Korben know what I'm talkin about. The air comes in because the building envelope was comprimised and not sealed (usually at the wall flashing). I understand Toadie thinks the air is at the connection between the vent pipe and unit and it could very well be both. If the installer didn't seal the vent he certainly didn't seal the wall flashing. And I'm not talking at the term cap thats easy to seal. The problem is on a dv fp when you slide the vent (attached to the fp) thru the wall flashing, How do you seal the wall flashing? Installers aren't doing it. Korben are you? If it is spring, summer or fall who cares? Nobody cares till it's the middle of winter and a draft flows thru. The customer points to the Buider, the buider points to the installer, the installer points to the drywaller, and the drywaller points to the insulator points back to the installer...and so on. The answer is it is the buiders responsibility to line up the trades so the wall is intact before the install. Then the installer can do his job (assuming he is equiped). In Toadies case he was screwd cause the installing company didn't do it properly and they should have to fix it. In Canada we he Technical Standards Saftey Authority that I would call in a second if I found a vent not sealed like Toadies.