A reverse osmosis tank is not a simple bucket. Inside the steel shell is a flexible bladder holding the treated water, with a pressurized cushion of air behind it. The membrane produces water slowly, the tank stores it, and the air charge is what pushes it back out to the faucet at a usable rate when you open the tap.
Why the tank exists at all
A membrane produces permeate a drop at a time, far too slowly to fill a glass on demand. The tank decouples production from use: the system runs quietly for an hour or two filling the bladder, then delivers a burst of water whenever you want it. As the bladder fills, the trapped air behind it compresses and the back pressure rises, and when that back pressure reaches a set fraction of the line pressure the automatic shutoff valve stops production. That relationship is why feed pressure affects how full the tank ever gets.
Checking the air charge, the single most useful maintenance task
The check needs the tank empty. Close the tank valve, open the treated faucet and let it drain completely, then disconnect the tank and put a tire gauge on the schrader valve under the blue cap. Compare against the figure printed on the tank label, and add air with a bicycle pump in small increments if it is low. Over-inflating is as bad as under-inflating: it reduces the volume of water the tank can hold. Do this yearly, and immediately whenever the faucet flow feels weaker than it used to.
The symptoms and what each one means
A tank that delivers only a trickle and stops has lost its air charge. A tank that feels heavy but gives almost nothing is the same fault further along. Water escaping from the schrader valve when you press it means the bladder has ruptured and the tank must be replaced rather than repaired. A tank that never fills, with the system running continuously to drain, points instead at the automatic shutoff valve, the check valve or low feed pressure. Diagnose in that order and you will rarely replace the wrong component.
Size, position and mounting
Tank capacity is quoted as a nominal volume but the usable draw is less, because a tank never fully empties and the useful volume depends on the air charge and the feed pressure. Households that fill large bottles or feed a refrigerator ice maker feel that difference and often want a larger tank or a second one. Tanks can generally stand upright or lie on their side, but check what the manufacturer allows, support the weight properly, and never let the tank hang from its own connection tubing under a cabinet.
Hygiene, and the tankless alternative
Treated water sits in the bladder with no disinfectant residual, so the tank should be sanitized whenever the filters are changed and drained fully if the house is left empty for a long period. Tankless reverse osmosis designs remove this concern entirely by using a pump to produce water on demand, at the cost of needing an electrical outlet in the cabinet and a slightly noisier operation. Our whole-house reverse osmosis guide compares how the storage question changes when the system serves more than one tap.
Questions people ask about reverse osmosis tank
How do I check the pressure in a reverse osmosis tank?
Drain the tank completely with the tank valve closed and the faucet open, disconnect it, then read the schrader valve with a tire gauge and compare with the figure on the tank label.
Why does my reverse osmosis faucet only give a trickle?
Most often a low air charge in the storage tank, sometimes a ruptured bladder. Loaded pre-filters and low feed pressure produce the same symptom, so check the tank charge first because it is quickest.
Can a reverse osmosis tank be repaired?
A low air charge can simply be topped up with a bicycle pump. A ruptured bladder cannot be repaired, and water coming out of the schrader valve is the sign that the tank needs replacing.
Can the tank lie on its side?
Many can, but check the manufacturer's instructions and support the weight on a proper surface. Never let a full tank hang from its connection tubing, which is a common cause of leaks in cabinets.