You're right Slim it does have the bypass but when I had the G400 and now the G200 I have always had smoke still come out the loading door. No where near the amount I had when I had my P&M boiler but it is still there. My unit is in an enclosed building so any smoke stays in the building and this vent hood works great at keeping it smoke free. Wife threatened me with physical harm so I had to solve the problem.
I will say the smoke when loading is mainly when I have a fair amount of wood left over from the previous filling and as the left over wood starts to burn off the accumulated carbon and creosote. If it is just coals left then almost no smoke comes out the loading door.
I had an Amish shop that does sheet metal work bend up the metal from plans I drew up. The fan is a circulating fan (all metal) from a junked out mobile home furnace and an 8 inch air duct to get it outside. Made a pendulum damper at the end of the duct that swings open when the blower comes on.
I replaced the door seal this summer and am drip free now. Started my G200 soonest ever this year, mid November, due to a good cold snap then. Love the endless hot water again with the breaker off to my electric hot water heater.