, because a new tank that only ever made four fifths of its published figure was either mis-charged at install or is the wrong model for that setting, and both are correctable while the ticket is fresh.
One worked pass, including a failure
Rural three bedroom, submersible pump, existing bladder tank confirmed failed on drawdown by the prior diagnostic visit. Setting to be run: 40 psi cut-in, 60 psi cut-out.
Sizing at step 2: pump delivery is 10 gpm at this setting, and the shop default minimum run is 1 minute, so required drawdown is 10 gpm times 1 minute, or 10.0 gallons. The tank selected publishes 12.0 gallons of drawdown at 40/60, on a shell labeled 44 gallons nominal, which is the number that would have been read as drawdown by anyone sizing off the label. Isolation is clean and the gauge falls to zero. The old tank, drained through the system, still gives up about 20 gallons when tipped, so at 8.34 pounds per gallon that is roughly 167 pounds of water alone and it goes out on a dolly with two people.
Step 8 fails. Measured drawdown comes in at 6.9 gallons against the published 12.0. That is 12.0 minus 6.9, or 5.1 gallons short, and 5.1 divided by 12.0 is 43 percent low, well outside the 20 percent gate. The timed run agrees rather than contradicting: 6.9 gallons at 10 gpm is 41 seconds per start, under the 60 second floor used to size the tank in the first place. Stop rule taken, back to step 6 rather than to the pump.
The cause is a step 6 shortcut. The charge had been read off the compressor regulator with the fill line still attached, which the acceptance clause forbids, and the tank was sitting at 48 psi rather than 38. A tank charged above cut-in gives up almost nothing before the switch closes again. The tank is re-isolated, drained to zero, recharged in short bursts, and read at 38 psi with a chuck gauge on the valve with the fill line off, holding through the full minute. Re-measured on the second pass: 11.4 gallons, which against 12.0 is 0.6 gallons short, or 5 percent low and inside the gate, and 11.4 gallons at 10 gpm is 68 seconds per start, over the 60 second floor. Signed off on the second pass.
References
- Tank manufacturer literature for the published drawdown table at each cut-in and cut-out pair, the stamped maximum working pressure, and the precharge rule used in step 6.
- Pump and motor manufacturer literature for delivery in gpm at the operating pressure and any published minimum run time or maximum starts per hour that overrides the one minute default in step 2.
- 29 CFR 1910.333(b)(2) for electrical safe work practices, with the 1910.147(a)(1)(ii)(C) carve-out, 1926.417 on construction, and NFPA 70E-2021, 120.5 for live-dead-live in step 3.
- See related: the Plumbing well pump service call SOP, which owns the verdict this procedure starts from, and the well water pressure complaint investigation SOP.