A under-sink reverse osmosis unit tends to decline gradually. The membrane slowly loses its ability to reject dissolved material, the storage tank refills more slowly, and the unit sends more water to drain for each glass it produces. Nothing dramatic happens, so most homeowners only notice when the glass of water tastes wrong or the tank is empty when they need it. The goal of this guide is to catch the decline while it is still small.
Start with the refill time
Note how long the storage tank takes to refill after a full draw, such as filling a pitcher or running a pot of water. Write that time down on the day the unit is in good working order and keep the note with the filter log. Repeat the same test every few weeks, using the same draw and the same tap. A refill that is clearly slower than your note is the most common early sign that the membrane, a prefilter, or the tank itself needs attention.
Do not assume the cause is the membrane. A clogged prefilter restricts flow ahead of the membrane, and a tank that has lost its air charge will also refill more slowly and push water out less forcefully. Those problems are often cheaper and simpler to correct, so check them first.
Watch the drain line
When the unit is idle and no one is drawing water, the drain line should be quiet. A thin, steady trickle of drain water during the fill cycle is normal. A drain that keeps running after the tank is full, or that runs when nothing else is happening, usually points to a flow restrictor or check valve that is not doing its job. Look at the drain tubing for a steady stream, and note whether the trickle stops when you close the feed valve. If it keeps going, the fault is inside the unit and the next step is a service call, not a filter change.
Also compare how much water goes to drain against how much reaches the tank. Many units have a fixed restrictor, so the ratio should be consistent over time. If the drain side starts to look heavier while the tank fills more slowly, the membrane is likely losing production.
Use a TDS meter as a rough check
A handheld total dissolved solids meter is an inexpensive way to compare the raw feed water with the product water in the glass. Take a reading from the cold tap before the unit, then one from the faucet at the unit. Record both on the same day. A product reading that climbs steadily over several checks, while the feed reading stays put, means the membrane is passing more dissolved material than it did when new. A meter does not tell you what is in the water, and it does not test for bacteria or specific contaminants, but a rising trend is a useful reason to call the company.
Pay attention to taste and smell
Changes in taste often come before any change in flow. A flat or faintly salty taste, or a stale smell from the tank, can mean the membrane is letting more through or that the tank has stood too long without fresh water moving through it. Check whether the tank has been drained recently, and whether a long vacation or a spell of low use has left water sitting in the system. Those causes are different from membrane wear, so note the circumstances along with the taste.
What to ask the company
When you call, ask what readings the technician will record at a service visit: tank air charge, the refill time, the drain flow, and the product water quality if they test it. Ask whether the technician will check the flow restrictor and check valve during the visit, and whether the membrane replacement schedule in your service agreement depends on the readings or only on the calendar. Ask how the company will confirm the fix worked, and what you should watch for in the first week afterward.
Keep your own log with the refill time, the drain observations, the TDS readings, and the date of the last filter change. A log that shows a steady trend is far more useful to a technician than a single complaint, and it gives you a clear basis for deciding whether the problem is routine upkeep or a sign the unit has reached the end of its useful life.