Oldtrucker,
I think what rehoppe was saying, is that the combustion chamber has a fan that sucks the air out thus pulling air in from the outside inlet. This is a failsafe design since if there is a leak in the combustion chamber air will also be sucked in through the leak thus preventing any carbon monoxide (CO)from escaping through the leak.
Imagine if the fan was blowing air into the chamber instead. There would be a positive pressure in the chamber so any leaks would allow CO to escape through the leak into the living quarters.
Battery boxes that are in living quarters are designed similarly--the fan pulls the air out of the box rather than pushing it in.
Still is a very good idea to have a CO monitor.
