To pressurize a reverse osmosis tank, you drain all the water from it, then add air through the tank’s valve using a bicycle pump until you reach 5 to 8 PSI. The process takes about 15 minutes, and it’s the single most common fix for slow water flow from an RO faucet. Here’s exactly how to do it, step by step.
How an RO Tank Works
Inside every RO storage tank is a rubber bladder surrounded by pressurized air. As your RO membrane slowly produces filtered water, it fills the bladder. When you open the faucet, the air pressure surrounding the bladder squeezes the water out and up to your tap. Over time, that air naturally leaks out. When it does, the tank can’t push water to the faucet with any force, even if the bladder is full of water.
Signs Your Tank Needs Pressurizing
The most obvious symptom is weak flow from your RO faucet. A healthy system should deliver at least one quart of water per 40 seconds. If your stream starts strong for a moment then quickly drops to a trickle, low tank pressure is almost certainly the cause.
Other signs include the tank feeling heavy (meaning it’s full of water it can’t push out), or the system running constantly because the tank never reaches the pressure needed to trigger the automatic shutoff valve. If you’ve recently replaced your filters and the flow is still weak, the tank pressure is the next thing to check.
Check for a Ruptured Bladder First
Before spending time pressurizing the tank, do a quick test. Remove the small cap from the air valve (it looks like a tire valve and sits on the side or bottom of most tanks). Press the valve pin briefly. If air comes out, the bladder is intact and you can proceed. If water leaks out of the air valve, the internal bladder has ruptured. A ruptured bladder can’t be repaired, and the tank needs to be replaced.
Another telltale sign of bladder failure: you get roughly one cup of water from the faucet at normal pressure, then flow immediately drops to almost nothing. That pattern, specifically, points to a bladder that has torn and is no longer separating air from water inside the tank.
Tools You’ll Need
- Low-pressure gauge: A standard tire pressure gauge works, but one that reads in 1 PSI increments is more useful in the 5 to 8 PSI range.
- Bicycle pump or air compressor: A hand bicycle pump gives you the most control. An air compressor works but can over-pressurize the tank quickly, so use short bursts.
- Towel and a bucket: Water will drain from the tank’s water connection, so have something to catch it.
Step-by-Step Pressurization
1. Shut Off the Water Supply
Turn off the cold water line feeding your RO system. This is usually a small valve on the tubing running into the first filter housing. Then close the valve on top of the tank itself by turning it clockwise until it stops.
2. Drain the Tank Completely
Open your RO faucet and let all the water run out. This is critical. You cannot get an accurate pressure reading or properly add air while water is still inside. Let the faucet run until the flow stops entirely, not just slows down. This can take several minutes depending on how full the tank is.
3. Disconnect the Tank
Once the tank is empty, disconnect the water line from the top of the tank. Place a towel underneath to catch any remaining drips. You don’t strictly have to disconnect the tank, but it makes it easier to work with the air valve, especially if the valve is on the bottom.
4. Check the Current Pressure
Remove the cap from the air valve and press your gauge onto it. Note the reading. If it’s well below 5 PSI, that explains your flow problems. Most tanks ship pre-charged to around 7 or 8 PSI, so anything significantly lower means air has slowly escaped.
5. Add Air to the Target PSI
Attach your bicycle pump to the valve and add air in small increments. Check the gauge after every few pumps. Your target is 5 to 8 PSI with the tank empty. Some manufacturers specify 5 to 7 PSI, others recommend 7 to 8. A good middle ground is 7 PSI. Going higher than 8 PSI can stress the bladder and reduce the tank’s total water capacity, since more air leaves less room for the bladder to expand.
6. Reconnect and Restore Water
Replace the air valve cap, reconnect the water line to the top of the tank, and open the tank valve. Turn the cold water supply back on. The tank will now begin filling, which typically takes 2 to 4 hours depending on your water pressure and membrane condition. Don’t worry if the faucet produces little water immediately. Give the system time to refill.
Why the Tank Must Be Empty
This is the step people most often skip, and it’s the reason the fix doesn’t work for them. When water is still inside the tank, it compresses against the air and gives you a falsely high pressure reading. You might see 20 or 30 PSI on the gauge and think the tank is fine, when the actual air charge behind the bladder is only 3 PSI. Draining the tank first lets you measure only the air pressure, which is what actually determines the tank’s ability to push water to your faucet.
How Often to Check Tank Pressure
Check your tank pressure once a year. The easiest way to remember is to do it whenever you replace your RO filters, since you’re already shutting down the system and can drain the tank as part of the process. If you notice flow slowing down between filter changes, check pressure then too. A small amount of air loss each year is normal. Losing pressure rapidly, within weeks of re-pressurizing, suggests the valve is leaking (try replacing the valve core, the same part used in bicycle tires) or the bladder is starting to fail.