You would think that the expert's who build a vessel would get the basic's right, but this does not always happen. After installing the sump tank and pump for the galley, we had a situation where the pump would cycle every hour or so, even though the sink was not being used.
When the thru hull valve was closed the pump did not cycle. After further investigation it was looking like something was not correct with the anti siphon loop.
The next move was to remove the washer-dryer combination unit and trace the line from the sump tank for the sink to the thru hull valve. Sure enough, the line went up about four feet above the water line, then back down to the thru hull, but I could not feel the top of the loop.
After cutting (yet again) an access panel, I found that the line did go up then come back down to the thru hull, but there was no vented loop installed.
Once an anti siphon loop was installed the system worked as it should. One more time it proved, never assume any thing.