Why is Holding Tank filling with seawater???

[gwp]

1988 C34 Hull#568
It seems that I read about this problem somewhere, but can't find it. Holding fills and start spitting from vent when flushing? Emptied holding and it is filling up waaaay too soon. When pumped out it looked like clear seawater. Seacocks under head sink are always closed except when underway. Macerator seacock has never been open because macerator doesn't work. Any help appreciated.

[Ron Hill]

gyp : Simple, the macerator sea cock is is not completely closing OR you are syphoning water into the holding tank from the head, OR a little bit of both!!

[Les Luzar]

The only seacock under the head sink that should be open underway is the raw water seacock. Are you keeping your head seawater intake seacock open underway? If so, close it and see if you still have this problem. When underway, all my seacocks are closed exept the raw water intake.

[Ralph Masters]

I would cycle the overboard valves for the macerator and the head direct discharge a few times to be sure they are both properly closed.

Ralph
Ciao Bella