Seems to me that the solution is simple: put a plug
in the sink when you want to flush the toilet. If you don't have a plug
, they're about $1.50 at any hardware
store. You do NOT want a valve in the system, it'll drive you crazy trying to switch back and forth when you want to use the sink or the toilet.
Fwiw, most head intake lines are 3/4" ID. It's tank vent lines that are 5/8"...leading me to question which line you relocated to the head intake thru-hull.
There's nothing strange or arcane about the way your system is plumbed...in fact, several boat builders plumb to use the same thru-hull for both sink drain and head intake 'cuz it eliminates one hole in the boat and also saves 'em the cost of a thru-hull and seacock. It also provides a safe source of fresh water to rinse all the sea water out of the system before the boat sits, eliminating odor
from stagnant sea water (it's not intended as a way to have fresh water flushing
"on the cheap") and also to add antifreeze
. It's an excellent system for a boat that always stays at the dock
, and for boats that want to go places.
However, if you keep running the Lectra/San without supplying adequate salt
to it, you'll destroy the electrode pack (if you haven't already done so). If you don't have an owners manual for it, I suggest you download and print one from the Raritan
site: Raritan Lectra_San MC manual