Low RO System Output
Updated February 20, 2026
RO system filling slowly, weak trickle, or running out of water? Clogged pre-filters, worn membrane, low feed pressure, or waterlogged tank. Most fixes are filter replacements or pressure adjustments.
Overview
RO membranes produce 50-100 gallons per day under ideal conditions -- inherently slow, which is why there is a pressurized tank. When output drops, the most common cause is neglected filters. Pre-filters clog and restrict flow to the membrane -- can cut production in half. Second: worn membrane (2-3 year lifespan). Third: low feed pressure (need 40+ PSI). Fourth: waterlogged tank with a failed bladder.
Symptoms
- Weak, thin stream that takes forever to fill a glass -- tank empty or bladder pressure too low
- System runs constantly (water flowing to drain) but the tank never fills -- worn membrane, failed auto-shutoff, or wrong tank pressure
- Strong flow initially, drops to a trickle after seconds -- tank not holding enough water or bladder pressure too low
- Taste changed or TDS reading higher than usual -- membrane passing contaminants, needs replacement
- Pre-filters discolored, brown, or visibly clogged -- past replacement schedule, restricting flow
- Production dropped in winter -- cold water reduces membrane efficiency. At 40 degrees, production can be half of rated capacity
Common Causes
- Clogged pre-filters -- sediment and carbon filters replaced every 6-12 months. As they clog, flow to the membrane drops. Severely clogged can reduce production to near zero. Sediment filter clogs first, especially on well water.
- Worn membrane -- 2-3 year lifespan. Pores clog with minerals, bacteria, organic matter. Produces less water and rejects fewer contaminants. Neglected pre-filters accelerate membrane fouling dramatically.
- Low feed pressure -- need 40 PSI minimum, 60-80 ideal. Below 40, production drops hard. Failing PRV, partially closed supply valve, or naturally low municipal pressure. Booster pump available for low-pressure situations.
- Waterlogged tank -- rubber bladder pre-charged with air (5-7 PSI empty). Bladder ruptures or loses charge, tank fills but cannot push water out. Strong initial flow that drops to a trickle.
- Failed auto-shutoff valve -- fails closed and restricts flow prematurely, or fails open and the system runs continuously wasting water. Less common but check if other causes are ruled out.
What You'll Need
How to Fix It
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Replace the Pre-Filters
Most common cause first. Feed water off, RO faucet open to depressurize. Towel and bucket under the housings. Filter wrench to unscrew each housing. Note the old filter condition -- brown sediment filter confirms overdue. New filters in correct housings (sediment in stage 1, carbon in 2 and 3). Reseat and hand-tighten. Water on, check leaks. 24 hours to fill the tank, flush the first tank to clear carbon fines.
Tip: Write the install date on each filter with a marker. Sediment: every 6 months. Carbon: every 12 months. High sediment or high chlorine? More frequent. -
Replace the RO Membrane
Pre-filters did not fix it (give 24-48 hours)? Membrane. Water off, depressurize, open the membrane housing. Rolled cylinder that slides in. Pull the old one out (grip and twist if tight). New membrane in with the O-ring end first (toward output). Reseat cap, water on. Discard the first two full tanks before drinking.
Tip: TDS meter before and after. New membrane should reduce TDS by 90-98%. Significantly less? May be installed wrong or there is a bypass letting unfiltered water through. -
Check and Adjust Storage Tank Pressure
Strong flow that drops fast? Tank bladder pressure. Close the tank valve, open the RO faucet to drain completely. Schrader valve at the bottom (same as a bike tire). Tire gauge: should read 5-7 PSI. Low? Bicycle pump to correct pressure. Will not hold air (drops to zero in minutes)? Bladder ruptured, tank needs replacing.
Tip: Always check with the tank completely empty. Water in the tank gives an artificially high reading. Close tank valve, drain at the faucet until empty, then check. -
Verify Feed Water Pressure
Pressure gauge on the feed line or nearby hose bib. Need 40 PSI minimum, 60-80 ideal. Below 40? Check the PRV, make sure the supply valve is fully open, check for restrictions. Chronically low? Booster pump ($50-100) on the feed line. Most manufacturers sell compatible pumps that activate automatically.
Tip: Cold water kills RO output. At 40 degrees, production can be half of rated. If low output is seasonal (worse in winter), a booster pump compensates. -
Check the Auto-Shutoff Valve
System runs constantly even with a full tank? ASO valve, located between membrane housing and tank. Uses tank back-pressure to stop flow. Test: close the tank valve, let the system run. Should stop draining within 1-2 minutes. Keeps running? ASO failed, needs replacement ($15-30).
Tip: Continuously running system wastes a lot of water to the drain. If you hear it running all the time, check the ASO promptly. -
Sanitize the System (Annual Maintenance)
Over a year without sanitizing? Bacteria colonize housings, tubing, and tank, reducing performance and affecting taste. Remove all filters and membrane. Sanitizer or 2-3 tablespoons unscented bleach in the first housing. Reinstall empty housings, tank valve open, feed water on. Fill until you smell chlorine at the faucet, close feed, sit 30 minutes. Drain completely. New filters and membrane in, flush two tanks.
Tip: Essential after extended downtime (vacation, seasonal home). Stagnant water develops bacterial growth fast. Always sanitize before putting the system back in service.
When to Call a Pro
Call a water treatment specialist if you are unsure about membrane or filter replacement on your system, if performance is still poor after filters, membrane, and tank pressure check, if you need a booster pump installed, if housings are leaking after filter changes (O-rings may need replacing), or if you want a professional water quality test.
Prevention Tips
- Pre-filters every 6-12 months. Single most important maintenance. Protects the membrane and maintains production.
- Membrane every 2-3 years, sooner if TDS rises. TDS meter ($15-20) makes monitoring easy.
- Tank pressure annually. Slowly deflating bladder reduces output so gradually you may not notice.
- Sanitize the system once a year. Prevents bacterial colonization in housings, tubing, and tank.
- Monitor feed pressure periodically. Low pressure reduces production more than any other factor.
- Very high sediment? Whole-house sediment filter upstream extends the life of RO pre-filters.