Normally the vented loop only goes in the discharge line from the MSD. This is the line from the MSD to the overboard
thru-hull/seacock. This side of the MSD system is not protected by any valves that you can control. Normally all there is in the discharge line is a "joker" valve that is made of rubber and irises open or closed with each pump of the discharge. Using sea water the "joker" valve tends to collect a crust and finally will not close properly. The Vented Loop prevents a "siphon" effect allowing seawater to flow backwards into the MSD (which is normally below sea level inside the boat) and flooding the boat.
- - The "vent" in the vented loop is usually another tiny "joker" valve orientated to allow air to enter the discharge loop but closes when there is any pressure in the discharge loop.
- - The seawater to flush the MSD bowl is controlled by a manual valve that you turn from "Flush" to "OFF" or whatever, so you do not need a vented loop in that line and in most MSD's it is not physically possible to install one.
- - With the new "holding tank" requirements, it is common now to directly plumb the MSD discharge to the holding tank
(which is vented) with no vented loop in that line. But from the holding tank
to the discharge thru-hull/seacock there is a vented loop to again prevent siphoning sea water back into the holding tank.
- - If your selector "Y-valve" is between the holding tank and the MSD it starts to get complicated as you end up with maybe two vented loops. One for the MSD discharge directly to the sea and another from the holding tank discharge to the sea. There is another "Y-valve" after the holding tank that allows selection of holding tank discharge directly to the sea or up to the deck
- - In order to simplify the mess, I plumbed my MSD's directly to the holding tanks
and then use a macerator pump and vented loop to discharge the holding tank to the sea. There is only one "Y-valve" and it selects the holding tank discharge to the macerator pump and overboard
or up to the deck