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Can You Add Air to a Pressure Tank Without Draining It?
You may be able to physically add air to a pressure tank while it still contains pressurized water, but you should not set or adjust the tank’s air precharge that way.
For a typical precharged bladder or diaphragm pressure tank, turn off the pump and release the water pressure before checking or adding air. The system pressure should be at 0 PSI, with the tank drained as required by the manufacturer.
The reason is simple: when water is pressurized inside the tank, it compresses the air chamber. A tire gauge on the air valve will then show the pressure created by the whole water system, not the tank's true precharge.
Pressure tank manufacturers commonly instruct owners to shut off the pump and drain the water pressure before measuring or adjusting the air charge.
What Is Pressure Tank Precharge?
The precharge is the air pressure inside a bladder or diaphragm pressure tank when there is no water pressure acting against it.
Inside the tank, air and water are separated by a flexible bladder or diaphragm. As the pump pushes water into the tank, the water compresses the air. That compressed air then helps push water back into the plumbing when you open a faucet.
The precharge must match the operating setup of the pump and pressure switch.
For example, many conventional steel well pressure tanks are set about 2 PSI below the pressure switch's cut-in pressure. A 30/50 PSI pressure switch would therefore commonly use about 28 PSI of precharge. Amtrol and several Pentair tank instructions use this relationship.
However, this is not universal. Some tank designs specify a different difference between cut-in pressure and precharge. Pentair's WellMate guidance, for example, specifies 4 PSI below cut-in for its air-cell tanks.
Always use the instructions for your particular tank rather than assuming that one setting fits every pressure tank.
Why You Should Release the Water Pressure First
Suppose your pump system is sitting at 40 PSI.
If you put a tire gauge on the tank's air valve, you may also see roughly 40 PSI. That does not mean the tank has a 40 PSI precharge.
The water inside the tank is already squeezing the air chamber.
To find the actual precharge, you need to remove that water pressure. Manufacturers therefore specify checking precharge with the pump off and the tank or system drained of pressure.
Trying to correct the precharge while the system is operating can leave you with too much or too little air once the tank is drained.
How to Add Air to a Pressure Tank Correctly
This procedure applies to a typical bladder or diaphragm tank used with a well pump, booster pump, or similar water system. Check your tank instructions before starting because designs vary.
1. Turn off power to the pump
Switch off the pump at its electrical disconnect or breaker.
Do not rely on the pressure switch alone to keep the pump from starting while you work.
2. Open a faucet or drain
Open a faucet or drain valve connected to the system.
Allow water to leave until the system pressure gauge reaches 0 PSI and water stops flowing under pressure.
Pentair instructions for precharged tanks specifically call for shutting off electrical power and draining the tank before checking its air charge.
3. Find the tank's air valve
Most precharged tanks have a Schrader-style air valve. It looks much like the valve on a car tire.
Remove its protective cap.
Do not loosen unrelated fittings, plugs, or nuts on the tank.
4. Check the air pressure
Use an accurate tire-pressure gauge to measure the precharge.
Compare the reading with the setting required for your pressure switch and tank model.
For many tanks, you first need to know the cut-in pressure. This is the pressure at which the pressure switch tells the pump to start.
5. Add air if needed
If the precharge is too low, add air through the Schrader valve with a suitable air pump or compressor.
Add small amounts and check the gauge often.
Do not exceed the tank manufacturer's pressure limits.
6. Restore the system
Replace the air-valve cap.
Close the faucet or drain you opened.
Restore electrical power to the pump and allow the system to pressurize normally.
Watch the pressure gauge through a pump cycle and check for unusual behavior or leaks.
Do You Have to Remove Every Drop of Water?
For normal precharge adjustment, the important point is that the tank is not under water pressure and is drained as directed by its manufacturer.
With a healthy bladder-type tank, opening a nearby fixture while the pump is off normally allows the compressed air to push most usable water out of the tank.
Looking closely at practical steps for putting air in the water tank can help evaluate tank materials for the water’s intended use.
You do not normally need to disconnect the tank from the plumbing just to check its precharge.
