If your reverse osmosis drinking faucet has slowed down, sputters, or the storage tank seems to empty faster than it used to, the tank's air charge is one of the first things to check. In Washington, the public water report describes very hard water, and some homes on private wells see iron. Either condition means a reverse osmosis unit may work harder and its filters may load up sooner. This guide covers what to check before you call a service person or order parts.

What the storage tank does

Most under-sink reverse osmosis systems use a small storage tank with a rubber bladder inside. Treated water fills one side of the bladder, while the other side holds air at a set pressure, called the precharge. When you open the faucet, that air pressure pushes the stored water out. If the air charge is too low, the tank delivers less water per fill and the faucet may spit. If the bladder has failed, water can fill the air side, and the tank may feel heavy or sound full even when it should be low.

Signs to note before testing

  • The faucet sputters or runs hard for a moment, then slows.
  • Flow fades quickly even though the filter cartridges look fine.
  • The tank seems to fill or drain on its own without faucet use, which can point to a leak or a stuck valve.
  • Taste or odor has changed recently. This is a separate question from air pressure, so write it down and check it on its own.

Safe steps for checking the air charge

  1. Shut off the cold water feed valve to the reverse osmosis unit. It is usually a small valve under the sink.
  2. Open the reverse osmosis faucet and let the storage tank drain completely. Keep the faucet open until the flow stops.
  3. Find the tank's air valve, the small stem on the bottom or side of the tank. It is usually covered by a cap.
  4. Press a tire-style pressure gauge onto the air valve. Compare the reading with the value printed on the tank label or listed in the manual for your model. Do not guess from the tank's color or shape.
  5. If the reading is below the manual's target, add air slowly and check often. If it is above the target, release a little air through the valve.
  6. Close the faucet, reopen the feed valve, and let the tank refill. Then test the flow again.

When the air is not the problem

If the air charge holds steady but the tank still delivers little water, look at the rest of the system. A clogged prefilter or membrane slows the whole unit, not just the tank. Low house water pressure can also reduce how fast the tank refills. Washington homes on municipal water and homes on private wells may see different sediment and iron loads, and those loads can clog filters faster than expected. Write down when each filter was last changed so you can tell whether the filters are overdue.

Signs the tank needs service

  • The air valve leaks or will not hold a charge after you have set it.
  • Water comes out of the air valve when you check it, which can mean the bladder has failed.
  • The tank shows cracks, bulges, or corrosion around the fittings.

In these cases, replacing the tank is usually the clearer next step. Do not force the valve or keep adding air to a tank that will not hold it. Refer to the manual for your model, or ask a qualified service person to inspect the tank before you decide.

Keep a simple record

Write down the date of each check, the reading you saw, and any changes you made. A short log helps you spot patterns, such as a tank that loses its charge over several months, and gives a service person useful history. Keep the notes near the unit or in the same folder as your water test results.

If you want to know what the system is removing

A tank check tells you about the equipment, not about the water. If you want to know what the system is removing, a lab test of your source water shows what is actually present. The public report for Washington describes hardness and iron as lines to watch, and a private well can carry different substances altogether, so test the source you are actually drinking from. Compare the lab results with your filter replacement history before you decide how often to service the unit.