Symptom: output has fallen, but the system runs normally
This is the most common complaint and it is almost never the membrane. In rough order of likelihood:
- Blocked prefilter. Cheapest, most likely, and it looks exactly like a dying membrane. Change it before anything else.
- Cold water. Output falls significantly as seawater temperature drops. A system rated in the tropics will make noticeably less in southern waters, with nothing wrong.
- Low feed pressure or flow. A tired feed pump, a partially closed seacock, a fouled intake strainer or a kinked hose.
- Air in the feed. Often shows as unsteady pressure and noise. Usually an intake placed where it can draw air, or a leaking fitting on the suction side.
- Operating pressure set low. On systems with a manual pressure valve, check it is where it should be before assuming a fault.
- Genuinely fouled or aged membrane. Last on the list, and often cleanable rather than replaceable.
Symptom: no pressure, or pressure will not build
Look at the feed side and the pressure-regulating side. A pump that cannot get enough water cannot build pressure: check the seacock, strainer, prefilter and every fitting on the suction side for air leaks. If feed is healthy, the pressure regulator or its valve is the next candidate. On energy-recovery systems the intensifier itself can stall if feed flow is inadequate.
A pump that is noisy, pulsing or vibrating unusually while failing to build pressure is telling you something. Stop and investigate rather than running it.
Symptom: product water tastes salty or reads high TDS
This is the one to take seriously, because it is about what you drink. Possible causes:
- Not yet up to pressure. Product quality is poor for the first minutes of a run, which is why systems divert to test before the tank. If you are sampling too early, that is normal.
- Operating pressure too low. Rejection depends on pressure; below the design point, salt passage rises.
- Warm water. Salt passage increases with temperature, so very warm tropical water gives slightly higher TDS than cold at the same pressure.
- A damaged membrane or a failed O-ring or brine seal. A bypass path lets feed water past the membrane. This is a real fault and needs attention.
- Residual preservation chemical after a lay-up. Flush properly per the manual before tanking anything.
Symptom: it will not start, or trips out
On a DC system, suspect voltage before anything else. Undersized cable, a poor crimp, a tired battery or a long run to the machinery space can all drop the voltage at the pump enough to cause low-voltage cut-outs or poor performance — and it will be worse when the bank is low, which is exactly when you want to make water.
On an AC system running from an inverter, check the inverter is genuinely sized for the surge, not just the running load. On petrol units, the problem is a small engine problem: fuel, spark, air.
When to call someone
Call a specialist when the fault is inside the high-pressure side — the pump, the intensifier, the membrane housings — or when product-water quality is wrong and you have eliminated pressure and running-in. Those are the areas where guessing is expensive and where a technician who knows your specific system is worth their travel fee.
Have the model, the symptom, your feed and operating pressures and your TDS reading ready before you ring. It shortens the conversation enormously and often produces an answer over the phone.