Water Maker Installation Considerations

Purchasing a water maker for your boat is the easy part. Installing the water making equipment on a boat can be an expensive and frustrating task.

This article strives to take some of the mystery out of the decisions related to the installation of a water making system. Specifically, this article looks at reverse osmosis systems. 

No matter what the hourly production rate or manufacturer, there are going to be a few basic components and tasks associated with every water maker installation currently sold today. Some of the current offerings combine several of the components into a common unit with peripheral components that connect to that common unit. Others are simply individual components that you mount individually then connect accordingly.

The basic components include:

  • RO membranes
  • Raw water source considerations
  • Feed Pump
  • High Pressure Pump
  • Filters
  • Electric power source
  • Piping or plumbing
  • Control system or methods

We will look at each component individually.

RO Membranes:

On most boats and smaller yachts the RO membranes will be 2.5 inches to 4.5 inches in diameter and have a length of 14 inches, 21 inches or 40 inches. Most membranes seen on boats and yachts under 100 feet will be 2.5 inches in diameter but the housings will be larger to accommodate the membranes. Typically a housing that accommodates a 2.5 inch membrane will be 3.25 inches to 3.5 inches on the outside diameter.

When calculating the total width of the housings in systems with multiple housings, add an additional inch per housing to allow for installation and servicing.

The chart below shows minimum space requirements for each of the common housing lengths that accommodate 2.5 inches membranes.

14 inch membranes minimum total length 19 – 21 inches
21 inch membranes minimum total length 26 – 28 inches
40 inch membranes minimum total length 45 – 47 inches

Choosing a location for the membranes/housings involves 4 factors

  1. How may housings/membranes are included with your system
  2. Access to the housing ends to facilitate connecting the piping
  3. Space for troubleshooting leaks should they occur
  4. Will membrane replacement will take place with the housing(s) mounted or are you going to remove the housing to change membranes

In reality the membranes in a system that is properly used and maintained should last 10 years, but should you have problems and when it comes time to replace them you will be glad you thought about servicing when you installed the system.

The 40 inch membranes can be difficult to service if the housing is not easy to remove from its mounting location. Many buyers now opt for the 21 inch membranes over the 40 inch units because of the amount of space required for mounting and the difficulty in servicing.

Some systems now come with specialized mounting brackets that make housing/membrane removal easy. The clamp style housing mounts that came with older systems are more difficult to install and service. The newer snap-in or slide-in rack systems make installation and service mush easier.

Raw Water Source:

The raw water pick up point should be below the water line and at least 5 feet from the engine exhaust. A thru-hull should be chosen that has a strainer or that can have a strainer added. Sometimes the thru-hull used for the air conditioning cooling water is used, but keep in mind if you make water while running your air conditioning, one or the other may be starved depending upon the diameter of that thru-hull.

Most water makers will require 2 – 4 gallons per minute flow rate from the thru-hull so a ½ inch thru-hull will most likely NOT accommodate your air conditioner and water maker at the same time. 

If you plan to make water while the vessel is moving, the water flowing over the hull can create capillary action which will limit the feed pump’s ability to pull sufficient water into the water maker to operate correctly. We do not recommend trying to make water while moving as the likely hood of picking up debris increases as does the possibility of system cavitation. We recommend a ¾ - 1 inch thru-hull and a ½ - ¾ inch inside diameter tube or hose from the
thru-hull to the water maker input point.

Feed Pump:

The feed pump pulls water from the raw water source and pushes it into the system. Whether the feed pump is before or after the sediment filters, the feed pump is there to make sure the high pressure pump never starves for water. Many feed pumps are self-priming so there is no need to bleed the line after installation. However, several systems are equipped with pumps that are not self-priming so you may need to bleed air from the lines to get that pump to prime. Since the raw water source is to be below the water line, the incoming tubing or hose to the feed pump will be flooded or easily flooded once the air is removed.

Most systems today are using continuous duty pump motors but if the system you purchase is using the older style pump-motor combinations that are not rated continuous duty, the system will likely be limited to a 1-2 hour run time. This should not be a problem for most boaters because the average run time of a water maker is 2 hours. Larger yacht owners and folks who want to make water for more than 2 hours should make sure the feed pump on their system is
continuous duty rated.

Cooler water can help the feed pump stay cool but you don’t want to rely on the cooling affect of the raw water.

If the feed pump is not included in a common unit of your water maker then usually, you’ll need to find a space of about 10 inches by 10 inches by 7 inches. That amount of space will accommodate the pump and give you some area around the pump for tubing or hoses and the wiring. Here again, space for future servicing will make your life easier down the road.

