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One of the easiest but totally forgotten maintenance items that should be on your checklist this spring is to service the vented loops on your boat. In case you have no idea what a vented loop is, several pictures might help.


vented loop

Vented loops are used in on board plumbing systems where a thru-hull fitting brings water on board below the boat’s water line. Typical uses are for head systems and engine raw water intakes. The idea behind the vented loop is to allow the water to enter the system it serves when needed, but prevent siphoning of water when the system is not in use. Without breaking the siphon, which the valve hidden under the cap at the top in the photo above is supposed to do, you run the risk of water siphoning in and well, maybe sinking your boat! These things are simple in design, but totally neglected when it comes to routine service. The details of the inside of the valve mechanism is shown below:


vented loops
Not much going on under that cap, just a small rubber one way valve. But here’s what happens to them. Over time they become contaminated with calcium deposits and or other small bits of debris in the water that flows through the loop on a daily basic. Eventually they can become permanently closed so that when its time to vent to prevent siphoning, they won’t work.

The fix is easy here and I recommend it as an annual maintenance item. Begin by threading the cap off to access the valve. You will probably need a set of water pump pliers or small pipe wrench to get the cap loose. Clean the valve, cap and the seat area the valve sits in with some warm water and vinegar. The vinegar will dissolve any calcium deposits. Once clean, re-install the duckbill valve and thread the cap back on.

Note that some boats today are using more sophisticated valve mechanisms, and they may have an additional hose attached to the vent to divert any water that spits out to your boat’s bilge, rather than squirting water all over system components near the valve. The photo below shows such an arrangement.


anti-siphon valve
The valve shown is a bit more complex than the units shown earlier, it’s actually easier to close the sea cock supplying the water and removing the valve mechanism from its mount for disassembly and cleaning. Again, vinegar works great for dissolving any calcuim build-up.

So, again, I’m hung on the small details that will help ensure a trouble-free boating experience. These vented loops / anti-siphon valves are installed for a good reason, to make sure sea water doesn’t back up into your boat when you least expect it. Make sure they are able to do their job, an annual bath is all they are asking for.

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