Reverse Osmosis Storage Tank Air Pressure and Feed Water Pressure Guidelines

Reverse Osmosis Storage Tank Pre-Charge Pressure (Empty Tank)

Recommended Pressure: Most standard Reverse Osmosis Storage Tanks should be pre-charged to approximately 0.5 – 0.8 Bar when empty. Many manufacturers specify a pre-charge in the range of roughly 0.5 – 0.8 Bar, with 0.8 Bar often cited as the optimal setting. This air charge, applied via the Schrader valve on the tank, provides the pushing force to dispense water and ensures the tank can fill with an appropriate volume.

• Why 0.8 Bar? A 0.8 Bar pre-charge maximizes water storage capacity while still providing enough pressure to push out nearly all the water when you open the faucet. In practice, 0.5 – 0.8 Bar is the standard pre-charge for normal operation. At this setting, the tank can fill efficiently and deliver water at good pressure. Always set the pre-charge with the tank empty of water and verify it using a low-pressure tire gauge.
• Effects of Improper Tank Pressure: Keeping the tank within the recommended pressure range is important. If the air pressure is too low or lost entirely, the tank becomes “water-logged”, it may fill with water but has insufficient force to push that water out. The result is weak or no flow at the Reverse Osmosis Faucet, especially as the tank empties. In a healthy tank, the compressed air pushes out water. With no air, the water just sits in the tank. Conversely, if the air pressure is too high, the incoming Reverse Osmosis water will encounter excessive resistance from the bladder. The tank will accept much less water before the internal pressure equals the Reverse Osmosis output pressure, drastically reducing the storage volume. Over pressurizing (for example, pumping the empty tank to 1+ Bar) means the tank reaches cutoff pressure very quickly with minimal water inside. In short, low air pressure = poor flow, and high air pressure = low capacity. Always use the manufacturer’s recommended pre-charge (around 0.8 Bar) for best results.

Feed Water Pressure Requirements for Reverse Osmosis Systems

Minimum and Optimal Feed Pressure: Reverse Osmosis Membranes need sufficient feed pressure to overcome osmotic pressure and push water through the membrane. In general, a minimum of 3 Bar feed pressure is required for most Reverse Osmosis Drinking Water Systems to produce water at a reasonable rate. If pressure drops much below 3 Bar, the Reverse Osmosis process will slow to a trickle or stop, water simply cannot push through the membrane adequately. For this reason, below 3 Bar is considered too low for standard Reverse Osmosis Units and will result in very poor production and water quality. On the other hand, pressures of 3.5 – 5.5 Bar are considered the optimal range for Reverse Osmosis operation in typical residential systems. Around 4 Bar is often cited as an ideal feed pressure, yielding good flow through the Reverse Osmosis Membrane and efficient contaminant rejection. At 3.5 – 4+ Bar, the membrane can produce water at its rated gallon per day output and achieve high rejection of dissolved solids.

Typical Pressure Range: Most residential Reverse Osmosis Systems are designed to work between about 3 Bar and 6 Bar of feed pressure. City municipal water supply typically falls in this range. Many Reverse Osmosis Unit manufacturers specify 6 Bar as the upper safe limit for feed pressure. Pressures significantly above that can risk leaks or damage to fittings, so if your household water pressure exceeds 6 Bar, it’s advisable to use a pressure regulator to protect the Reverse Osmosis System. In summary:

• Minimum: 3 Bar – Below this, the Reverse Osmosis system will struggle, very slow production and low output.
• Recommended: 3.5 – 5.5 Bar – This range provides optimal Reverse Osmosis performance. 4 Bar is ideal for many systems, ensuring good flow and efficiency.
• Maximum: 6 Bar – Generally the highest feed pressure allowed by Reverse Osmosis manufacturers. Operating in the 5 – 6 Bar range is fine if the system is rated for it, but pressures beyond 6 Bar should be reduced to avoid overstressing the components.

If your feed pressure is on the low end of 3 Bar, you may need to boost it (see Booster Pumps below). Adequate feed pressure not only improves water production rate but also improves the Reverse Osmosis Membrane’s rejection of contaminants and its waste water ratio. At very low pressure, waste water increases and filtered water output and quality decrease, so maintaining at least the minimum pressure (or boosting it) is critical for system performance.

Relationship Between Tank Pressure and Feed Pressure (ASOV Operation)

Reverse Osmosis Systems use an Automatic Shut-Off Valve (ASOV) to control when the system stops making water. The Automatic Shut-Off Valve senses the line feed pressure and the tank back pressure, and it closes to stop the flow of inlet water once the tank is almost full. In standard under sink Reverse Osmosis Units, the Automatic Shut-Off Valve is designed to shut off production when the tank pressure reaches approximately 2/3 (≈ 65–66%) of the feed water pressure. In other words, the Reverse Osmosis Tank will fill until its internal pressure is about two-thirds of the incoming line pressure, then the Automatic Shut-Off Valve closes and the system stops producing water. This 2/3 rule is a built-in ratio for most residential Reverse Osmosis shut-off valves and prevents the tank from trying to equalize fully to line pressure (which would waste a lot of water down the drain for very little additional stored water).

