RO System Keeps Running or Draining
If your RO system keeps running, it is sending water to the drain without taking a break. A reverse osmosis system naturally creates drain water while it purifies drinking water. However, the flow should shut off once the system finishes its work.
When the draining sound never stops, the unit is likely struggling with a full tank signal, a worn part, or an old membrane. You can check a few simple parts on your own before calling a professional.
Some drain water is normal
A reverse osmosis system has a membrane and a small drain line. An under-sink filter that is not RO connects to the cold water line only, with no drain line. Because of how reverse osmosis works, running water down the drain is part of the regular filtering cycle.
The membrane separates pure water from rinse water. The rinse water carries rejected minerals away through the drain tube. When the system makes clean water, you will hear a soft hiss or trickle in your plumbing pipes. The sound should stop once clean water production is complete.
Tank systems and the shutoff valve
Under-sink RO systems that use a storage tank rely on water pressure to control the flow. As clean water enters the storage tank, pressure builds inside the tank. When the tank becomes full, this backpressure pushes against an automatic shutoff valve.
The automatic shutoff valve closes the incoming water line. If this valve fails, incoming tap water continues to push across the membrane and out the drain line. A storage tank that cannot build sufficient internal pressure will also fail to trigger the shutoff valve. In both cases, the system continues to run constantly.
Tankless systems
Tankless RO systems do not store water in a pressure tank. Instead, they make fresh water on demand whenever you open the dedicated faucet. They use built-in sensors and electronic valves to control water flow.
A tankless unit should run only while you dispense water, plus a brief rinse cycle right after you close the tap. If a tankless unit keeps draining long after you turn off the faucet, an internal flow switch or solenoid valve may be stuck open. Countertop units operate on a similar direct demand concept and should also shut off promptly after filling a container.
Filter and membrane age
Clogged pre-filters can starve the system of incoming water pressure. Reverse osmosis membranes require good line pressure to push pure water through the tiny pores. If the pre-filters are clogged, pressure drops, making it hard for the unit to produce clean water quickly.
A filter is built to handle a set amount of water, its rated capacity. Change it sooner if water flow slows or the filter light comes on. Typical change intervals are rough guidance: under-sink cartridges every 6 to 12 months, RO pre and post filters every 6 to 12 months, and RO membranes every 2 to 3 years. The interval on the product page and in the manual always wins.
- RO pre and post filters: typical change interval is every 6 to 12 months.
- RO membranes: typical change interval is every 2 to 3 years.
- Filter lights track time or water used, but do not test water quality directly.
Step-by-step checks to find the cause
You can run a few simple checks to narrow down why your RO system keeps running. Work slowly and observe how the system responds to pressure changes.
- Shut the blue valve on top of your storage tank if your unit has one. This simulates a full tank quickly.
- Wait a few minutes and listen to the drain line. If the draining stops, your automatic shutoff valve works, and the issue is likely low tank pressure or slow water production.
- If the draining continues after closing the tank valve, the shutoff valve or check valve may have failed.
- Check the age of your filter cartridges. If your pre-filters have been in place for more than 6 to 12 months, replace them to restore proper water pressure.
- Turn off the water supply before changing an under-sink, inline or whole house filter.
When to call a plumber
Some plumbing issues require specialized tools and skills. If you have inspected the filters and the drain line still runs nonstop, an internal valve may need replacement. Water running constantly can waste water and raise utility bills.
If you notice leaking pipes, cracked housings, or water dripping beneath your sink cabinet, shut off the cold water line immediately. A licensed professional can test your incoming water pressure, verify internal valves, and ensure your drain saddle connects properly without leaks.
When to call a pro
If you are not sure about plumbing work, hire a licensed plumber. Stop work and call a pro right away if you find leaking fittings or cannot stop the constant water flow.
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Questions
How long should an RO system run after use?
An RO system runs until it refills its storage tank or completes its clean cycle. Tankless systems stop shortly after you close the faucet. If water flows down the drain all day without stopping, the system requires attention.
Does a non-RO under-sink filter have a drain line?
No, it does not. An under-sink filter that is not RO connects to the cold water line only, with no drain line. Only reverse osmosis systems use a separate drain line to carry away rinse water.
How often should I change my RO filters and membrane?
Typical change intervals are rough guidance. RO pre and post filters are usually changed every 6 to 12 months, while RO membranes last about 2 to 3 years. Always check the interval on the product page and in the manual.
What should I do before changing my under-sink filters?
Turn off the water supply before changing an under-sink, inline or whole house filter. Always run some water through a new filter before you drink it, following the flush steps on the box or product page.
Keep reading
General information, not a substitute for your system or fridge manual. Our filters and systems are made by Waterdrop. Brand names are used only to show fit.