RO Low Production - Pressure vs Membrane vs Prefilter Decision Tree

Why this matters

"My RO is slow" is a five-minute call if you know the order to check, and a two-hour call if you do not. Membranes are the most expensive consumable and the most over-replaced part on a residential RO. Most low-production calls are not the membrane - they are prefilter clog, low feed pressure, tank air pressure, or a stuck check valve. Replacing a membrane on a fault that was an empty tank bladder buys a callback. The tree below sequences the diagnosis in the order of field probability so the call closes with the right replacement.

Symptom presentation

Pattern A: faucet flow at the RO tap is trickle compared to last month. Pattern B: tank fills slowly or never fully. Pattern C: TDS at the RO tap is high (180-plus ppm where it used to be 20). Pattern D: tap flows initially then stops fast - tank not holding. Pattern E: RO produces but constantly drains to the brine line (high reject ratio).

Quick checks - 12 minute pass

1. Confirm city water pressure at a sink near the RO. Below 40 psi feed pressure, an unpressurized residential RO will struggle. Below 30 psi it will barely produce.
2. Confirm the type of RO. Standard tank-style (most resi installs), tankless permeate-pump-driven, or commercial multi-stage. Each has different troubleshooting steps.
3. Confirm tank air pressure with an empty tank. Disconnect tank line, drain to faucet until faucet stops dripping, schrader check at the tank: 5-7 psi is the standard pre-charge.
4. TDS test at three points: feed, post-membrane permeate (before tank), tap. A jump from feed to permeate is a membrane rejection number; a jump from permeate to tap is a tank or post-carbon problem.
5. Check filter change history. Customer's last filter change date and what was changed.

Isolation tree

Step 1 - Tank pre-charge (Pattern A, B, D)

A tank with a flat bladder cannot push water out the faucet. Empty the tank, schrader test:

• 0-2 psi: bladder failed (replace tank) or pre-charge bled through schrader. Re-pressurize to 7 psi, retest after 1 week.
• 2-7 psi: low. Re-pressurize to 7-8 psi.
• 7-10 psi: normal, move on.
• Above 10 psi: over-pressurized, will give weak flow as bladder cannot displace fully. Bleed to 7 psi.

A waterlogged tank (bladder failed) feels heavy as a brick when shaken empty, sounds dead. Replace.

Step 2 - Prefilter check (Pattern A, B)

Most resi ROs have 1-3 prefilters: sediment 5 micron, carbon block, sometimes a second sediment. Field-typical life is 6-12 months. Pull, hold up to light:

• Brown/black sediment cart: at end of life. Replace.
• Original-white sediment cart: still good.
• Compressed pleat (sucked into cart center): paper-style cart failed under pressure. Replace.

A heavily clogged sediment filter starves the membrane and the diagnosis can look like membrane failure. Replace prefilters first, retest after 24 hours of tank refill.

Step 3 - Membrane rejection test (Pattern C)

Test TDS feed and TDS permeate (drilled tee or sample line at the membrane housing output, before tank). Calculate rejection percent:

• Rejection = ((TDS feed - TDS permeate) / TDS feed) x 100.
• Healthy membrane: 90-99 percent rejection.
• Failing membrane: 70-85 percent rejection.
• Bypassed or holed membrane: below 50 percent.

A membrane failing rejection but still producing flow is a membrane swap. A membrane refusing to produce but with no holes is fouled and may be salvageable with citric-acid clean for commercial systems; for resi the cost-effective answer is replace.

Step 4 - Auto-shut-off (ASO) and check-valve check (Pattern B, D)

The ASO valve closes the brine drain when the tank reaches approximately 2/3 feed pressure. A failed ASO either:

• Stuck closed: RO never produces, no drain flow, no permeate flow. Test by disconnecting drain line and listening for brine flow during a tank refill.
• Stuck open: RO drains constantly even with the tank full. High utility bills and slow refill.

The check valve between membrane and tank prevents back-flow from tank into the brine. Failed open allows brine to siphon back into the tank, causing high TDS at the tap and a slow refill. Test by isolating the tank and checking for back-flow.

Step 5 - Flow restrictor (Pattern E)

The brine flow restrictor sets the recovery ratio. A failed-open restrictor sends all the water to drain and the membrane never makes permeate. A failed-closed restrictor backs pressure up and over-stresses the membrane. Match restrictor GPD to membrane GPD per manufacturer chart (50 GPD membrane plus 400 ml/min restrictor is common).

Step 6 - Post-carbon TDS climb (Pattern C with healthy membrane)

If feed-to-permeate rejection is fine but tap TDS is high, the post-carbon filter is leaching. A spent or never-changed post-carbon can release TOC and bump tap TDS. Replace.

Step 7 - Long stagnation (Pattern A after vacation)

A system that sits 2-plus weeks unused develops biofilm in the tank and lines. Tap flow may be normal but quality drops. Drain tank fully, sanitize with manufacturer-approved sanitizer, refill 3x to flush. Replace post-carbon if biofilm odor persists.

Confirming the diagnosis

TDS meter calibrated against a known standard (342 ppm NaCl reference). Pressure gauge on the inlet. Schrader gauge on the tank. Stopwatch on the brine line flow during refill (compare to membrane spec). Documented rejection calc.

Remediation

• Tank pre-charge wrong: re-pressurize to 7 psi.
• Tank bladder failed: replace tank.
• Prefilters spent: replace as a set, document date on the housing.
• Membrane rejection failed: replace membrane, replace prefilters at the same time so new membrane is not loaded by old prefilter debris.
• ASO failed: replace.
• Check valve failed: replace.
• Flow restrictor wrong size: match to membrane GPD spec.
• Post-carbon spent: replace.
• Biofilm in stagnant system: sanitize per manufacturer.

Reset the customer's calendar reminder for prefilter change after the call. Annual prefilter change plus 3-5-year membrane plus annual sanitize is the maintenance cadence that keeps an RO out of trouble.

References

• NSF/ANSI 58 Reverse Osmosis Drinking Water Treatment Systems.
• NSF/ANSI 42 Drinking Water Treatment Units, aesthetic effects.
• WQA Reverse Osmosis Operation and Maintenance technical bulletin.
• Dow FilmTec Residential Membrane Element data sheet, rejection and flow specs.
• AWWA M46 Reverse Osmosis and Nanofiltration manual.