An under-sink reverse osmosis system normally sends some water to the drain while it produces filtered water. The warning sign is not simply hearing drain flow. It is hearing the same flow long after the storage tank should have refilled, especially when nobody has used the drinking-water faucet.
Do not begin by ordering a replacement membrane. A restricted prefilter, low incoming pressure, an undercharged storage tank, a leaking faucet or a faulty automatic shutoff valve can all keep the system producing water. Work through the system in order so you can tell whether the membrane is actually the problem.
Confirm that the drain flow is continuous
Choose a period when nobody will use filtered water, an ice maker or another appliance connected to the reverse osmosis system. Close the drinking-water faucet and let the system refill. Check the drain connection later without using any treated water in between.
A faint sound near the sink is not enough evidence. A dishwasher, condensate line or ordinary faucet leak may use the same drain area. Locate the small reverse osmosis drain tube and confirm that water is moving through that specific line.
Record whether the flow is steady, intermittent or limited to the period after water use. Also note whether the storage tank feels full and whether the faucet still delivers a useful stream. These observations give a service technician more to work with than a report that the system is noisy.
Run the tank-valve test
Once the tank has had a chance to refill, close the valve on top of the storage tank. Leave the feed-water valve open and keep the drinking-water faucet closed. The drain flow should stop after the system responds to the closed tank valve.
If it stops, the automatic shutoff assembly is probably reacting to pressure. The problem may instead involve the tank, a small downstream leak or treated water being used somewhere else. If the drain continues without slowing, the shutoff valve, membrane check valve, flow control or feed conditions deserve closer inspection.
Reopen the tank valve after the test. Do not leave it closed and assume the system is ready for normal use.
Look for hidden demand on the treated-water line
A reverse osmosis system cannot shut down if treated water keeps leaving the system. Dry the drinking-water faucet and watch for a slow drip. If the system supplies a refrigerator, temporarily close the refrigerator branch valve if the installation includes one. Then repeat the observation.
Inspect accessible tubing and fittings for moisture. A very small leak may not create a puddle, but it can prevent pressure from rising enough to trigger the automatic shutoff. Pay particular attention to push-fit connections, the faucet stem and tubing near the tank.
Check feed pressure before blaming the controls
Reverse osmosis production depends on adequate pressure entering the membrane. Low pressure can make production slow and keep the drain flowing because the tank takes much longer to fill.
Make sure the cold-water supply valve feeding the system is fully open and that the tubing is not kinked. Consider whether the symptom appeared after plumbing work, a valve adjustment or installation of another treatment device. A clogged sediment or carbon prefilter can also reduce pressure reaching the membrane even when pressure elsewhere in the house seems normal.
If you have a suitable pressure gauge and know where the manufacturer permits it to be connected, compare the measured feed pressure with the operating range in the system manual. Do not rely on the kitchen faucet stream as a pressure test. A faucet can appear normal while a restricted filter limits the much smaller line feeding the reverse osmosis unit.
Check the prefilter service history
Find the filter labels, service tag or installation paperwork. Identify which cartridges are upstream of the membrane and when they were last changed. Do not assume that every cartridge under the sink is interchangeable.
If the prefilters are overdue or visibly loaded, replace them with cartridges that match the housing and system requirements. Flush and restart the unit according to its instructions, then repeat the drain-flow test after the tank refills. Changing the membrane first will not correct a pressure loss caused by a clogged prefilter.
Separate a tank problem from a membrane problem
The storage tank contains both water and a pressurized air chamber. If its air charge is wrong or its internal bladder has failed, the tank may hold little usable water. The system then starts producing again after only a small draw, which can look like continuous operation during normal household use.
Turn off the feed water, open the reverse osmosis faucet and observe how the flow changes. A tank that gives only a brief stream may need its air charge checked while completely empty. Use the tank manufacturer's procedure and specified pressure, not a generic setting from another system. If water comes from the tank's air valve, the bladder has failed and the tank should be replaced.
A weak faucet stream can also come from clogged postfilters, pinched tubing or a partly closed tank valve. Check those items before concluding that the membrane produces too little water.
Inspect the drain connection and flow control
The drain saddle should connect to an open section of drainpipe in the position required by the installation instructions. A clogged or misaligned saddle can create noise and backpressure. Remove accessible tubing only after shutting off the system and preparing for spilled water.
The small flow-control component in the drain line limits wastewater flow and maintains pressure across the membrane. A missing, damaged or incorrect control can allow excessive drain flow. Do not substitute a random restrictor based only on tubing size. It needs to match the membrane and system design.
Know what the shutoff and check valves do
The automatic shutoff valve stops feed water when pressure on the treated-water side rises as the tank becomes full. A check valve prevents stored water from pushing backward through the membrane. Trouble in either component can keep water moving to the drain after the tank is full.
These parts may be separate fittings or built into the membrane housing or manifold. Before buying anything, identify the system model and obtain its flow diagram or parts list. If a technician recommends a valve replacement, ask which test showed that the valve failed and whether the check valve was tested separately.
Evaluate the membrane with measurements
A membrane can become fouled, damaged or exhausted, but drain noise alone does not prove it. A useful evaluation compares feed water, treated water quality, production rate, feed pressure and water temperature under the conditions specified by the manufacturer.
If water quality is the concern, use an appropriate water test rather than taste alone. A handheld dissolved-solids meter can show relative membrane performance when used correctly, but it does not identify every contaminant or determine whether water is safe. When comparing service providers, ask how they diagnose equipment and support their recommendations. The site's review methodology and editorial criteria explain the kinds of documentation and service practices considered in provider reviews.
What to ask before approving a repair
Ask the provider to identify the confirmed failure, not merely the part most commonly replaced. Request the measured feed pressure, the condition of the prefilters, the tank test result and what happened during the tank-valve test. If the membrane is being replaced, ask for the old and new membrane specifications and the measurements that support the change.
Also ask what will be checked after the repair. A complete handoff should include confirmation that the tank fills, drain flow stops, the faucet delivers normally and all disturbed fittings remain dry. If sanitation or flushing is required after parts are opened, the scope should say who performs it.
Shut the system down when leakage is possible
Continuous water inside the drain tube is different from water leaking into the cabinet. If a housing, fitting, faucet connection or storage tank is wet, close the reverse osmosis feed valve and protect the cabinet until the leak is found. If the small valve will not stop the water, close the appropriate upstream supply valve.
The practical goal is to determine why the system never reaches shutoff pressure. Confirm continuous drain flow, remove hidden water demand, verify feed conditions, test the tank and then evaluate the control parts and membrane. That sequence makes it much less likely that you will replace an expensive component while leaving the actual fault in place.