An under-sink reverse osmosis system normally sends some water to the drain while it refills its storage tank. The warning sign is not simply hearing drain water. It is hearing water long after the tank should be full, noticing that the system restarts without anyone using filtered water, or finding that faucet flow has become weak while drain flow continues.
Do not start by ordering a membrane or adjusting random valves. Several faults can create the same symptom, and replacing the wrong part may leave the system running continuously. A few controlled observations can help you describe the problem accurately and decide whether it needs a filter change, a tank adjustment or professional service.
First, confirm that the sound is coming from the reverse osmosis drain
Dishwashers, refrigerator ice makers, humidifiers, leaking toilet valves and ordinary plumbing can all produce intermittent water sounds. Open the sink cabinet and locate the small drain tube running from the reverse osmosis unit to the sink drain connection. Touch the tube gently while listening. Flowing water may create a slight vibration.
Do not disconnect the drain tube merely to see whether water comes out. That can spill water into the cabinet and disturb a connection that was not leaking. Instead, note whether the sound begins after filtered water is used and whether it eventually stops.
If water is present on the cabinet floor, treat that as a leak rather than a drain-flow diagnosis. Shut off the reverse osmosis feed valve if you can identify and operate it safely. Dry the cabinet, protect stored items and arrange service if the source is not obvious.
Run a simple fill-cycle check
Use enough water from the reverse osmosis faucet to make the system begin producing water. Close the faucet and listen near the drain connection. Drain flow during tank refill is expected. Leave the filtered-water faucet closed and avoid using any connected refrigerator dispenser or ice maker while you observe the system.
The important question is whether the drain flow stops after the storage tank refills. Refill time varies with incoming water pressure, water temperature, membrane condition, tank size and the amount of water used. Instead of relying on a universal time limit, compare the behavior with the system's normal pattern. Continuous flow through a long period with no filtered-water use deserves further investigation.
Also check whether the system repeatedly starts and stops. Rapid cycling can point to unstable pressure, a small downstream leak or a shutoff component that is not holding consistently.
Check for filtered-water use that is easy to miss
A reverse osmosis system cannot remain shut down if treated water is leaving the storage tank. Before assuming an internal failure, check every connection downstream of the system.
Look for a dripping reverse osmosis faucet. If the system serves a refrigerator, inspect the supply tube, connection fittings and area behind or below the appliance. An ice maker may draw small amounts automatically, so turn it off temporarily if its controls are accessible. A damp fitting, slow faucet drip or hidden refrigerator-line leak can keep the system refilling.
Dry each accessible connection with a paper towel, then check it again after the system has been operating. Do not tighten plastic fittings aggressively. Overtightening can damage tubing, threads or sealing surfaces.
Notice what the faucet tells you about the storage tank
Open the reverse osmosis faucet after the system has had time to fill without any water use. A healthy storage tank usually provides an initially strong stream that gradually declines as the tank empties. The exact flow depends on the faucet, tubing and system design.
If only a small amount comes out before the stream becomes weak, the storage tank may have lost its air charge, developed an internal bladder problem or failed to receive enough treated water. Any of those conditions can make the system run frequently while delivering little usable water.
Do not add air to the tank until you have the manufacturer's procedure and an appropriate low-pressure gauge. Tank pressure must be checked with the water side empty. Measuring or adjusting it while the tank contains water can produce a misleading result and make performance worse.
Consider clogged prefilters before blaming the membrane
Sediment and carbon prefilters protect the membrane and help the system operate at an adequate feed pressure. As they load with debris, pressure reaching the membrane may fall. The system can then produce water slowly, take much longer to fill the tank and send water to the drain for extended periods.
Review the service label or maintenance record. Check which cartridges were last replaced and whether all required prefilters were changed. A cartridge that looks clean from the outside can still be restricted, so appearance alone is not a reliable test.
If faucet production weakened gradually and filter service is overdue, prefilter restriction is a reasonable possibility. Use the cartridge types and replacement sequence specified for the equipment. A cartridge that physically fits the housing is not automatically the correct cartridge.
Look for a shutoff failure
Most tank-style reverse osmosis systems have a mechanism that stops feed and drain flow when tank pressure rises sufficiently. Depending on the design, this may involve an automatic shutoff valve, a check valve and other flow-control parts.
If the tank is full and provides a normal amount of water, but the drain continues flowing, the automatic shutoff system becomes a stronger suspect. A check valve that allows pressure to bleed backward can cause similar behavior. Mineral deposits, debris, worn seals, incorrect tubing connections or mismatched replacement components may prevent proper shutdown.
These parts should be diagnosed as a system. Replacing only the most visible valve can miss the actual cause. Ask a technician to verify that the tank reaches shutoff pressure, the check valve holds and feed flow stops when the tank is full.
Inspect the drain flow control and drain connection
The membrane's drain line normally includes a flow restrictor. It maintains the pressure and waste-flow balance required by the system. A missing, damaged, clogged or incorrectly sized restrictor can change production, rejection performance and drain behavior.
Check service records for previous membrane or tubing work. If constant draining began immediately after maintenance, ask whether the original restrictor was reinstalled and whether it matches the membrane. Do not insert a generic restrictor based only on tubing size.
Also inspect the drain connection for kinks, pinched tubing and buildup where the line enters the sink drain. A blocked drain path may create unusual noise, leakage or poor operation. If the system uses an air-gap faucet, water emerging from the faucet's vent opening often indicates a restricted drain path rather than a membrane failure.
Check incoming pressure without guessing
Reverse osmosis performance depends heavily on feed pressure. Low pressure can result from a partly closed supply valve, a clogged prefilter, a pressure regulator issue or the home's plumbing conditions. Systems equipped with booster pumps add more possible failure points, including the pump, transformer and pressure switches.
Make sure the reverse osmosis feed valve is fully open unless the equipment instructions specify otherwise. Look for bent tubing and recently moved items pressing against a line. If pressure must be measured, use a gauge at the appropriate point instead of judging pressure from another faucet in the house. Household faucet pressure does not show how much pressure remains after restricted prefilters.
Know what evidence to collect before requesting service
A clear description can shorten the diagnosis. Record whether the drain runs continuously or cycles, whether the storage tank feels full, how faucet flow behaves, whether the refrigerator is connected and when the filters were last serviced. Note whether the problem started after a cartridge change, membrane replacement, plumbing work or a supply interruption.
Ask the provider to identify the failed condition before authorizing parts. Useful questions include:
Does the system shut off when the tank reaches the required pressure? Does the tank hold its air charge? Are the prefilter pressure drop and membrane production within the equipment specifications? Is the check valve holding? Is the installed flow restrictor correct for the membrane? Is any treated-water connection leaking?
If the diagnosis includes replacing the membrane, ask for the measurements supporting that conclusion. Continuous drain flow alone does not prove that the membrane has failed.
When to stop troubleshooting
Shut off the system and seek service if water is leaking into the cabinet, a housing is cracked, electrical pump components are wet, tubing repeatedly disconnects or a valve will not operate normally. Also stop if accessing a connection would require moving hard-wired equipment or disturbing plumbing you cannot isolate.
For non-emergency symptoms, compare providers by how clearly they explain diagnosis, parts compatibility and post-repair testing. The site's review methodology and editorial criteria describes practical factors used to evaluate water treatment providers.
The central check is simple: drain flow should correspond to water production and should stop when a sound, leak-free system has filled its tank. When it does not, observe tank delivery, downstream leaks, filter condition, feed pressure and shutoff behavior before approving a repair.