The flow restrictor is a small, cheap part on the drain line of your RO, but it has a big job. It controls how much water goes to the drain, and in doing so it creates the pressure the membrane needs to work properly.
What does a flow restrictor do?
- It limits the drain (reject) water so the membrane stays under pressure and produces purified water.
- It keeps a steady flow across the membrane surface, which carries away salts and slows down scaling.
- Without it, almost all the water would run straight to the drain and very little purified water would come out.
Choosing the right size
Flow restrictors are rated by the drain flow they allow, often marked in ml/min (for example FR 300 or FR 450). The rating should match the membrane capacity:
| Membrane | Commonly used restrictor |
|---|---|
| 75 GPD | around 300 ml/min (FR 300) |
| 80–100 GPD | around 400–450 ml/min (FR 400 / FR 450) |
| Higher-capacity membranes | as recommended by the membrane maker |
Signs of a blocked or wrong restrictor
- Too little drain water and TDS of purified water rising quickly – restrictor blocked or too small; the membrane is not being flushed.
- Too much drain water and very slow purified flow – restrictor too large, missing, or damaged.
- No drain water at all while the pump runs – blocked restrictor.
- A new membrane that wears out unusually fast.
When to replace it
Restrictors can get blocked by scale, especially with hard or high-TDS water. It is good practice to replace the flow restrictor whenever you change the membrane. It is one of the cheapest parts in the system.
How to replace
- Unplug the purifier and close the inlet.
- Find the restrictor on the drain line coming out of the membrane housing (often inline in the drain pipe or a small cartridge on the housing).
- Release the push-fit pipes, fit the new restrictor in the same direction, and push the pipes fully back in.
- Run the purifier and check the drain flow and for leaks.
Shop RO flow restrictors (from ₹35).
Frequently asked questions
Can I remove the flow restrictor to get more water?
No. Without a restrictor the membrane loses pressure – you get less purified water, much more waste, and higher TDS.
Why is my RO wasting too much water?
A missing, oversized or damaged flow restrictor, or a solenoid valve that stays open, are the common causes.
Does a 100 GPD membrane need a different restrictor?
Usually yes – a larger one than a 75 GPD membrane, commonly around 400–450 ml/min.
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