Also, if the feed pump is not part of a common unit, keep in mind the type of water connections you use. Water connections that are easily taken apart will make life easier down the road. We are not fans of hose barb fittings that use hose clamps to hold the hose or tubing onto the fitting. Rust or corrosion seems to be the enemy of hose clamps, even if they claim to be stainless.

High Pressure Pump:

The high pressure pump takes the water from the feed pump and raises the pressure to the point where the H2O molecules are pushed through the membrane. Typically the operating pressure of these pumps is 750 – 850 psi with 800 psi being the normal operating pressure of most systems on the market today. The pumps themselves will produce much higher pressures but the membrane and housing ratings are typically 1000 psi and water production above 800 psi is limited, so it is not worth the extra wear and tear on the pumps and other components. Most of these pumps are diaphragm or piston type pumps but there are a few push-pull style pumps on the market that work very well.

No matter which style is included with your system, they are expensive. The pump body which houses the components actually moving the raw water should be brass, stainless, or ceramic. With that said we have seen some aluminum pump bodies last for years if they are given proper care while operating them. The fresh water flush cycle is important to the longevity of your system no matter which style of pump is included but it is especially important for the
aluminum pump body.

High pressure pumps are larger than the feed pumps so the pump-motor combination will require a space that is 16-20 inches in length, 10-12 inches in width and 12-15 inches in height. These pumps are heavy so give yourself plenty of room to make handling and future maintenance easier.

There are a few DC pumps on the market but the vast majority are 120 volt AC. Most of these AC pumps utilize capacitors to help start the motor and keep it running after it gets started. A centrifugal switch inside the motor disconnects the start capacitor once the motor is up to speed. High in-rush starting current is typical of these capacitor motors and a 1.5HP motor will need 14-16 amps at full operating load but can require over 30 amps for a second during start up. That in-rush current will limit your ability to run the system on an inverter unless the
manufacturer has installed a Soft-Starter to get the pump running.

Recently a few manufacturers have begun using a type of motor controller called a VFD or variable frequency drive. These units will take 120 volts and turn it into 3 phase 220 volt. The motor associated with these controllers does not use capacitors and does not have that centrifugal switch since there is no capacitor to disconnect. These VFDs control the speed of the motor by accelerating the motor slowly and limiting the current to the motor. This method
virtually eliminates the current in-rush spike seen with the capacitor motors. As a result, an inverter can be used to run the water maker. We know of a NEEL trimaran that uses this system to run a 40 gallon per hour water maker directly from his 4KW inverter with no generator.

Since the acceleration of the high pressure pump motor is controlled and the motor is now a 3 phase unit, the overall noise is also dramatically reduced.


Most systems use 2 filters in line to remove the contaminates from the raw water before it gets to the high pressure pump. Usually a 2.5 inch by 10 inch housing with a filter cartridge rated at 20-25 micron, followed by a 2.5 inch by 10 inch housing with a filter cartridge rated at 5-10 micron are used to remove the majority of contaminates from the raw water. On systems that can also make water from brackish water, a third 25-30 micron filter is installed ahead of the
first filter. Brackish water tends to have more ‘junk’ in it than sea water, hence the need for the extra filter. We recommend using 3 filters even on systems only making water from sea water. A mounting arrangement using ‘L’ brackets, capable of holding the three filters will require a space on a wall or bulkhead about 15 inches wide by 14 inches high by 7 inches deep.

Flushing the water maker with fresh water after each water making cycle will help ensure the longevity of the system, especially the membranes. However, since most flushing uses the water from the vessel’s storage tank there is the possibility of chlorine being in the water in that tank. Chlorine will destroy the membranes in an RO water maker, so we need to remove the chlorine from the water used for flushing. Chlorine removal is accomplished by passing the
water through a carbon filter before it gets to your membranes. Usually, the carbon filter is placed just before the fresh water connection used for flushing. In this way the carbon filter is only used when flushing. Most carbon filters are good for 2000-3000 gallons so considering a flush cycle uses only 6-8 gallons of water it should be good for a minimum of 250 water making cycles. However, we recommend changing the carbon filter every 6-8 months to reduce the possibility of bacteria buildup. The space required will be about 6 inches wide by 14 inches high by 7 inches deep.

