Well Pressure Tank Replacement: Signs, Steps and Precharge

A well pressure tank needs replacing when its diaphragm or bladder has failed. The clearest sign is water, not air, coming out when you press the air valve. The usual symptoms are a pump that clicks on and off every few seconds while a tap runs, and pressure that swings fast between high and low. Before you buy a tank, rule out the two other common causes of short cycling, a leaking check valve and a faulty pressure switch. Then fit a correctly sized tank with its precharge set 2 psi below the switch’s cut-in pressure.

Is the tank actually bad?

Franklin’s AIM manual lists four causes for a pump that “starts too often”: a pressure switch problem, a check valve stuck open, a waterlogged tank, and a leak in the system. Only one of those is fixed by a new tank. Use these tests to tell them apart.

The air-valve test comes from InspectApedia’s bladder diagnostic guide, which notes that water squirting out of the valve shows the bladder has ruptured in nearly all bladder tanks. The same guide calls waterlogging the common cause of short cycling. Amtrol’s own replacement symptoms are rapid pump cycling and low system pressure.

Why not just add air?

On a galvanized tank without a diaphragm, adding air is routine maintenance. On a diaphragm tank, Amtrol’s instructions say otherwise. Do not adjust or add pressure if the tank has lost air, if there is visible exterior corrosion, or if pump cycle time or precharge has dropped compared with the original setting. Amtrol’s reasoning is that a loss of air can mean internal corrosion, and re-pressurizing a weakened tank can make it rupture or explode. Amtrol limits precharge adjustments to a new tank, or one whose integrity is confirmed, and says only licensed professionals should check or recharge precharge.

So “it lost its air” is a reason to replace the tank, not to top it up.

Choosing the replacement

• Size by drawdown, not by the old label. Use Pentair’s formula: pump gpm x minimum run time / drawdown multiplier. At a 1-minute run, a 10 gpm pump needs about 34.5 gal of tank volume on a 30/50 switch (multiplier 0.29) and 40 gal on 40/60 (multiplier 0.25). If the old tank was undersized, this is the time to fix it. The pressure tank sizing guide walks through the math.
• Match the connection and mounting. Amtrol’s in-line models use 3/4 in or 1 in male threads, vertical stand models 1 in or 1-1/4 in female threads. Vertical tanks must be mounted vertically.
• Check pressure and temperature ratings. Well-X-Trol tanks are rated to 150 psig maximum working pressure and 130°F. Pentair’s small Wellmate in-line tanks are rated 100 psi and 90°F.
• Plan for the weight. Amtrol says the surface must carry the tank’s full operating weight including water.

Step-by-step replacement

These steps follow the Amtrol Well-X-Trol and Goulds instructions. Check your new tank’s manual, because fittings and precharge vary by model.

1. Kill the power. Turn off and lock out the pump breaker. Goulds’ manual warns in capital letters that failing to disconnect and lock out power before maintenance can cause shock, burns or death.
2. Relieve all pressure. Amtrol says to release all water pressure in the tank and pumping system before work. Open a faucet near the tank, or the tank tee’s drain, until flow stops and the gauge reads zero.
3. Disconnect the old tank. Break the union or fitting at the tank tee. Expect water trapped in a failed tank, and a lot of weight: a waterlogged tank can be full.
4. Set the precharge on the new tank before it sees water. Remove the air valve cap, read the factory precharge, and add or release air to 2 psi below your switch’s cut-in (28 psi for 30/50, 38 psi for 40/60). Do this at room temperature.
5. Install the new tank. Use the same pipe size from pump to tank and switch, which Amtrol says prevents switch cycling. Keep the tank as close to the pressure switch as possible. Use lead-free solder and flux on any sweat joints.
6. Fit or keep a relief valve. Amtrol calls for one at the tank connection, set to open at 125 psig or more and sized to the pump’s capacity at 125 psig. Goulds says it is mandatory on systems producing over 100 psi.
7. Start up and fine-tune. Following Amtrol’s start-up steps, restore power and let the system pressurize until the pump cuts off. Then open fixtures to start a pump cycle and check operation. Make any further precharge changes only with the tank empty and the system off.
8. Check for leaks at every joint over the next few cycles, and confirm the pump now runs longer and starts less often.

Setting the precharge correctly

Source: Amtrol well tank FAQ. Goulds gives the same 2 psi rule and notes you lose 1–2 psi each time you check with a gauge, so check once and replace the valve cap.

If you change the switch setting later, Amtrol warns that raising the precharge on a tank already in service carries the same rupture risk as re-airing. Follow the manual, or have a pro do it.

DIY or call a pro?

Amtrol’s instructions state that the product must be installed by a licensed professional and that only licensed professionals should check or adjust precharge. Following the manufacturer’s instructions is also what keeps the warranty intact; Amtrol says failure to follow them voids the warranty. Check your local code as well, since Amtrol cites state and local codes, or the National Plumbing and Electric Codes where none exist.

The job itself involves pressurized plumbing, a pump circuit that is often 230 V (Goulds factory-wires its larger jet pumps for 230 V), and a tank that can weigh as much as the water it holds. A pro is the safer call if any of these apply: the tank is corroded, the new tank is a different size or connection, the pressure switch also needs replacing, or you are not sure whether the real fault is a check valve down the well. That last case turns a tank job into a pump pull; see well pump replacement cost for what drives that bill.

For the full system picture, start at the well pumps and pressure tanks guide.