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Basic Troubleshooting Guide for Watermakers

Your Watermaker is not making good product water anymore. What do you do?

If your water maker (or desalinator) is no longer making good water, there are several possible causes. I will try to break it down into logical troubleshooting steps for you, so you don’t spend a lot of money replacing parts or membranes hunting for the solution.

Every desalinator needs to have an operating log of some sort. You don’t have to record every time you use it, but it best practice to document the operating conditions when newly installed, at every seasonal startup, after cleaning chemicals are used, and whenever major components are replaced. This information is critical to understanding whether your system is performing as it should.

First of all, every seawater desalinator needs the following in order to function:

  • Filtered feed water, supplied under pressure
  • High-pressure pump
  • Reverse Osmosis (RO) membrane in a pressure vessel
  • Pressure regulator (back-pressure type or needle valve)
  • Pressure gauge to measure RO system pressure

Optional features that make operation and troubleshooting easier:

  • Flow meter for product water
  • Flow meter for feed water or brine (I prefer to measure feed water)
  • Pressure/vacuum gauge to measure feed water pressure
  • TDS meter to measure product water dissolved solids (mostly salt)
  • A method to separate the product flow in multi-membrane systems
  • Helpful, but rarely seen, is a pressure gauge on the feed inlet side of the pressure vessels
  • Thermometer to measure feed water temperature
  • Refractometer or high-concentration TDS meter to measure salinity of feed water

Check Your Calibrations

Before you assume that something is seriously wrong with your reverse osmosis system, make sure you check the calibration of your TDS meter, and inspect all pressure gauges and flow meters.

As the battery in a TDS meter fails, or the electrodes become fouled, the readings can change dramatically. Make sure the electrodes are clean, and the battery has plenty of juice in it. Borrow a handheld TDS meter from a neighbor and compare the reading you get with their’s.

Inspect your pressure gauges for damage or bent pointers. Has the damping fluid drained out? Does the pointer move when you tap the gauge? Is there any leakage or rust around the fittings?

Inspect your flow meters for fouling and damage. A dirty flow meter will not be accurate. Most can be taken apart and cleaned. Be careful not to scratch the inside of the float chamber or bend the guide rod. Note that all flow meters that use a “floating” ball or slug need to be installed vertically, and they are read at the widest point of the float.

Check Your Product Water from Each Pressure Vessel

If you have more than one pressure vessel in your system, check the product water flow rate and TDS from each vessel. Typically, the first vessel in the set will yield more product flow at lower TDS than then next vessel. This is because the feed water for the second vessel is brine from the first vessel – lower flow and higher salinity.

Check Your Strainer, Pre-Filters and Boost Pump

High-pressure pumps require the feed water to be supplied under pressure to prevent cavitation. Make sure your sea strainer (you do have a dedicated thru-hull and strainer for your watermaker, right?) is clear, and the sediment filters are clean enough to allow good flow to the high-pressure pump.  Make sure your boost pump is working properly. This is why we strongly recommend a pressure/vacuum gauge between the last pre-filter and the high-pressure pump. Don’t make your high-pressure pump “suck” the feed water, or it will soon “suck” at making pressure!

Check Your Feed (Source) Water

Feed Water Temperature

Before you send an email or make a phone call, you need to know what your feed water temperature and salinity are. A simple thermometer will tell you the feed water temperature.

Effect of Feed Water Temperature on Product Water:

  • Increasing Temperature:
    • Higher TDS
    • Higher product flow rate
  • Decreasing Temperature:
    • Lower TDS
    • Lower product flow rate

Feed water temperature can make a significant  impact on your product water flow rate. Reverse osmosis systems are rated based on feed water conditions of 32,000 ppm, 77° F, and operating at 800 psi. At 90° F, you can expect to be making approximately 25% more water, and at 48° F, you should expect 50% LESS!

Feed Water Salinity

Measuring the salinity of the feed water is quite simple with the right tool. The Boundless Outfitters Yacht Services department uses a refractometer to measure feed water salinity. A refractometer is a  simple-to-use device that uses light to measure the salinity. You just dip the end into the feed water source and hold it to the light. Look into the viewer and read the salinity directly. The reading will not be as precise as a high-concentration digital TDS meter, but at one tenth the cost, it is close enough for our purposes.

Effect of Feed Water Salinity on Product Water:

  • Increasing Salinity:
    • Higher TDS
    • Lower product flow rate
  • Decreasing Salinity:
    • Lower TDS
    • Higher product flow rate

Feed water salinity can make a significant impact on your product water flow rate. At 35,000 ppm, you can expect to be making approximately 10% less water, and at 25,000 ppm, you should expect 25% MORE!

If you are cruising and don’t have access to a refractometer or high-concentration TDS meter, you can still get an approximate value for feed water TDS. It just requires a some simple math, and some careful measuring. You do need to have a handheld TDS meter for this.

Carefully measure 1/2 cup of feed water. Use a good measuring cup – the Pyrex types used for cooking are not very good, as the thick glass and printed markings make for inaccurate measurements. Molded plastic measuring cups have the markings molded in, so they are more accurate than the glass ones.

In a clean container (a 6 quart pan works well) add the 1/2 cup of feed water to one gallon of distilled water. Mix it well and let it sit for about five minutes. Then measure the salinity with your handheld TDS meter. Multiply the reading by 33 to get the approximate salinity of the feed water.

