I would be concerned leaving the vent open on the Vetus because in the past, we've had to start the engine somewhat unexpectedly. For example, a rain-less squall came and resulted in a huge wind shift that the autopilot couldn't cope with, resulting in us getting backwinded. I fired up the engine and quickly got us around the right way. 

Might be a bit more complicated if you're dumping seawater and CO into the engine room.

I'll stick to running it for 15 minutes daily and enjoy the hot water showers!
--
Scott 
2007 A54 #69
SV Tengah
http://www.svtengah.com