Reverse Osmosis System Not Working — Low Pressure, Membrane Fouling, Filter Replacement, Tank Bladder Failure & TDS Creep

Common Symptoms

• RO faucet produces no water or only a very slow trickle (less than 1 cup per 2–3 minutes)
• Storage tank feels empty (very light) even after running the system for 8–12 hours
• Water from the RO faucet tastes and tests the same as tap water (TDS creep — no reduction in dissolved solids)
• RO system drain line runs continuously — water draining nonstop even when the tank should be full
• Filters or membrane have not been replaced in over 12 months (pre-filters/post-filter) or 2–3 years (membrane)
• Storage tank feels full and heavy but water flow from the RO faucet is still very slow
• Measured TDS of RO water is within 50–100 mg/L of tap water TDS (normal RO rejection is 90–99%)
• Air pressure tested in storage tank reads 0 PSI — tank bladder has failed

Most Likely Causes

1. 1

   Clogged Pre-Filters (Sediment and Carbon Block Stages 1–2)

   Standard five-stage RO systems have two or three pre-filter stages before the RO membrane: a 5-micron sediment filter (Stage 1) that traps particulate matter, a granular activated carbon (GAC) filter (Stage 2) that removes chlorine and chloramines, and often a carbon block filter (Stage 3) that provides fine chlorine removal. These pre-filters are designed to protect the delicate RO membrane. When they become saturated — typically every 6–12 months depending on source water quality — incoming water pressure drops across the blocked filters, reducing the pressure available to drive water through the RO membrane. A clogged sediment filter alone can drop feed pressure from 65 PSI to 30 PSI — below the minimum 40–45 PSI required for adequate RO production. Replace all pre-filters annually as a complete set rather than individually.

2. 2

   RO Membrane Fouling or Chlorine Damage (TDS Creep, Near-Zero Production)

   The RO membrane (typically a thin-film composite polyamide membrane, 75 GPD or 100 GPD residential size) is the heart of the system and the most expensive component to replace ($30–$80). RO membranes fail in two distinct ways: (1) Fouling — mineral scale (calcium carbonate, silica), biological fouling (iron bacteria, sulfur bacteria), or organic fouling (humics, tannins) forms a layer on the membrane surface that reduces water flux through the membrane. Fouled membranes produce very little water (near zero GPD output) while TDS rejection may still be acceptable. (2) Chemical damage — chlorine and chloramines, even at normal municipal water treatment concentrations, will destroy a polyamide RO membrane within days to months if the carbon pre-filter has failed or been bypassed. A chlorine-damaged membrane passes water readily but provides little or no TDS rejection — TDS of product water matches tap water TDS.

3. 3

   Insufficient Feed Water Pressure (Under 40 PSI at the RO Unit Inlet)

   Reverse osmosis is a pressure-driven process — residential RO membranes require a minimum feed pressure of 40–45 PSI to produce water at the rated GPD output, and they produce water proportionally to the pressure above this threshold. Houses with low water pressure (older homes with 1/2" galvanized supply pipes, homes on well systems with pressure tank issues, or homes at the end of a long distribution main) may have line pressure below 40 PSI. Also, the RO system is typically connected via a small-diameter saddle-clamp fitting on the cold water supply under the sink — these fittings can partially close or become kinked, reducing pressure at the RO inlet even when line pressure is adequate. Measure pressure with a gauge at the RO unit's supply line, not at the main pressure gauge, to identify a localized restriction.

4. 4

   Storage Tank Bladder Failure (Zero Tank Pressure — System Runs Continuously)

   The RO storage tank contains a rubber bladder that is pre-charged with air pressure (typically 7–8 PSI from the factory). As the bladder fills with RO product water, the bladder compresses the air charge, and it is this rising air pressure that delivers water under pressure when the RO faucet is opened. If the rubber bladder fails (tears, perforates, or separates from the tank wall), the air charge is lost (tank pressure drops to 0 PSI) and the tank can no longer deliver water through the RO faucet at usable flow rates — the faucet runs barely at a trickle because only gravity pressure is available. Test tank pressure: disconnect the supply tube from the tank, open a faucet to drain the tank fully, and press the Schrader valve on the bottom of the tank. A bladder-failed tank will spray water from the Schrader valve instead of air.

5. 5

   TDS Creep — Failed Membrane Bypass Check Valve or Blending Valve

   TDS creep is the gradual increase in RO product water TDS over time, eventually reaching near-tap-water levels, without a corresponding drop in water production volume. It has three causes: (1) A failed or absent check valve between the membrane and the post-filter allows concentrated reject water (brine) to backflow through the membrane during off cycles, contaminating the product water storage. (2) Some RO systems include a blending or remineralization stage that adds back a controlled amount of minerals — if this valve fails open, it adds tap water directly to the product water line. (3) The RO membrane has reached end of life and its TDS rejection has declined from 97–99% to below 50% — often without any change in water production volume, making this the most insidious failure mode.

