When an under-sink reverse osmosis system starts giving less water or water that tastes different, most owners go straight to the faucet or the membrane. Start instead with the storage tank, the filter record, and the drain. Those three parts are easy to overlook and quick to check.

Check the storage tank air pressure

The storage tank holds treated water under pressure, with an air charge on one side of an internal diaphragm. When that air charge drops, the tank fills more slowly and the faucet can run weak or sputter, even when the membrane is fine. To check it, shut off the feed water to the system, open the faucet until the tank is empty and the flow stops, then press a tire-style gauge onto the small valve on top of the tank. Compare the reading with the figure printed in the owner's manual or on the tank label. If the pressure is low, the tank can be recharged, but if it will not hold air or the shell is bulging, the tank needs replacing rather than more air.

Keep a filter log

Write the install date and the approximate water volume or time on each filter stage: the sediment prefilter, the carbon prefilter, the membrane, and any polishing filter. Manufacturer labels give replacement intervals, but your own water decides when a filter is really finished. If a prefilter clogs much sooner than its label suggests, the cause is often sediment or a supply problem that a single filter change will not fix. Keep the log with the manual so whoever services the system next can see what was done and when.

Look at the drain and the shutoff

The membrane sends a steady stream of wastewater to the drain. A kinked line, a drain saddle that has slipped, or a clogged air gap can slow production and build back pressure against the membrane. Once in a while, look at the drain line for kinks and for water pooling at the connection. Also find the feed shutoff valve for the system and confirm that it turns freely. You want to stop water to the system quickly during a leak, without hunting for the valve.

Decide who does each task

Write down which tasks you handle and which need a technician. Owners usually handle the filter log, a quick faucet check, and a look at the drain. A technician handles membrane replacement, tank replacement, and any repair to the feed or drain piping. If an installer gave you a schedule, ask them to put it in writing with the part names on it, so a filter change is never a guess.

When output drops suddenly

A sudden drop in output or a change in taste is worth a service call rather than weeks of guessing. Before you call, have the filter log, the tank pressure reading, and the part labels ready. Those three facts shorten the call and tell the technician where to start.