Well Pressure Tank Air Pressure: How to Check and Set It
The air pressure in a well pressure tank should sit a little below your pressure switch's cut-in: Amtrol says 2 psi below, and Boshart suggests about 15 percent below to allow for gauge error and temperature swings. You can only check it accurately with the pump off and the tank drained of water, using a tire gauge on the air valve on top of the tank.
Why the air charge matters
A bladder or diaphragm tank has water on one side and compressed air on the other. The air pushes water out to the house between pump cycles. With too little air, the tank holds less water, the pump short cycles and the switch chatters. With the air set at or above the cut-in, the water at the tap can drop to nothing for a moment before the pump starts.
The air charge also has to follow the switch. Raise the switch setting and the tank needs more air too.
What to set it to
Amtrol's instruction is the simple one: set the pre-charge 2 psi below the cut-in, so a 40/60 switch gets 38 psi. Boshart says 2 psi is the minimum for a 20-40 switch and isn't enough on a 40-60, which should be about 6 psi below. Its middle-ground target is 15 percent below the cut-in, within a 10 to 20 percent range.
| Switch setting | What the makers say |
|---|---|
| 20/40 | 2 psi below cut-in is the minimum |
| 30/50 | No more than 25 to 28 psi |
| 40/60 | 38 psi in Amtrol's example, or about 6 psi below cut-in per Boshart |
Either works on most systems. If you have a low-pressure cutoff switch, lean toward Boshart's wider gap, since a pre-charge too close to the cut-in is a common cause of nuisance trips.
New tanks don't always ship at your number. Amtrol's Well-X-Trol tanks, for example, leave the factory at 38 psi, which suits a 40/60 switch and nothing else. Check a new tank before you install it.
Tools
- A tire pressure gauge (a digital one reads more precisely at these pressures)
- A small air compressor or a bike pump
- A hose to drain the tank, if there's a drain valve or boiler drain near the tank
- A rag for the air valve
Step by step
Turn off the pump breaker
Close the main valve to the house
Boshart says this keeps the whole plumbing system from draining back while you work.
Drain the tank
Open the drain or a faucet on the tank side of the main valve and let it run until it stops. Amtrol says to drain the tank of all water before checking the pre-charge.
Confirm it's empty
Boshart's tests: the system gauge reads zero, no water comes out, and the tank sounds hollow when you knock on it.
Read the air valve
Take off the cap and check the pressure with your tire gauge.
Add or let out air
Close the drain, then pump air in or press the valve core to bleed it down until you hit your target. Amtrol says to make the adjustment at room temperature only.
Cap the valve
Put the cap back on firmly so the valve doesn't slowly leak.
Restart
Reinstall the switch cover if you removed it, turn the breaker on and let the pump fill the tank to cut-out. Then slowly open the main valve.
Flush the lines
Boshart says to purge air and sediment starting with laundry tub faucets and taps without aerators, then the rest of the house. Some sputtering is normal.
The one rule people break
If the tank has lost air, find out why before you refill it. Amtrol's manual says in capital letters not to adjust or add pressure if there has been a loss of air. A healthy tank holds its charge for a long time. A tank that was way low, or that loses air again within weeks, usually has a leaking valve or a failed bladder.
Here's a quick test. Press the valve core on the air valve for a second. Air is good. Water spitting out means the bladder has failed, and no amount of air will fix it. Boshart's other waterlogging clues are a cold or sweating tank top and a switch that clicks on and off in quick succession.
Mistakes to avoid
- Checking with water in the tank. You'll read system pressure, not the air charge.
- Setting it in a hot garage at noon. Air pressure rises with temperature, and Boshart notes that a gap set in winter can disappear in summer.
- Matching the old switch, not the current one. If someone changed the switch setting, the tank was never updated.
- Adding air to a tank with a dead bladder. It just pushes air into your pipes.
When to call a pro
Call a well contractor if the tank keeps losing air, water comes out of the air valve, the tank is rusting, or the pump still short cycles after the charge is right. Those mean a new tank, and our guide to replacing a well pressure tank covers the job.
FAQ
How much air pressure should be in a bladder tank?
Set it just below your pressure switch's cut-in. Amtrol says 2 psi below, and Boshart suggests about 15 percent below, and about 6 psi below on a 40/60 switch. Measure it with the tank drained.
How do I add air to a well pressure tank?
Shut off the pump, drain the tank, and pump air in through the valve on top with a compressor or bike pump. Stop at your target pressure, cap the valve, and refill the system. Don't add air while the tank is full of water.
What should my pressure tank be set at for a 30/50 switch?
Boshart says a 30 psi cut-in calls for a pre-charge of no more than 25 to 28 psi. Amtrol's 2 psi rule lands at the top of that range. Either is fine for most homes.
Why does my pressure tank keep losing air?
A slow leak at the air valve core is the easy one to fix. More often, a tank that keeps losing air has a torn bladder or diaphragm, and Amtrol's manual says not to keep adding air in that case. Check for water at the valve, then see our pressure tank troubleshooting page.
Sources
- AMTROLWell-X-Trol Residential Pre-Pressurized Well Tanks Installation and Operation Instructions (opens in a new tab)
- AMTROLWell Systems FAQs (opens in a new tab)
- Boshart IndustriesWhat PSI should a tank's pre-charge pressure be set to? (opens in a new tab)
- Boshart IndustriesAdjusting the Pre-Charge Pressure of a Captive Air Tank, Bladder or Diaphragm Type Tanks (opens in a new tab)
- Boshart IndustriesWhy is my low-pressure cut-out switch nuisance tripping? (opens in a new tab)
- AMTROLWell-X-Trol Well Tanks (opens in a new tab)
- Boshart IndustriesHow can I tell if my pressure tank is waterlogged? (opens in a new tab)