Re: Head Backfill
Sounds like he already has vented loops.
"Entering the head through the toilet" is unclear to me, since to most people the head is the toilet, though you could mean it is overflowing into the compartment. Is it waste or seawater? I assume from the title you mean backflow of waste.
It is very likely a simple matter of not flushing enough water. That is the root cause of most backflow problems; urine sits on the joker valve, deposits build, and the valve becomes blocked open. This can happen as quickly as a few months but generally a few years. If enough is flushed, deposits don't build since the process requires reaction time. Additionally, if waste comes back, by definition, not enough was flushed.
If it is doing more than refilling the bowl then there must be a siphon issue. Assuming there is a vented loop, I would guess the vent is clogged, common enough on waste lines. It can also be a matter of wave impact; on my boat the shower sump loves to backfill on one tack if it's rough enough long enough. The waves and pounding are just enough to make the check valve flop around enough, and the waves are striking the through-hull.
One tack vs the other may be the location of the tank or some asymmetry of the plumbing, rather than the actual cause. Couldn't say without a lot of detail.
(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")
"Well, I just climb up to them."
by Joe Brown, English rock climber
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