An under-sink reverse osmosis system sends some water to the drain while it produces filtered water. That discharge is normal. A steady drain flow long after the storage tank should be full is not.
The difficult part is that several faults can produce the same symptom. A restricted prefilter, low supply pressure, a tank problem, a downstream leak or a failed shutoff component can all keep the system producing wastewater. Replacing the membrane before identifying the cause can leave the original problem untouched.
First, confirm that the drain flow belongs to the RO system
Listen near the drain saddle or air-gap faucet while no sink water is running. If you can hear or feel water moving through the RO drain tube, close the system’s feed-water valve. The flow should stop after the remaining pressure dissipates. If the sound continues, investigate the sink drain, dishwasher or another connected appliance instead.
Reopen the feed valve after this brief check. Do not leave it closed if the RO system supplies a refrigerator, ice maker or another fixture that someone expects to use.
Decide whether the system has had a chance to fill
An RO system may drain for an extended period after someone empties the storage tank. Before calling that a failure, stop using filtered water and allow the system to refill according to its manual. Include every connected outlet in this pause. A refrigerator drawing small amounts of water can keep the system cycling even when nobody uses the dedicated faucet.
Once the tank appears full, draw a small amount from the RO faucet and close it. Drain flow should resume while the system replaces that water, then stop when storage pressure again activates the automatic shutoff valve. Record what happens instead of relying on a sound noticed at an unknown point in the fill cycle.
Look for demand that never ends
A slow leak after the RO system can prevent the storage tank from reaching shutoff pressure. Dry the faucet base, tubing connections and refrigerator connection, then check them again. Watch for a faucet that drips only after use, a loose push fitting or an ice maker that cycles more often than expected.
If the installation has separate valves for the faucet and refrigerator line, isolate one branch at a time. A system that shuts down with one branch closed has given you a useful clue. Restore the valves to their normal positions when the test is complete.
Check feed pressure before blaming the membrane
The automatic shutoff valve depends on pressure. If the incoming pressure is below the equipment manufacturer’s operating requirement, the system may make water slowly and struggle to reach the pressure needed to stop the drain flow.
Ask for a pressure reading taken while the RO system is producing water, not just a static reading taken with every fixture closed. Also note whether the problem appears when another appliance runs or several fixtures are open. That pattern can point to a supply-pressure problem rather than a failed RO component.
A booster pump, if present, adds more items to check. Confirm that it starts during production, stops when the tank is full and has the pressure controls specified for the system. A pump that runs continuously is a symptom to diagnose, not proof that the pump itself is defective.
Check for restricted prefilters
Sediment and carbon prefilters protect the membrane, but they also create pressure loss as they load. A badly restricted filter can leave adequate pressure at the cold-water pipe and inadequate pressure at the membrane.
Find the filter model numbers and maintenance record. Ask for pressure readings before and after the prefilter stages while water is flowing. Replacing overdue filters may be appropriate, but the useful evidence is the pressure reaching the membrane after the filters are installed.
Test the storage tank correctly
A storage tank with the wrong air charge can hold less usable water or send misleading pressure back to the shutoff valve. Do not check the air valve while the tank contains water. The water side must be depressurized and emptied, and the air charge must be compared with the tank or system manufacturer’s procedure.
Water coming from the tank’s air valve indicates a damaged internal bladder. That tank generally needs replacement. If only air escapes, the tank may still require adjustment, but adding air without first emptying the water side produces an unreliable reading.
Separate a shutoff fault from a production fault
Most tank-style RO systems use an automatic shutoff valve and a check valve to stop production when storage pressure rises. If the check valve allows pressure to move backward through the membrane, or the automatic valve cannot close, water may continue going to the drain after the tank fills.
A technician can isolate the storage side, verify feed pressure and watch whether the system reaches shutoff. Ask the technician to identify which valve failed the test. “It keeps running” is an observation, not a diagnosis.
A worn or fouled membrane can also produce water too slowly to complete a normal fill cycle. Before approving membrane replacement, ask for the product-water flow, drain-water flow and relevant water-quality readings under stated pressure and temperature conditions. Those measurements should be compared with the specifications for that exact membrane and system.
Do not confuse a drain problem with an air-gap problem
Water emerging from the small opening in an air-gap faucet often means the drain path is blocked, kinked or poorly connected. That is different from the RO system producing wastewater continuously. Inspect the drain tube for a sag, pinch or clogged connection at the drain saddle. The saddle opening should align with the hole in the drainpipe and should not be placed where sink debris routinely collects.
Do not pour drain cleaner into RO tubing. If the drain connection must be opened, shut off the RO feed water first and protect the cabinet from spills.
What to require from a service visit
A useful service report should state whether the system shut off during the test, the pressure reaching the membrane, the condition of the prefilters, the empty-tank air charge, whether any downstream demand was isolated and which component failed a repeatable check. It should also distinguish necessary repairs from optional maintenance.
If you are comparing companies, use the site’s award methodology to understand the service qualities considered in evaluating providers. For this specific problem, favor a proposal that explains the shutdown test and the evidence behind each recommended part.
Until the cause is known, close the RO feed valve when the system will be unattended if the drain flow is continuous or water is appearing inside the cabinet. Reopen it only when someone can watch the system. Early isolation limits wasted water and keeps a small drain or fitting problem from becoming cabinet damage.