I'm installing a marine sanitation device type III (holding tank, then Y-valve, then (a) macerator pump to thru-hull, or (b) hose to pump-out deck plate) on my 1978 Catalina 30. The diagram from Catalina shows the hose going directly from the macerator pump to the thru-hull. Other diagrams that I've seen (such as the one on the macerator pump instructions) indicate the need for a vented loop above the water line, between the pump and thru-hull. Is the vented loop really necessary, if I don't open the thru-hull valve except when I'm pumping through it? (Besides, the "vented loop" sounds like a good way to get undesirable odors inside the cabin.)
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It depends...
If you mount the macerator pump above the tank--which should also put it well above the waterline--you can do without the loop. But if you have to put it at or below the waterline, you'll have to install one. Odors should not be a problem, because the air valve in the loop only allows air INTO the line, nothing out. But no matter where you put the macerator, the thru-hull should always be kept closed except when dumping the tank. Btw, I suggest that you specify two discharge ports in the tank--one for a hose going directly to the pumpout, the other for the hose to the macerator. That eliminates the need for a y-valve. And, you want both the inlet and the vent fitting to be away from the hull, toward the centerline of the boat...that prevents waste in the tank from spilling into the vent line--which can result in a clog--and head discharge line to run back toward the toilet when you're heeled.
If you mount the macerator pump above the tank--which should also put it well above the waterline--you can do without the loop. But if you have to put it at or below the waterline, you'll have to install one. Odors should not be a problem, because the air valve in the loop only allows air INTO the line, nothing out. But no matter where you put the macerator, the thru-hull should always be kept closed except when dumping the tank. Btw, I suggest that you specify two discharge ports in the tank--one for a hose going directly to the pumpout, the other for the hose to the macerator. That eliminates the need for a y-valve. And, you want both the inlet and the vent fitting to be away from the hull, toward the centerline of the boat...that prevents waste in the tank from spilling into the vent line--which can result in a clog--and head discharge line to run back toward the toilet when you're heeled.
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