So, I'm working on all of those firewall penetrations with a constant eye on sealing everything so firewall gases can't enter the cabin. I'm looking at the Van's standard cabin heat shuttle and it's designed such that it seals when you have cabin heat closed. However, when cabin heat is open, there are significant gaps along 3 edges of the valve plate as well as another gap on the other side. Now I understand that the cabin heat airflow will probably be a whole lot more than the leaks around the edges, but isn't this still a pathway for bad air into the cabin?