Electrical Power:

Power requirements can vary greatly but are usually determined by the size of the high pressure pump. A 20 gallon per hour water maker will likely use a 1 HP pump, and a 40 gallon per hour system will require a 1.5hp – 2hp pump. Our suggestion is to install a 20 - 25 amp breaker for your system unless the manufacturer is asking for something larger. That means you will need to run #12 AWG wire size for the 20 amp breaker and #10 AWG wire size for the 25 amp breaker. 

The water maker MUST be wired to a dedicated circuit. No other appliances can be on that circuit. Also, the electrical ground MUST be carried through the entire water maker circuit. Those newer systems using a VFD to drive the high pressure pump will normally require a 20 amp breaker and are not tolerant of ungrounded electrical power.

We have seen older systems utilizing 5hp high pressure pumps and feed pumps in the 2hp range so those water makers will have special electrical requirements. Consult your manufacturer for their specifications.

It is important to note that longer runs between the power source and your water maker could require you to jump up one wire size. However, all of the systems we have installed utilized the 20-25 amp breaker and associated wire size for the water maker circuit.

Electrical voltage and capacity should be reviewed with the manufacturer of your water maker prior to installation. A 220 volt water maker will not run on a 120 volt supply and a 120 volt water maker connected to a 220 volt source will end up severely damaged so make sure of the requirements and know if that matches your boat electrical system.

Additionally, we never recommend aluminum conductors and we always recommend stranded wire not solid wire.

Piping or Plumbing:

We recommend ‘push-to-connect’ piping and tubing fittings such as Shark-Bite, DMFIt, Sungator, or John Guest. We are not fans of hose barb and hose clamp connections. Keep in mind most tubing is measured using the outside diameter so purchase the correct size.

When installing the tubing or hose be careful not to allow it to rub against something that will rub through and cause a leak. If necessary, use an overwrap of hose a couple sizes larger to protect the tubing. Also, secure the tubing or hose with tie wraps and padding to reduce the possibility of leaks caused by vibration.

We always recommend ball valves be installed at any connection point so you can isolate the water maker from the rest of the vessel.

When connecting the output from the water maker to the vessel you do not need to make a separate run to the storage tank. Simply tee into the suction side of the vessel fresh water pump. The system will push the water back to the tank as it produces water. We usually suggest a sampling valve at that tee so you can sample the water before it is sent to the tank. Similarly, the flush water is usually a tee on the pressure side of the fresh water pump.

You should always have a water quality meter in the line to the fresh water tank. Whether your system has an upscale sensor and meter or one of the simple sensors seen on Amazon, make sure it is positioned so you can easily read it and again install a ball valve to be able to isolate the sensor.

You will need to install a brine overboard line for the waste water from the water maker. This line should connect to a thru-hull that is above the water line. Some folks simply allow the brine to dump into their bilge and allow the bilge pump to take it out. This method works but remember the brine is very salty so make sure the bilge pump can handle that water.

Control System or Panel:

Water making systems can be manual, semi-automated or fully automated. Sometimes the flushing function is the only automation available with the system.

Manual systems require the user to turn on the pumps, turn valves, adjust and set the system pressure, and monitor the flow and water quality as it is produced. Most of these manual systems will have an option to purchase a simple control panel with a pressure gauge, the pump start switches, a flow indicator, a TDS meter/sensor for water quality and some method of starting the flushing process all incorporated into the design. We highly recommend purchasing that panel. Having all of those functions at your finger tips will make your life easier during start up and during the day to day operation of the system. If you have purchased a manual system, you should have no problem making water but you will need to monitor the system as it runs. System operating pressure and the TDS meter need continuous monitoring
to ensure the system is always operating within its specifications and the water quality is good. That control panel is usually about 14 inches by 14 inches and will require about 4 inches of depth for the piping behind it.

Automated or semi-automated systems will perform most if not all the tasks involved in operating the water maker and a few will also monitor the water quality, control system pressure, and perform a flush when the cycle is completed. The automated systems make operation more simplistic so the non-technical person can easily operate them, and improves reliability since the system will protect itself should a problem arise. Utilization of a touch screen makes monitoring the operation very easy with animated graphics and alarms. A few of the automated systems place the operator touch screen in the galley or helm so the user doesn’t need to go to the engine room or lazarette to turn on the system and make water. Automated systems will either have the control panel or touch screen built in or require it to be remotely mounted. You will need to ask the manufacturer or seller how much space is required.

Flushing the system with fresh water after each water making cycle is important for the longevity of the system and for your health. One of the first questions to ask your manufacturer or seller is, “How do I flush it after making water.” It doesn’t matter if the system is automatic or manual, it needs flushed after each water making cycle. 

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