Ever use a double-basin sink where the drains are connected with a tee directly under one of the drains? That is, the straight part of the tee is vertical? The difference in draining speed is very great. The push for water's draining is the same as the push for hot air's tendency to go up the vent, its weight, where the hot air has negative weight. The push is greatest when the flow is aligned with gravity, vertical. This, of course, is why the amount of horizontal travel in a atmospherically-vented vent is limited. The only reason the combustion gases are moving up the vent is that they are marginally lighter than the surrounding air. It doesn't take a whole lot to screw this up and have some of the combustion gases come out the huge hole at the draft hood. So as the combustion gases pass out of the appliance, through the draft hood, and into the vent, we want as much push, with as little friction, as possible. A bend, going into a section that is not vertical, right above the draft hood, will make the upward draft force less than what it would be with a straight, vertical section. Maybe it is a small difference, but when outside winds are set up for backdrafting, or if the combustion air is insufficient, I wouldn't want venting system slowing down the combustion gases just where they can come out into the room.

So I believe that is why there are reccommendations for a straight section of pipe above the draft hood.

They may be only recommendations, but if an installation doesn't start out with a straight section of vent where it could, I would call it out. There are advantages in our not being code inspectors. We can, and should, point out where safety could be improved even if the installation meets code. If I were acting for the AHJ I would require a straight section at the draft hood if the installation allowed it based on the installation instructions mentioned in this thread.

- BOB