6. 6

   Automatic Shut-Off (ASO) Valve Failure — Continuous Drain or No Refill

   The ASO valve is a four-port pressure-actuated valve that stops RO production when the storage tank reaches approximately 2/3 of line pressure. It fails in two ways: (1) Failed closed — the ASO valve closes and never reopens, so no water reaches the membrane and no product water is produced. Diagnosis: disconnect the tank pressure tube from the ASO valve — if water immediately flows through the system, the ASO valve's tank port was keeping it closed. (2) Failed open — the ASO valve never closes when the tank is full, so the RO system runs continuously and wastes water down the drain. The ASO valve is typically a $10–$25 replacement part and is one of the more commonly replaced RO components after the pre-filters and membrane.

Quick DIY Checks

1. 1Step 1 — Replace all pre-filters and post-filter (the first and most important maintenance step): If the pre-filters and post-carbon filter have not been replaced in the past 12 months (or 6 months in high-sediment or high-chlorine water), replace them first before any other diagnosis. A severely clogged sediment filter can drop the pressure reaching the membrane by 30–40 PSI — enough to reduce water production to near zero without any membrane or tank issue. Standard residential RO filter sets (Stage 1 sediment, Stage 2 GAC, Stage 3 carbon block) cost $20–$40 for all three and install in 10 minutes with the system pressure relief button. After filter replacement, run the system for 4–6 hours to allow the storage tank to refill, then test water flow and TDS. A surprising number of 'dead RO system' calls are resolved entirely by filter replacement.
2. 2Step 2 — Measure feed water pressure at the RO unit inlet: Purchase or borrow a 0–100 PSI water pressure gauge with a 1/4" NPT male fitting (available at hardware stores, $10–$20). Locate the 1/4" compression tubing that connects the cold water supply valve (the small blue or white saddle clamp valve under the sink) to the RO system inlet port. Disconnect this tubing at the RO unit, attach the pressure gauge via a compression fitting adapter, and open the cold water supply valve. Read the pressure with no demand on the system (static pressure): it should be 50–80 PSI for a municipal supply. Also open the RO faucet and read the pressure while the system is producing water (dynamic pressure): it should not drop below 40 PSI. If static pressure is below 45 PSI, install a RO booster pump ($60–$120) to raise pressure to 65–80 PSI. If static pressure is fine but dynamic pressure drops sharply, the supply saddle valve or inlet tubing is partially blocked.
3. 3Step 3 — Test TDS before and after the membrane using a TDS meter: A TDS (total dissolved solids) meter ($10–$25) is essential for RO system diagnosis. Test in sequence: (A) Tap water TDS from the cold water supply under the sink — this is your baseline. (B) RO faucet TDS — this is the membrane output. Normal RO rejection: product TDS should be 90–99% lower than feed TDS. Example: if tap water is 400 mg/L, RO product should be 4–40 mg/L. If RO product TDS is over 100 mg/L on 400 mg/L feed water, TDS rejection is below 75% and the membrane requires replacement. (C) If your system has a post-remineralization stage, test at a point before it as well. Also test immediately after the system's pressure relief button is pressed to purge the membrane housing — stagnant water in the housing can show elevated TDS from concentration polarization, while fresh product water will test lower.

