Many of our boats (Catalinas) came with the shower sump
pump OUTLET connected to the head inlet. Works just fine.
A better solution, if you can do it based on how your head compartment is laid out, if your head sink is close to the head inlet thru hull
, is to connect the sink outlet to the head inlet and use the separate thru hull
for the shower sump outlet.
Like this (it's looong and takes some time to read, but goes through the whole issue, please read all three pages, thanks)
Head Odors 101 Head Odors 101 & Fresh Water to Head from the Sink in the Head FLIX