Preferred source for seawater to head

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I have a 2017 C-28. My boat differs from the Owners Manual in that there are two different sources available to draw sea water to the electric toilet. (There are a significant number of other build discrepancies from what’s shown in the Owners Manual but that’s a different post.)
The Owners Manual shows a line coming of the starboard side sea strainer going to the head. (And yes, my boat has both a port side sea strainer and a starboard side sea strainer.) My C-28 has this line coming from the sea strainer with a 90 degree ball valve next to the sea strainer to shut the line off.
I also have a separate through-hull with a line that goes to the toilet. This line also has a 90 degree ball valve installed to shut the line off. The through-hull is NOT referenced anywhere in the Owners Manual.
The two lines merge after the respective ball valves giving me the choice of selecting which source to use - sea water from the sea strainer OR sea water from the independent through-hull.
When winterizing it’s easy to run RV antifreeze down the sea strainer to the electric toilet with the ball valve to the independent through-hull turned off.
For normal operation I’m assuming I should be using sea water from the independent through-hull. I think the through hull has zero pressure at cruising speeds and therefore won’t flood the electric toilet.
The “scoop” on the through-hull used by the starboard side sea strainer has a positive pressure to it at high speeds and has forced sea water into the toilet when at high cruising speeds. And yet, that’s what the Owners Manual states as the proper source for sea water to the head!
So, just only use the sea strainer sea water feed for winterizing and use only the independent through-hull for normal operations????

PS: There is no check valve visible anywhere in these lines.

 

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Photos of the two through-hulls. Top is the independent one and bottom is the “scoop” through-hull going to the starboard sea strainer.

 

[bob daily]

Are you sure it’s factory installed and not an after market upgrade?
I remember reading that the water intake for the inboard diesels were marginal at best, and using for flushing and raw water wash down made the problem worse.
I believe it was a post by BB Marine that explains the problem and the fix.
Bob

 

[scross]

Bob, thanks for your reply. It is possible that this was a factory or dealer installed modification. I am the second owner.
I think our C-28 was being built as a LE and midway through construction was switched to a NW edition. A port side sea strainer is typically used for the AC on a LE. Our port side sea strainer is dedicated to the raw water wash down pump at the stern instead of going to a non-existent AC.
With the ball valve on the line to the head turned off at the starboard sea strainer the full flow from the “scoop” through-hull is going through the sea strainer to the engine. Never had any issues with engine temps.