Some manufacturers specifically state that precharge should be checked with no water in the tank, while others describe the procedure as draining the tank completely or draining the system pressure to zero.
Follow your tank's manual where its instructions are more specific.
What Happens If You Add Air Without Draining the Tank?
Adding a little air while the tank is pressurized does not tell you what the final precharge will be.
You could end up with several problems.
Too much precharge
If the air pressure is too high compared with the pump's cut-in pressure, the tank may accept less water than intended.
In a badly adjusted system, water delivery can become uneven as the tank empties.
Too little precharge
Low precharge reduces the useful amount of water the tank can deliver between pump cycles.
The pump may start more frequently.
Frequent starting and stopping is often called short cycling. It can indicate a pressure-tank problem, although other pump and plumbing faults can cause it too.
A misleading pressure reading
This is the biggest reason not to adjust the tank while it is pressurized.
You are trying to measure the tank's air precharge, but the water system is also applying pressure to the air chamber. The reading therefore cannot be used as the unloaded precharge setting.
What If the Pressure Tank Keeps Losing Air?
If you correctly set the precharge but it drops again, do not simply keep adding air indefinitely.
Check for the cause.
A leaking Schrader valve can slowly release air. Manufacturers sometimes recommend checking the valve with a soap solution for continuous bubbling.
A failed bladder or diaphragm can also cause pressure problems.
One useful warning sign is water coming out of the air valve when you briefly check it. In a conventional bladder or diaphragm tank, water at the air valve usually means the barrier separating the water and air has failed.
That generally means the tank or its replaceable internal air cell needs service or replacement.
What If the Pump Is Short Cycling?
Low tank precharge is one possible cause, but do not assume adding air will always fix the problem.
Short cycling can also be related to:
- A damaged bladder or diaphragm
- A leaking air valve
- Incorrect pressure-switch settings
- A tank that is too small for the pump
- Plumbing leaks
- Pump-control problems
- A blocked or restricted connection to the tank
Start by checking the precharge correctly with the pump off and the water pressure released.
If the tank will not hold its air charge, water comes through the air valve, or the pump continues to cycle rapidly after the precharge is corrected, the system needs further troubleshooting.
Bladder Tanks and Traditional Air-Over-Water Tanks Are Different
The draining procedure above is mainly for modern precharged bladder, diaphragm, or air-cell tanks.
Older galvanized pressure tanks may not have a bladder separating the air and water. They can use an air-volume-control system and are managed differently.
Do not apply bladder-tank precharge instructions automatically to an old air-over-water tank.
If you are unsure which type you have, look for the tank's model information and installation manual before adding or releasing air.
Frequently Asked Questions
Can I check my pressure tank PSI without draining it?
You can take a reading, but it will not give you the tank's true precharge while the water system is pressurized. Shut off the pump and release water pressure before checking precharge.
Should the pressure tank be empty when adding air?
For a typical precharged bladder or diaphragm tank, yes. The pump should be off and the tank drained of water pressure according to the manufacturer's instructions before you set the air precharge.
How much air should I put in my pressure tank?
Use the setting specified by your tank manufacturer. Many conventional well pressure tanks use a precharge about 2 PSI below the pressure switch cut-in setting, but some tanks require a different setting.
What should the precharge be for a 30/50 pressure switch?
Many tanks designed for the common 2-PSI-below-cut-in setup would use 28 PSI. Do not assume that value applies to every tank. Check the tank manufacturer's instructions.
Can I use a bicycle pump to add air to a pressure tank?
A suitable manual air pump can be used on many tanks with a Schrader valve, provided it can reach the required pressure. Whatever air source you use, add air gradually and check the pressure with an accurate gauge.
Why does my pressure tank show the same PSI as my water-pressure gauge?
When the system contains pressurized water, the water compresses the tank's air chamber. The pressures therefore tend to equalize. That is why you need to depressurize the water system before measuring the tank's actual precharge.
What does it mean if water comes out of the pressure tank air valve?
On a bladder or diaphragm pressure tank, water coming through the air valve usually indicates that the internal bladder or diaphragm has failed. Adding more air is unlikely to provide a lasting repair.