• Example – 4 Bar feed pressure: If your feed water supply is 4 Bar, the Reverse Osmosis Membrane will continue to fill the storage tank until the compressed air in the tank reaches roughly 2.7 Bar, which is about two-thirds of 4 Bar. At about 2.7 Bar tank pressure, the Automatic Shut-Off Valve closes and water production ceases. The tank is considered “full” at this point, even though it’s not 100% full of water, it’s full in terms of pressure. This leaves the tank water at about 2.7 Bar available to push out to your faucet.
• Lower feed = lower tank pressure: If the feed pressure is lower, the shut-off will occur at a correspondingly lower tank pressure. For instance, at only 2.7 Bar feed, the Automatic Shut-Off Valve will shut off when the tank reaches roughly 1.8 Bar. That 1.8 Bar in the tank is two-thirds of 2.7 Bar, so the system stops, but note that 1.8 Bar is a fairly low water pressure for delivery, the flow to the faucet will be slower and the usable volume of water is smaller in this case. Low feed pressure thus leads to the tank filling to a lower cutoff pressure, giving you less stored water and weaker flow when you dispense it.
• Higher feed = higher tank pressure: If feed pressure is higher, say you have 4.4 Bar incoming, as might be the case with a booster pump or high city pressure, the tank will fill to a higher pressure before the Automatic Shut-Off Valve triggers. Two-thirds of 4.4 Bar is about 2.9 Bar, so the tank could reach about 3.4+ Bar before shutting off. This means more water is stored, the tank can push more water out before the pressure drops too low, and you’ll have a stronger faucet stream initially. In general, the higher the feed pressure, within safe limits, the more water the tank will store before shutting off.

The Automatic Shut-Off Valve’s 2/3-pressure mechanism is crucial for efficiency. It ensures the system turns off once the tank is sufficiently full, to avoid continuously sending water to the drain. When the tank pressure later drops, after you’ve used some water, and falls to a certain differential below line pressure, the Automatic Shut-Off Valve opens again and the Reverse Osmosis System restarts to refill the tank. The cycle repeats automatically, maintaining the tank at roughly 2/3 of line pressure when full.

Impact of Tank Pressure on Automatic Shut-Off Valve Operation and System Performance

The pressure in the Reverse Osmosis Tank, especially the air pre-charge setting, directly affects how the system behaves. If the tank pressure is not set correctly, it can cause issues with both water delivery and the Automatic Shut-Off Valve’s shut-off function:

• Over Charged Tank (Too High Air Pressure): If the tank’s pre-charge air pressure is set too high, above the recommended range, the Reverse Osmosis Unit will shut off prematurely. The tank’s internal pressure will reach the The Automatic Shut-Off Valve’s cutoff, 2/3 of feed, when the tank has only a small amount of water in it, because the over inflated bladder resists incoming water. In effect, the system “thinks” the tank is full and the The Automatic Shut-Off Valve closes too soon. This leaves you with very little usable water in the tank.

The Key Point: Do not over pressurize the Reverse Osmosis tank’s air side. It will greatly reduce performance and does not increase the water pressure at the faucet. The tank should have just enough air to push out water, but not so much that it prevents water from filling the tank.

• Under-Charged or Zero-Pressure Tank: If the tank’s air pre-charge is too low or completely empty, different problems arise. With little to no air cushion, the Reverse Osmosis Tank can fill with water but cannot deliver it effectively. The water pressure at the faucet will be very weak, especially as you start to empty the tank. In some cases a tank with no air charge will become nearly full of water yet only a dribble comes out of the faucet, a clear sign of a lost pre-charge. Additionally, a tank with a failed bladder or zero air pressure can affect the Automatic Shut-Off operation.  The Reverse Osmosis System may fail to shut off because the tank never provides the back pressure needed to reach the 2/3 threshold. The system could continue running water to the drain continuously since the Automatic Shut-Off Valve doesn’t close. This might seem counterintuitive, a waterlogged tank will actually often not trigger the Automatic Shut-Off Valve properly. Thus, a limp or under-charged tank can lead to both low faucet flow and continuous waste water. The remedy is to drain the tank and re-pressurize the air bladder to 0.8 Bar, or replace the tank if the bladder has ruptured. Reverse Osmosis Tanks should be checked  annually to ensure the pre-charge is around the recommended 0.5 – 0.8 Bar.

Note: If you observe continuous running water to the drain of your Reverse Osmosis System even when the tank is full, it usually indicates the Automatic Shut-Off Valve isn’t closing. One common cause is insufficient pressure buildup in the tank, due to low feed pressure or an under charged tank, preventing the 2/3 threshold from being reached. Other causes include a faulty Automatic Shut-Off Valve or a leaky Check Valve (Non-Return Valve), but always verify tank pressure and feed pressure first.