1. 4Step 4 — Test storage tank air pressure and diagnose bladder failure: Empty the storage tank by opening the RO faucet until flow stops completely. Close the cold water supply valve to the RO system. Use a standard tire pressure gauge and press it on the Schrader valve (air valve) on the bottom or side of the storage tank. A healthy bladder tank should register 7–8 PSI air pressure when completely empty of water. If the gauge reads 0–2 PSI, either: (A) The air charge has leaked down over time — attempt to re-pressurize to 7–8 PSI using a bicycle pump or tire pump, then reopen the supply and allow the tank to refill. If the re-pressurized tank holds air and the system refills and delivers good flow from the faucet, a simple air charge top-off was all that was needed. (B) If the gauge reads 0 PSI and water squirts from the Schrader valve (instead of air), the bladder has failed and the tank must be replaced — a bladder-failed tank cannot be repaired. Tank replacement cost: $30–$80 for a standard 3.2-gallon tank.
2. 5Step 5 — Diagnose and replace the ASO (automatic shut-off) valve if drain runs continuously: Locate the ASO valve in your RO system — it is a small plastic four-port block (typically 1" × 1.5") connected to: the RO membrane concentrate port (drain), the product water line from the membrane, the storage tank pressure tube, and the drain saddle. With the system running normally and the tank confirmed partially full, feel the drain flow restrictor output line — if water is draining continuously (not just intermittently during production), the ASO valve is not closing. Test: use your finger to lightly press closed the product water output tube from the ASO valve — if drain flow stops immediately, the ASO valve's normally-open drain path is stuck open. Replacement ASO valves are standard 1/4" push-connect fittings ($10–$25 at RO parts suppliers). Unplug all four tubes (note orientation — label each port), remove the old ASO valve, and install the new one with tubes reconnected to the corresponding ports. Run the system through a complete fill cycle and confirm the drain stops when the tank is full (approximately 2/3 of supply pressure).
3. 6Step 6 — Replace the RO membrane if TDS rejection is below 85%: Before replacing the membrane, confirm pre-filters are fresh (Step 1) and feed pressure is adequate (Step 2) — a membrane cannot be fairly evaluated if either pre-condition is not met. To replace the membrane: close the cold water supply valve, open the RO faucet to depressurize the system, and locate the RO membrane housing (the tallest housing in the filter assembly, typically 12" tall by 2.5" diameter). Unthread the housing end cap with a filter housing wrench or strap wrench — counterclockwise when facing the end cap. The membrane slides out from the housing — it is a cylindrical element approximately 11" long by 1.8" diameter. Note the orientation (inlet end vs. outlet end — most membranes are marked). Install the new membrane (push firmly until seated — the membrane is a friction fit), reinstall the end cap, reopen the supply, and discard the first full tank of product water (flush cycle) before testing TDS and consuming the water. The membrane requires 2–3 tank refill cycles to reach full rejection efficiency.
4. 7Step 7 — Check the check valve between the membrane and product water storage: If TDS is elevated but the membrane tests within acceptable rejection limits (tested with fresh feed water), a failed check valve is allowing brine water to backflow through the membrane during off cycles and contaminate the product water in the storage tank. The check valve is a small one-way valve (usually a 1/4" push-connect inline valve) installed in the product water line between the membrane housing outlet and the post-filter. Push-connect check valves are inexpensive ($3–$8) and directional — ensure the arrow on the valve body points toward the storage tank (away from the membrane). Replace the check valve and re-test TDS after the system runs two complete fill cycles to flush the contaminated water from the storage tank and post-filter.
5. 8Step 8 — Measure water production rate to confirm system performance after service: After completing filter and membrane replacement, re-pressurizing the storage tank, and verifying the ASO valve is cycling correctly, confirm the system is producing water at the rated GPD. Drain the storage tank completely. Open the cold water supply. Time how long it takes to produce one cup (8 oz) of water at the RO faucet while the tank is empty (gravity feed from the membrane directly to the faucet, bypassing the tank). One cup in 60 minutes = 0.13 GPD — far below rated output, indicating a membrane or pressure issue still present. One cup in 3–5 minutes = approximately 75 GPD — normal for a 75 GPD membrane at 65 PSI feed pressure. If production rate is significantly below rated, and filters are fresh and pressure is adequate, the membrane is the bottleneck — replace it if it has been over 2 years since last replacement.

Repair vs Replace

Recommended Tools & Parts

• RO System Pre-Filter Replacement Set — Sediment + GAC + Carbon Block (1, 2, 3 Stage)

  Annual replacement filter set for standard under-sink RO systems. Includes 5-micron PP sediment filter (Stage 1), GAC coconut-shell carbon filter (Stage 2), and CTO carbon block filter (Stage 3). Fits most major brands including APEC, iSpring, Watts, Pentair, and Home Master with standard 10" × 2.5" housings. Replace every 6–12 months depending on water quality.

  $20–$40

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• Filmtec TW30-1812-75 Reverse Osmosis Membrane — 75 GPD

  Industry-standard Filmtec (Dow) TW30 thin-film composite RO membrane, 75 GPD at 65 PSI feed pressure. 97–98% TDS rejection. Fits standard 1812 membrane housings used by APEC, iSpring, Watts Premier, Express Water, and most residential under-sink RO systems. Replace every 2–3 years. Higher 100 GPD version also available for faster fill rate.

  $25–$60

  Buy on Amazon →

• RO Storage Tank — 3.2 Gallon (Standard Under-Sink Size)

  Replacement 3.2-gallon pressurized storage tank for residential under-sink RO systems. Steel tank with food-grade butyl rubber bladder, pre-charged to 7–8 PSI. Fits all systems with standard 1/4" tank ball valve. Replace when tank bladder fails (water comes from Schrader valve) or tank holds less than 1 gallon before pressure drops and flow stops.

  $30–$60

  Buy on Amazon →

• Automatic Shut-Off (ASO) Valve — 1/4" Push Connect RO Systems

  Replacement automatic shut-off valve for 1/4" tubing RO systems. Stops membrane production when storage tank is full; allows refill when tank pressure drops. Replace when drain runs continuously (ASO stuck open) or system never produces water (ASO stuck closed). Universal fit for APEC, iSpring, and most standard 5-stage RO systems.

  $8–$20

  Buy on Amazon →

• RO Booster Pump — 24V DC Permeate Pump / Pressure Pump

  24V DC booster pump for reverse osmosis systems with low feed water pressure (under 45 PSI). Raises feed pressure to 65–80 PSI to restore rated membrane production rate. Compatible with most standard under-sink RO systems. Installs on the cold water supply line before the RO inlet. Requires 24V DC power adapter (usually included).

  $50–$120

  Buy on Amazon →

• Inline Check Valve — 1/4" Push Connect (Product Water Line)

  Replacement 1/4" push-connect inline check valve for the RO product water line between the membrane housing and storage tank. Prevents brine water backflow through the membrane during off cycles, which causes TDS creep. Replace when TDS is elevated despite a healthy membrane. Directional — install with the flow arrow pointing toward the storage tank.

  $3–$10

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Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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