Know how: Freshwater Flushing for Electric Toilets

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There’s nothing like freshwater to cure what ails a marine toilet

There’s nothing like freshwater to cure what ails a marine toilet

Both of the heads on Britannia, my Downeast 45 schooner, are Raritan Elegance electric models that I installed nearly four years ago. They have performed flawlessly, and one of the reasons for that is that for about 90 percent of the time, they are flushed with clean freshwater.

When I bought the Raritans they were raw (sea) water models, but I devised a modification that allows them to be easily interchangeable between freshwater and salt. Raritan also offers a kit that converts an Elegance raw water flush toilet into freshwater flush at the press of a button. It operates in a very similar way to my method but does not have a second one-way check valve or a shutoff valve. The Raritan kit costs $190. My method cost me $80.

Unlike the factory system, which is specific to the Elegance head, my method will work with any marine head, whether electric or manual. The reason for this is that it’s fitted on the incoming pipework, not in the actual body of the toilet like Raritan’s installation.

Of course, the corrosiveness of seawater is well known, and the brackish water in the Intracoastal Waterway does not help either. Worse yet, even “clear to the bottom” seawater does not prevent calcification of pipes and valves or the frequent offensive odor. It isn’t so much human waste that causes the foul stink commonly emitted by marine heads. It’s the seawater-borne microorganisms that die in the pipes and are then pumped into the bowl, usually during the first few flushes after a period of nonuse.

Controls and operation remained the same

Controls and operation remained the same

Fortunately, flushing a marine head with freshwater eliminates these problems, and most manufacturers recommend seawater flush toilets be flushed with freshwater on occasion for this very reason, especially when leaving the boat for any length of time. As an added benefit, freshwater also prevents that nasty brown ring from forming in the bottom of the bowl.

My objective in devising my system was to be able to flush the toilets with freshwater from either a marina shore water supply or the boat’s potable water tanks. I also wanted to ensure the boat’s freshwater system was totally protected from any possible saltwater incursion. I achieved these goals by using a 12-volt stainless steel water solenoid valve, which is engineered to remain closed and watertight by default and only opens when current is applied. Not wishing to rely entirely on this mechanical/electrical device to prevent back-flushing, I also incorporated a manually operated shutoff ball valve, as well as a one-way check valve as a third protection: all this is in addition to closing the saltwater seacock when using freshwater flush.

Like most boats of her size, Britannia has an on-demand electric pump that draws from her freshwater tanks and pressurizes the boat’s entire water system. Basically, I just interconnected these valves to each of my two heads, so that now when one of the solenoid valves is activated the toilet flushes with freshwater. The method can also be used to flush manually operated heads with freshwater, even if the boat does not have a pressurized system.

When I’m in a marina I usually connect to the pressurized dock water supply with a water hose taken to a pressure-reducing regulator into the boat. I then switch the electric pressure pump off and have constant silent water pressure throughout the boat, including the heads. (I have also incorporated an automatic solenoid shutoff to this water inlet in case of an internal pipe failure: see SAIL March, 2019.) Even when underway I now normally leave both heads on freshwater flush most of the time, especially in the ICW and on short passages offshore. I can do this because Britannia has a large freshwater storage capacity of 325 gallons. Aboard other boats, the decision to change over to seawater on passage will depend on the capacity of your freshwater tanks and the number of people using the heads. Aboard Britannia, a “non-solids” flush uses about a quarter of a gallon of water, while a full flush uses about three quarters of a gallon.

Normal seawater operation requires pressing a button on the control panel in the bathrooms, which activates a dedicated saltwater diaphragm pump that draws seawater from a through-hull seacock, through a filter and into the toilet bowl. Manual toilets are the same, except sea water is sucked in by operating the toilet’s hand pump.

With my new freshwater system, the actual flushing operation is the same as for saltwater, except the boat’s freshwater pressure pump now pumps water into the bowl (or pressurized water comes in through the dock supply). In the case of a manual head, freshwater is sucked in with the manual pump instead of saltwater.