Check Your High-Pressure Pump

There are components inside your high-pressure pump that are susceptible to wear and tear. Valves and seals are critical elements for ensuring consistent flow and pressure. You should keep a manifold rebuild kit in your spare parts locker. Make sure there pump oil is at the proper level.

Your high-pressure pump should move the same amount of water regardless of the pressure it is producing, as long as it is within its design limits. Measure the feed flow rate with the system pressure at Zero (open your bypass valve, or back off the regulator fully). A feed or brine flow meter is great for this, but if you don’t have one,  you can use the empty jug from the distilled water you just used for the last test. You will also need a stopwatch. Measure the time it takes to fill the jug from the overboard (brine) discharge of your watermaker.

Now, increase the pressure of your system to 800 psi (or whatever pressure is appropriate for your conditions). Measure the feed flow rate again. This is why I prefer to have a flow meter measuring feed water before it gets to the high-pressure pump. You just read it directly. This meter should never vary during operation. If you have a flow meter measuring brine discharge, you need to add this flow rate to the product water flow rate to get the total feed flow. I have seen many systems with brine flow meters, and most use different units from the product flow meters. Make sure you convert to consistent units before you add the product flow to the brine flow.

If you don’t have a flow meter for feed or brine, you will need to get your jug and stopwatch out. Make sure the product water is being rejected back into the brine stream so you measure total flow.

If there is any difference between the feed flow rate at Zero pressure and the total flow rate at operating pressure, you will need to carefully inspect the high-pressure pump, motor and wiring, and repair as needed. Some small DC-powered watermakers may run slightly slower under load due to resistance in the wiring, and battery draw-down. Please keep this in mind while troubleshooting.

Check the Voltage at the High Pressure Pump

Over time, vibration and salt air can cause increased resistance in electrical connections. The results are reduced voltage for equipment, and temperature rise in electrical components. A further complication for motor circuits is that the current draw increases as well. This is because most electric motors use a constant amount of power (Watts). Since power consumption is the product of voltage and current (P = V x A), if the voltage goes down, the current goes up. Current is what causes heat to build up in areas of high resistance.

Ideally, the voltage measurement at the connections nearest the high-pressure pump motor should be at least 97% of the voltage at the power source in all running conditions. Worst case, the motor voltage should be no less than 90% of the source voltage. Reduced voltage causes motors to run slower, which reduces the flow rate of your high-pressure pump.

Effect of Feed Water Flow Rate on Product Water:

  • Increasing Flow Rate:
    • Lower TDS
    • Higher product flow rate
  • Decreasing Flow Rate:
    • Higher TDS
    • Lower product flow rate

Check Your Pressure Regulator

If your filters, pumps and all of your measuring equipment is in good working order, you can check your pressure regulator. Does the system pressure vary? Some systems use a “set it and forget it” pressure regulator arrangement. While running your system at operating pressure, adjust the regulator up and down (do not exceed 950 psi!), and watch the pressure gauge. Does it increase and decrease smoothly? Can you set it to any pressure and have it hold steady? Most regulators can be taken apart and inspected/cleaned.

Effect of Feed Pressure on Product Water:

  • Increasing Pressure:
    • Lower TDS
    • Higher product flow rate
  • Decreasing Pressure:
    • Higher TDS
    • Lower product flow rate

Check Your Pressure Vessels and Membrane Elements

Make sure all the plumbing is in good condition and free of leaks. A cracked or corroded end cap in your pressure vessel can cause leaks, and on rare occasions, brine contamination of the product water.

If you are having trouble with a new membrane, open up the pressure vessel and check the installation.

  • Verify that the brine seal is installed correctly
    • The brine seal prevents the feed water from bypassing the membrane
    • The sharp edge of the brine seal faces upstream (toward the feed inlet)
    • There should only be one brine seal
    • The brine seal can be at either end of the pressure vessel as long as it is facing the correct direction
  • Inspect the membrane product tubes for scratches or nicks
  • Inspect the o-rings, and replace as needed
  • Use proper o-ring lubricant for assembly

If the brine seal is not oriented correctly, feed water can bypass the membrane. This will reduce product flow rate and increase TDS.

Be very careful when changing the product water o-rings (the ones on the inside of the end plug, where the membrane product tube enters). If you scratch or nick the grove where the o-ring sits, you can create a path for brine to contaminate the product water.

Effects of Failures Inside Pressure Vessel:

  • End Plug Crack or Corrosion
    • Higher TDS
    • Higher product flow rate
    • Leaks
  • Brine Seal Damaged or Installed Backwards
    • Higher TDS
    • Lower product flow rate
  • Product Water O-ring or O-ring Groove Damage
    • Higher TDS
    • Higher product flow rate
  • Membrane Product Tube Damage
    • Higher TDS
    • Higher product flow rate
  • Membrane Failure
    • Higher TDS
    • Higher product flow rate


Reverse Osmosis Watermakers are complicated systems that require careful maintenance. Many cruising boaters are quick to blame the membranes if the product water flow is reduced, or TDS increases. As you can see from the above troubleshooting tips, there are several possible causes for reduced (or increased) product flow and increased TDS.

Happy Cruising!
Tim Allen