Well pumps · how-to
Pressure tank precharge: how to check and set it
A well pressure tank holds a cushion of air behind a diaphragm or bladder. That air pressure — the precharge — decides how much water the tank can deliver between pump starts. When precharge is wrong, the pump short-cycles, pressure swings get choppy, and the pump and switch wear out early. Checking it takes a tire gauge and about twenty minutes.
Signs the precharge is off
- Rapid cycling: the pump kicks on and off every few seconds while a faucet runs.
- Pressure gauge jumps quickly from cut-in to cut-out and back instead of moving steadily.
- Clicking at the pressure switch every time someone opens a tap.
- The tank sounds solid all the way up when you knock on it (a healthy tank sounds hollow near the top, where the air is).
- Your pressure switch or pump relay has burned out sooner than expected — see replacing a 40/60 pressure switch.
What you need
- A reliable tire gauge that reads accurately at 20–60 psi — AstroAI digital tire pressure gauge (0–150 psi range per listing).
- An air source: a small compressor or a cordless inflator such as the AstroAI L7 cordless inflator.
- Your pressure switch settings — usually printed inside the switch cover (30/50 or 40/60 are common).
- A hose to drain the system from the tank tee or the nearest faucet.
How to check and set precharge
- Turn off the pump at the breaker.
- Drain the system to zero. Open the drain valve on the tank tee or a nearby faucet until water stops and the system gauge reads 0 psi. Precharge readings taken with water pressure in the tank are meaningless.
- Find the air valve. It is a tire-style Schrader valve, usually on top of the tank under a plastic cap.
- Test for water first. Briefly press the valve core. Air only is good. Water or a wet spray means the diaphragm or bladder has failed — stop here and plan a tank replacement.
- Measure the precharge with the tire gauge.
- Adjust: add air to raise it, or press the valve core to bleed it down, until it reads about 2 psi below cut-in (38 psi for 40/60, 28 psi for 30/50).
- Close the drain, restore power, and watch a full cycle. The pump should start near cut-in and stop near cut-out, and running a faucet should give a noticeably longer stretch before the pump comes back on.
- Recheck in a few weeks. If the precharge keeps dropping, the air valve or the diaphragm is leaking.
Common precharge targets
| Pressure switch | Cut-in | Typical precharge (water at 0 psi) |
|---|---|---|
| 20/40 | 20 psi | About 18 psi |
| 30/50 | 30 psi | About 28 psi |
| 40/60 | 40 psi | About 38 psi |
| Constant-pressure system (for example Grundfos SQE with a CU 301-class controller) | Set point | Follow the pump and controller manual — it is set relative to the controller set point, not a switch |
Tanks often ship pre-charged for a 40/60 switch. Amtrol’s Well-X-Trol listing, for example, says the tank comes pre-pressurized at 38 psi and should be adjusted to 2 psi below cut-in for other settings. If you change your switch settings, reset the precharge to match the new cut-in.
Waterlogged tank vs low precharge
Low precharge is fixed with air. A waterlogged tank — a failed diaphragm or bladder, or an old air-over-water tank that has lost its air — cannot hold a charge and needs replacing. If the tank holds air for a day and then loses it again with no leak at the valve, assume the diaphragm is leaking. A larger tank lengthens pump run times, which is easier on the motor; sizing depends on pump flow and household demand, so see sizing by GPM and total head and ask your well contractor.
When to call a pro
- The tank is rusted, leaking, or needs replacing and you are not comfortable cutting and re-plumbing the tank tee.
- Precharge is correct but the pump still short-cycles — that can mean a leaking check valve, a faulty switch, or a pump problem.
- The pump cannot reach cut-out pressure, or the breaker trips on start.
- You are thinking about moving to constant pressure — see SQ vs SQE (constant pressure / CU).
For tank replacement, electrical work, and well service, use a licensed plumber or licensed well contractor. If the pump itself is wearing out from years of short-cycling, R.E. Merrill & Associates is a Grundfos dealer — we can talk through SQ and SQE options with you and your contractor. Contact R.E. Merrill & Associates.
Related across the site
- Iron in well water — sediment and iron also clog switch nipples and tank tees
- Water softener resin cleaner — maintenance for softeners on well water
Sources
- Amtrol Well-X-Trol WX-202 Amazon listing — pre-pressurized at 38 psi; adjust precharge to 2 psi below pump cut-in; butyl diaphragm with polypropylene liner.
- Pressure tank manufacturer guidance — measure precharge with the pump off and water pressure at zero.
- AstroAI digital tire gauge Amazon listing — 0–150 psi range, Schrader valve.