Modern marine toilets require minimal maintenance, but ignoring it completely puts you at risk.
Cleaning Lines
Limescale builds up in heads washed with salt water in the discharge ducts and hoses. Limescale deposits cause the head to gradually become more difficult to flow, and it is the limescale on the valves that allows water in the discharge line to leak back into the bowl. Calcium deposits eventually lead to total blockage, which is a very unpleasant prospect.
Avoiding this problem is as easy as putting a pint of white vinegar on your head once a month. Swirl the vinegar through your head slowly, giving your head one pump every 4 or 5 minutes. The mildly acidic vinegar dissolves fresh limescale deposits inside the head and hoses. When the vinegar has gone through the entire system, pump a gallon of fresh water through it to sanitize the lines.
If you suspect limescale buildup is already present, dissolve it with a 10% muriatic acid solution, available at most hardware stores. The acid will not harm porcelain, plastic or rubber parts. It attacks metal, but the collateral damage takes too long. The greatest risk is to the eyes and skin, so be sure to observe all precautions on the label.
Lubrication
To keep your pump running smoothly, follow your monthly vinegar wash with a dose of oil. The best option is a lubricant specifically designed for marine toilets, but you can also use mineral oil. The oil lubricates the pump wall and helps keep the leather and rubber interior flexible.
The usual treatment is to get a little water into the bowl, pour in a few ounces of lube, and flush it down the toilet. This method is sufficient, but less than ideal because it lubricates the discharge side of the pump only. To also lubricate the intake side, disconnect the intake hose from the enclosed seacock and pour oil into that hose. Pumping the head will draw this oil through both pump chambers.
While you service the head, lightly coat the piston rod with Teflon grease. This will extend the piston rod seal life.
Scent
Marine toilets don't necessarily have to be smelly, but they often are. A drain hose is by far the most common culprit. To check, rub the hose with a clean, damp cloth, then smell the cloth. If you pick up a smell, the hose is permeable and you will never be able to get rid of the smell until you replace that hose with a suitable sanitation hose.
Leaky connections are another source of odor, and you can use your rag in the same way to determine where the leak is. Also check the seal around piston rod. On some heads, tightening the seal will stop leaking; In other cases, the piston rod seal should be replaced.
Another common source of head odor is grass and other marine life trapped within the passage of water flowing below the edge of the bowl. You can prevent this by installing a strainer in your suction line.
The anti-siphon valve in the discharge line can also release odors into the boat toilet. The valve is installed correctly outside the cab area.
Repair
If your toilet smells bad but isn't leaking, or if it's difficult to flush but the vacuum hose isn't clogged, or if it just isn't working properly, it's time for a complete overhaul. The exact procedure for rebuilding your head depends on the type and model of marine toilets, but marine toilets are simple machines and you're not likely to encounter many difficulties. Rebuild kits are available that contain new valves, springs, gaskets, and often bolts – in short, everything you need to renovate a toilet. The set will also provide a detailed instruction sheet.
What the instruction sheet may not tell you is that repairing a toilet is always more fun and almost always easier when you remove it from the head chamber before disassembling it. It may also fail to instruct you in placing the parts in order while disassembling the toilet so you know which bolts go where, or how each valve should be oriented.
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