An under-sink reverse osmosis system normally sends some water to the drain while it produces filtered water. The drain flow should eventually stop after the storage tank fills. If water continues running long after anyone has used the RO faucet, something may be preventing the system from reaching its shutoff point.
This problem is easy to miss. The drain connection is usually hidden inside the sink cabinet, and a thin stream may be difficult to hear over a dishwasher, refrigerator, or furnace. The first signs may be a faint hiss, a recurring trickle in the drain, weak flow from the RO faucet, or a system that seems to refill constantly.
Do not choose a replacement membrane, storage tank, or complete new system from the sound alone. Several different problems can produce the same symptom. Start by confirming when the drain runs and whether the storage tank can fill.
First decide whether the drain flow is actually abnormal
Open the RO faucet and draw enough water to make the system begin producing more. Close the faucet, note the time, and listen near the drain connection. Production and drain flow should continue while the tank refills, so immediate silence is not the goal.
Check again after the system has had plenty of uninterrupted time to refill. Avoid using the RO faucet or refrigerator water during this test. If the drain is still flowing, close the storage tank valve without changing any other valve. Wait several minutes and listen again.
If closing the tank valve causes the drain flow to stop, the system may be responding to the tank or the pressure around it. If drain flow continues, the problem may involve feed pressure, prefilters, the membrane, the automatic shutoff valve, or the check valve. This is a screening test, not a final diagnosis.
Restore the tank valve to its original position after the check. If you are unsure which valve controls the tank, do not turn several valves at random. Photograph the system and ask the installer to identify each connection.
Make sure the sound is coming from the RO drain
A leaking faucet, refrigerator valve, dishwasher connection, or sink drain can sound surprisingly similar inside a cabinet. Dry the cabinet floor and inspect each tube with a flashlight. Look for droplets, damp fittings, mineral marks, and water tracking along a tube before it falls.
You can also place a dry paper towel under the RO assembly and around the storage tank base. Moisture on the towel indicates a leak, which is a different and more urgent problem than water flowing through the intended drain tube.
If the RO system uses an air gap faucet, occasional gurgling can be normal during production. Water coming out of the faucet's air gap opening is not normal. That commonly points to a restricted drain tube, a blocked drain saddle, or a household drain that is backing up. Stop using the system until the drain path is checked, especially if water is entering the cabinet.
Check whether the storage tank ever becomes full
A system cannot shut off normally if the tank never reaches the required pressure. After leaving the system unused long enough to refill, lift the tank gently without pulling on its tubing. A full tank should feel substantially heavier than an empty one.
If the tank stays light while the drain continues, the system may be producing very little filtered water. Possible causes include a closed feed valve, low incoming pressure, clogged prefilters, a fouled membrane, cold feed water, or a kinked product-water tube.
If the tank feels heavy but the drain still runs, ask the service provider to test the automatic shutoff valve and the membrane housing check valve. A failed or obstructed valve can prevent the pressure signal that should stop production.
Use the RO faucet flow as another clue
Open the RO faucet after the tank has had time to fill. Strong flow that quickly becomes weak may point toward a storage tank problem. Weak flow from the beginning may instead indicate an empty tank, restricted tubing, clogged filters, low feed pressure, or a partly closed valve.
Do not judge membrane performance from faucet pressure alone. The tank supplies much of the pressure you feel at the faucet. A tank can have an air charge problem even when the membrane is still reducing contaminants, and a membrane can perform poorly while a full tank initially delivers a strong stream.
If the faucet serves a refrigerator too, temporarily avoid using the refrigerator dispenser and ice maker during troubleshooting. Hidden demand from that line can keep the system refilling and make a normal production cycle appear continuous.
Check the simple restrictions before replacing major parts
Confirm that the cold-water feed valve is fully open and that flexible tubes are not pinched behind stored items. Look at the maintenance label or service record to identify which cartridges are installed and when they were last changed. Do not rely only on cartridge color because appearance does not show every restriction.
Prefilters protect the membrane and need to match the housing and system specifications. A heavily restricted sediment or carbon cartridge can reduce membrane feed pressure enough to extend production and drain time. Replacing only the membrane while leaving restricted prefilters in place may fail to solve the problem.
If the system includes a booster pump, confirm that it starts and stops as intended. Ask whether its pressure switch, transformer, and shutoff controls were tested together. Replacing the pump without checking its controls can leave the same continuous-drain symptom.
Do not adjust the storage tank by guesswork
RO storage tanks contain an air charge separated from the water by a bladder. The air charge must be checked with the water side empty and the feed water turned off. Measuring it while the tank contains water produces a misleading reading.
A technician should also determine whether the tank bladder can still hold pressure. Adding air may temporarily change faucet flow, but it will not repair a ruptured bladder. Before approving a new tank, ask for the empty-tank air-pressure reading and the reason the existing tank cannot be adjusted or reused.
Ask for measurements before approving a membrane
A membrane should not be condemned solely because the system runs for a long time. Ask the provider to record feed-water pressure, product-water flow, drain flow, and dissolved-solids readings from both the feed and treated water. Those measurements help separate a restricted membrane from low feed pressure, an incorrect flow restrictor, or a faulty shutoff component.
The flow restrictor in the drain line must be appropriate for the membrane. If someone previously installed mismatched parts, the system may send too much water to the drain or may not flush the membrane correctly. Ask the provider to identify the installed membrane capacity and confirm that the restrictor matches it.
If water quality is part of the complaint, use an appropriate water test rather than taste alone. Continuous draining and poor contaminant reduction can occur together, but one does not prove the other.
Give the service provider a useful problem record
Before the appointment, write down when the drain flow begins, how long it continues, whether the tank feels full, and what happens when the tank valve is closed. Record any recent filter changes or plumbing work. Take clear photos of the membrane label, cartridge labels, valves, tubing, drain saddle, and any pump or electronic controller.
Ask the provider to explain which condition prevented shutoff and how the repair corrected it. The final check should include filling the tank, drawing water, allowing the system to refill, and confirming that drain flow stops. A brief test immediately after installing a part is not enough if the system has not reached a full-tank condition.
If replacement is being recommended, compare systems by the water problem they address, their required feed pressure, their storage and delivery capacity, their filter schedule, and the local service included. The Compare Water Treatment system finder and provider grid can help organize those choices. For details about how systems and providers are evaluated, review the comparison methodology.
The most useful early warning is not simply that an RO system makes drain noise. It is that the drain never stops after the tank should be full. Confirm that pattern first, then require pressure, flow, and water-quality checks before paying for major replacement parts.