The system I created is actually easier to install, than it is to describe. Again, there are three main parts—a one-way check valve, a regular ball shutoff valve, and a 12-volt solenoid. The check valve and ball valve can be bought from most hardware stores. I bought the solenoid online (see “What You’ll Need” sidebar).

First I tested the one-way check valve by connecting it to a dock water hose. In its flow direction water flowed without restriction, but when connected to the opposite end of the check valve absolutely no water came out. That’s how it should be.

I connected the parts together into an inline assembly consisting of the solenoid, the shutoff valve and the one-way check valve. I then installed them, using T-connectors, between the boat’s cold-water system and the seawater inlet pipe that feeds each toilet. You can see in the picture above that the manual ball valve is connected next to the toilet pipe. After that comes the one way valve, then the solenoid. This arrangement ensures no saltwater can ever get to the solenoid or percolate back into the freshwater system. The installation was under an easy-to-reach floorboard. Interconnecting the pipes would be the same for a manually operated toilet.

A 12-volt wire from each toilet’s control box normally activates the saltwater inlet pumps, so I fitted each with quick-change connectors on the pump and solenoid wires. Now, when on saltwater flush the wire is connected to the saltwater inlet pump, but when using freshwater I disconnect the wire and plug it into the solenoid. The two negatives (returns) from the pump and solenoid go to ground.

Early on, I considered making this wiring connection using a double-pole, double-throw switch, mounted somewhere near the toilets. (That’s what Raritan uses.) However, since my system has the added safety of the manual ball shutoff valve, it is also necessary to physically open and close the valve to change the flushing method. I therefore decided it was just as easy to switch the pump and solenoid power over at the same time. One less switch also means one less thing to go wrong, and we don’t change the flushing method that frequently anyway.

In the case of a manually operated toilet, since there is no pump, it is only necessary to connect the solenoid to a straightforward 12-volt supply through a switch near the toilet that is permanently connected to the solenoid. When the valves are turned to freshwater and the switch is activated, the solenoid opens and permits freshwater to be drawn into the bowl. When switched off, the solenoid closes again.

The solenoid/shutoff assembly (left); a closeup of the solenoid before installation

The solenoid/shutoff assembly (left); a closeup of the solenoid before installation

Operation Underway

When changing over from seawater to a freshwater, first the saltwater inlet seacock is closed, after which the wire to the saltwater pressure pump is changed over to the solenoid. That done, the freshwater supply ball valve is opened. On a manually operated toilet, there is no pressure pump and the switch to the solenoid can be permanently connected.

When the control panel button is operated (or the switch, in the case of a manual head), the solenoid instantly opens and the water pump (or shore water pressure) pumps freshwater into the bowl, exactly the same as if a faucet has been opened. It takes only a few seconds to fill the bowl and when the button is released the solenoid closes and the water stops. With a manual toilet you do the same pumping by hand. After flushing, the switch would be returned to “off” and the solenoid closed.

To switch back to seawater the process is reversed—the freshwater ball valve shut-off is closed, the electrical wire from the solenoid is changed over to the saltwater pump, and the sea water seacock is opened.

Finally, freshwater flushing has a great secondary advantage that will only be appreciated when it comes time to dismantle the head to install new parts or replace pipes (which is as certain as the wind is always on the nose). You will no longer need an oxygen apparatus to do the job! Bottom line, I consider freshwater flushing to be a major improvement to a very necessary, but often neglected, (until a problem occurs), piece of my boat’s equipment. 

What You’ll Need

12 volt water solenoid: Available through and manufactured by U.S. Solid (, Part # USS2-SV00069, $42.99 each

Ball valve shutoffs, one-way check valves, pipework and hose clamps: These bits of gear are available at any hardware store; total cost will be in the neighborhood of $40

TOTAL COST: About $80 for each toilet

July 2019




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