The RO tank is the small pressurized cylinder that sits beside an under-sink reverse osmosis system and holds the treated water until you open the faucet. A membrane produces water far too slowly to serve a tap directly, so the tank is what turns a trickle into a usable stream, and almost every complaint about slow reverse osmosis water traces back to it.
How the tank stores and delivers water
Inside the steel shell is a rubber bladder or diaphragm with water on one side and compressed air on the other. As the membrane produces water, the bladder expands and squeezes the air, raising its pressure. When you open the faucet, that compressed air pushes the water back out. Nothing about this is powered, so the entire delivery pressure at your faucet is the air the tank was charged with, minus whatever the tank has slowly lost through its valve over the years.
Precharge is the number that matters
An empty tank carries an air precharge set at the factory, measured at the Schrader valve under the blue cap with an ordinary tire gauge. Too low and the tank fills with water but pushes it out weakly, so the faucet dribbles. Too high and there is little room for water, so the tank runs dry after a glass or two. The check is simple: drain the tank fully at the faucet, then measure the air pressure and adjust it to the maker's figure with a hand pump.
Diagnosing slow or short delivery
Work through the tank first because it is the cheapest thing to rule out. A full tank that delivers weakly points at low precharge. A tank that gives a little water then stops points at high precharge or a ruptured bladder, and a ruptured bladder usually spits air at the faucet. A tank that never fills points upstream: a fouled membrane, a clogged prefilter, low feed pressure, or a closed tank valve, which is a surprisingly common outcome of a filter change.
Size, placement and the plumbing around it
Tanks come in several sizes, and the useful capacity is always less than the nominal one because the air side occupies space and the tank never empties completely. A larger tank means more water on demand but more cabinet space and a longer refill. Tanks can stand upright or lie on their side, and they connect through a single line with a shutoff valve. Vendor product pages list tank sizes offered with each system. Our reverse osmosis system installation guide covers fitting one into a crowded cabinet.
Life, hygiene and replacement
Tanks last many years but not forever: the bladder perishes, and the steel can corrode where condensation sits on the outside. Because water stands inside for hours, the tank is also the part of an under-sink system where taste can develop if the household is away for weeks. Draining and refilling after a long absence clears that, and sanitizing the system when the membrane is replaced is good practice. A tank spitting air or delivering less each month is a tank at the end of its life.
Questions people ask about ro tank
How do I check the pressure in an RO tank?
Turn off the feed, open the faucet until water stops, then measure at the Schrader valve under the cap with a tire gauge. Compare it to the precharge figure the manufacturer publishes.
Why does my RO faucet run slowly?
Most often the tank precharge has dropped over the years, so there is not enough air to push the water out. Clogged prefilters, a tired membrane and low house pressure are the other common causes.
Can I use an RO system without a tank?
Only with a tankless system built for it, which uses a pump to produce water on demand. A standard membrane makes water far too slowly to serve a faucet without storage.
Does the water in an RO tank go stale?
It can pick up a flat or plastic taste if it stands for weeks, since nothing in the tank is disinfected. Draining the tank after a vacation and letting it refill fixes it.