A reverse osmosis system pushes water through a semipermeable membrane that holds back many dissolved substances. The membrane and the filters around it do not fail with a bang. They lose performance a little at a time, and the change is easy to miss if you are not watching for it. This explainer shows what to notice and how to tell whether the decline is normal or a sign of trouble.
How the decline usually starts
Prefilters catch sediment and chlorine before the membrane sees them. When a prefilter clogs or is overdue for replacement, flow drops and the membrane is exposed to material it was not designed to handle. Many owners blame the membrane when the real problem is an overdue prefilter, so always check the prefilters first.
Signs to track at home
Watch how long the storage tank takes to refill after a draw. A tank that refills more slowly than it used to is one of the clearest early signals. Notice whether the faucet flow has fallen, and whether the unit runs more often than it did. Listen for a drain line that stays running longer than usual, and look for drips at fittings, which can point to a worn seal or a restricted drain.
Taste is the last sign, not the first
A change in taste can come late in the decline, and it is hard to judge by taste alone because people adapt to changes gradually. Use taste as a confirming clue, not your main warning.
Use a simple meter as a check
A handheld total dissolved solids meter can compare the water entering the unit with the water coming out of the faucet. Take readings at the same point, under the same conditions, and write them down. Over time, a rising reading on the treated side tells you the membrane is passing more than it should. Treat a single reading as a snapshot, not a verdict, and repeat the check before you act.
Ask the provider how the unit reports trouble
Some systems include a warning light, a flow indicator or a service reminder. Others depend entirely on the owner noticing. Ask how your system shows a problem, which parts need replacement on a schedule, and what the replacement schedule depends on. A good provider will explain the answer in terms of your water and your use, not only a fixed calendar.
Keep a short log
Write down the refill time, the prefilter change dates, any unusual sounds or drips and each meter reading. When you call for service, the log turns a vague complaint into specific information the technician can act on, and it shows whether the decline is gradual or sudden.