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For most bladder or diaphragm water pressure tanks, the air precharge should be set about 2 psi below the pump pressure switch cut-in setting.
For example, a system that turns the pump on at 40 psi usually needs about 38 psi of air in the tank. But the pressure must be checked with the pump off and the water pressure fully drained from the tank.
The right amount of air depends on what kind of water tank you have. A normal rainwater storage tank or cistern is different from a pressurized well or pump tank.
How Much Air Should Be in a Water Pressure Tank?
A common starting point for a bladder or diaphragm pressure tank is:
| Pressure switch setting | Typical tank precharge |
|---|---|
| 20/40 psi | 18 psi |
| 30/50 psi | 28 psi |
| 40/60 psi | 38 psi |
The first number is the cut-in pressure. That is the pressure where the pump turns on.
The second number is the cut-out pressure. That is where the pump turns off.
So, with a 40/60 pressure switch:
- Pump starts at 40 psi.
- Pump stops at 60 psi.
- Tank air precharge is usually set near 38 psi.
Always check the tank manufacturer's instructions because some systems may call for a different setting.
Check the Air Pressure With the Tank Empty
Do not simply put a tire gauge on the tank while the water system is running.
Water pressure inside the tank can affect the reading.
For a typical bladder or diaphragm pressure tank:
- Turn off electrical power to the pump.
- Open a faucet or drain valve.
- Let the water pressure fall to zero.
- Confirm the water side of the tank is drained.
- Check the air valve with an accurate pressure gauge.
- Add or release air as needed.
- Close the faucet or drain.
- Restore power to the pump.
The air valve usually looks similar to the valve on a car or bicycle tire.
If you are working around a pump, pressure switch, or electrical wiring, keep the power disconnected until you are done with the pressure check. Electrical repairs should be left to someone qualified to do them.
Why the Air Pressure Matters
A pressure tank uses compressed air to push stored water into your plumbing.
In a bladder or diaphragm tank, a flexible membrane separates the air from the water. As the pump fills the tank, the water compresses the air. That compressed air then helps move water when you open a faucet.
The tank also keeps the pump from turning on every time you use a small amount of water.
Correct precharge helps provide:
- Better pump cycling
- Useful water storage between pump starts
- More stable pressure
- Less unnecessary pump operation
Too much or too little air can reduce the amount of usable water the tank provides.
What Happens If There Is Too Much Air?
If the precharge is too high, the tank may hold very little water before the pressure drops enough to start the pump.
In some cases, you may notice:
- The pump starting soon after a faucet opens
- Pressure changing quickly
- Very little usable water between pump cycles
- Unusual pressure behavior around the pump's cut-in point
Do not solve these symptoms by randomly bleeding air from the tank. First determine the pump's cut-in setting and check the precharge correctly with the water pressure at zero.
What Happens If There Is Too Little Air?
Low air pressure can also reduce the useful performance of a pressure tank.
Possible signs include:
- Frequent pump cycling
- Large pressure swings
- Poor tank drawdown
- The pump starting more often than expected
Drawdown is the amount of water you can use from the pressure tank between the pump shutting off and turning back on.
Adding a lot of extra air does not increase the tank's useful capacity. The air pressure needs to match the pump system.
Is the Gauge on My Plumbing Showing Tank Air Pressure?
Usually not.
The gauge installed on the water line measures water-system pressure.
The air precharge is checked separately at the tank's air valve, normally while the water side has no pressure.
For example, seeing 50 psi on your plumbing gauge does not mean your tank should contain 50 psi of precharge.
A 40/60 system could show 50 psi during normal operation while the tank's empty precharge remains around 38 psi.
What About a Rainwater Pressure Tank?
Rainwater systems often use a pressure tank after the storage tank and pump.
A simple layout may look like:
Rainwater tank → pump → pressure tank → irrigation or plumbing
Factor in adding the correct air charge to a water tank to compare airflow and overflow provisions around the storage vessel.
The large rainwater tank stores the water. The smaller pressure tank manages pump cycling and water pressure.
