A reverse osmosis system sends some water to the drain while it produces filtered water. That is normal. What is not normal is drain flow that continues long after the storage tank should be full, returns without anyone using water or runs through the night.
Do not start by replacing every filter. First determine when the drain flow starts, whether the storage tank can become full and whether the system can shut itself off. Those observations narrow the problem more reliably than the sound of running water alone.
Confirm that the sound is coming from the reverse osmosis drain
Under the sink, identify the small tube that runs from the reverse osmosis assembly to the drain connection. Do not confuse it with the filtered water line leading to the faucet or refrigerator.
Listen near the drain connection, then touch the drain tube lightly. Flowing water may produce a steady vibration. If the system uses an air gap faucet, you may also hear water at the faucet body while the unit is producing water.
Check for other possible sources before opening the system. A dripping faucet, leaking toilet or dishwasher drain can create a similar sound. If you cannot identify the tubing confidently, stop and ask the installer to label the feed, product and drain lines.
Give an emptied tank time to refill
If someone recently used most of the stored water, drain flow is expected while the membrane refills the tank. The refill can take a while because reverse osmosis production is intentionally slow.
Close the reverse osmosis faucet and avoid using connected refrigerator water or ice. Note the starting time. Return periodically and listen at the drain. The useful question is not whether it stops within a guessed number of minutes. The question is whether it eventually stops after the tank has had an uninterrupted refill period.
Then open the faucet and check the delivery pattern. A full storage tank should provide an initial pressurized flow. The flow will eventually slow as stored water is depleted and the membrane becomes the main source. If the faucet never produces a useful initial flow, the tank may not be filling properly.
Use the tank valve to separate tank trouble from shutoff trouble
Find the valve on top of the reverse osmosis storage tank. Before moving it, take a photo of its original position. Close the tank valve, leave the feed water on and make sure the drinking water faucet is closed.
Wait and listen to the drain line. A properly working system should respond when product water pressure reaches the shutoff point. If drain flow continues indefinitely with the tank isolated, the problem may involve the automatic shutoff valve, check valve, flow restrictor or membrane assembly rather than the tank's usable capacity.
If drain flow stops with the tank valve closed but resumes and never ends when the tank is connected, investigate the tank and any downstream demand. Reopen the tank valve after the check.
Do not force a stiff valve. A cracked fitting or twisted tube can create a leak that is more urgent than the original drain problem.
Check for water use that the system cannot see you noticing
A refrigerator connection can draw small amounts of water without anyone standing at the sink. An ice maker may refill automatically. A leaking refrigerator line or a dripping reverse osmosis faucet can also keep reducing product water pressure, causing the system to restart repeatedly.
If the system supplies more than one outlet, isolate those branches one at a time where proper shutoff valves are available. Do not kink tubing as a substitute for closing a valve. Watch whether the drain finally stops when a particular branch is isolated.
Also inspect the faucet outlet. A slow drip can be easy to miss when it falls into a wet sink. Dry the sink and place a clean, dry cup beneath the outlet without opening the faucet. Any collected water indicates a demand that should be corrected.
Look for low feed pressure and clogged pretreatment
An automatic shutoff valve depends on pressure. If feed pressure is too low, or if clogged sediment and carbon cartridges restrict flow before the membrane, the system may produce water slowly and struggle to reach shutoff pressure.
Check whether a whole-home filter, softener or partially closed valve sits upstream. Confirm that all intended supply valves are fully open. Review the service record for the reverse osmosis prefilters, but do not assume age alone proves they are clogged.
A technician can compare feed pressure before and after the prefilters while water is flowing. Ask for the actual measurements and the equipment manufacturer's required operating range. That test is more useful than replacing cartridges without knowing whether they caused the pressure loss.
Do not overlook the storage tank's air charge
The storage tank contains an air side and a water side separated by a flexible bladder. Incorrect air charge can reduce usable storage and interfere with the pressure conditions that control system operation.
A tank that delivers only a brief burst before the faucet slows may have an air charge problem, a damaged bladder or very little stored water. The air charge must be checked with the water side completely empty. Measuring it while water remains in the tank gives a misleading result.
Use the pressure specified by the tank or system manufacturer. Do not copy a setting from an unrelated tank. If water comes out of the air valve, the bladder has failed and adding air will not repair it.
Ask for a shutoff test before approving parts
If professional service is needed, describe the pattern rather than saying only that the unit runs too much. Tell the technician whether the drain stops with the tank valve closed, whether the faucet drips, how the tank delivers water and whether any refrigerator branch was isolated.
Ask the technician to show:
• Feed pressure while the system is producing water
• Pressure loss across the prefilters
• Whether the tank holds the specified empty air charge
• Whether the membrane check valve prevents reverse pressure loss
• Whether the automatic shutoff closes when product pressure rises
• Whether the drain flow restrictor matches the installed membrane
Request separate findings for each part. A failed shutoff valve does not automatically mean the membrane is bad. A clogged prefilter does not prove the storage tank needs replacement.
Verify the repair with a complete operating cycle
After service, empty enough stored water to make the system begin producing again. Confirm that water reaches the drain during production. Close the faucet, allow the tank to refill without other use and verify that drain flow stops.
Inspect every fitting that was disturbed. Dry the tubing and connections with a towel, then check again under pressure. Test the faucet and any refrigerator connection before considering the work complete.
The clearest early warning is a change in the system's cycle: drain flow lasts longer, restarts when nobody uses water or never reaches a quiet stopped condition. Catching that pattern early gives you a specific problem to document and a focused set of checks to request.