If that smaller tank has a bladder or diaphragm, its precharge is normally matched to the pump's pressure control settings.
For example, if a rainwater pump operates with a conventional 30/50 pressure switch, a typical precharge would be around 28 psi.
However, some modern rainwater pumps use electronic controllers or variable-speed controls instead of a traditional pressure switch. Their pressure tank requirements can be different. Follow the pump and tank manufacturer's instructions rather than automatically using the 2-psi rule.
Do Not Put Pressure Into a Normal Rainwater Storage Tank
A normal rain barrel, IBC tote, polyethylene storage tank, or atmospheric cistern is generally not a pressure vessel.
Do not connect an air compressor to one and attempt to pressurize it unless the tank was specifically designed and rated for that use.
Normal rainwater storage tanks need a way for air to enter and leave as the water level changes. Their vents also help prevent damaging pressure or vacuum conditions.
If you need pressurized water from a rainwater tank, use equipment designed for that job, such as an appropriate pump and pressure tank.
What About Older Tanks Without a Bladder?
Some older water systems use plain steel pressure tanks without an internal bladder or diaphragm.
These tanks work differently. Water and air are in direct contact, so air can slowly dissolve into the water. The system may use an air-volume control or another method to maintain the correct air cushion.
There is not always a simple air-valve pressure setting that can be treated exactly like a bladder tank.
If you have a large plain steel tank with no bladder, identify the tank and its control system before adding air.
How to Find Your Pump's Cut-In Pressure
Watch the water-pressure gauge while using water.
The pressure will gradually fall. When the pump starts, note the gauge reading. That is approximately the cut-in pressure.
You might see the pump start near:
- 20 psi
- 30 psi
- 40 psi
Then watch the pressure rise until the pump stops. That is the cut-out pressure.
A pump that starts around 40 psi and stops around 60 psi is commonly described as a 40/60 system.
Do not adjust the pressure switch just to make the tank precharge match an arbitrary number. Pump pressure settings must also stay within the limits of the pump, tank, plumbing, controls, and connected equipment.
When Adding Air Will Not Fix the Problem
A pressure tank that keeps losing its air charge may have another problem.
Stop troubleshooting by adding air if you find:
- Water coming out of the tank's air valve
- A tank that will not hold air pressure
- Severe corrosion
- Visible tank damage
- A pump that cycles on and off every few seconds
- Pressure that rises beyond the intended system range
- A pressure switch that does not reliably turn the pump off
Water coming from the air valve on a bladder tank often points to a damaged bladder or diaphragm. Simply adding more air will not repair it.
A badly damaged or corroded pressure vessel should be taken out of service and inspected or replaced rather than patched or repeatedly repressurized.
Frequently Asked Questions
Should my water pressure tank have 40 psi of air?
Only if that matches the requirements of your pump system. With a traditional 40/60 pressure switch, a bladder tank is commonly precharged to about 38 psi rather than 40 psi. Check it with the pump off and the water pressure at zero.
How much air should be in a 30/50 pressure tank?
A bladder or diaphragm tank used with a conventional 30/50 pressure switch is commonly set to about 28 psi when the tank is empty of water pressure.
How much air should be in a 40/60 pressure tank?
A typical starting precharge is about 38 psi. Check the tank manufacturer's instructions before changing it.
Can I check my pressure tank while it is full of water?
Not accurately for the normal precharge check. Shut off the pump and release the water pressure to zero before checking the air valve.
Can I use a tire pump to add air to a pressure tank?
A suitable hand pump or air compressor can often be used to adjust a bladder tank's precharge. Add air carefully and check the pressure frequently. Do not exceed the tank manufacturer's limits.
Why does my pressure tank keep losing air?
Possible causes include a leaking air valve or a damaged bladder or diaphragm. Older non-bladder tanks can also lose their air cushion because air dissolves into the water.
Should I add air to my rainwater storage tank?
No, not if it is a normal atmospheric rain barrel, IBC tote, cistern, or storage tank. These containers are normally vented rather than pressurized. Use a pressure-rated tank after the pump if your system needs stored pressure.
