The stack was lower than my house and it would set off a smoke detector in my attic. The town elected to work with him and after he put in a 30' extension on his stack it became far less of an issue. Obviously making your home tighter will reduce the smoke getting inside. If you have a chimney and are not using it, that is logical thing to seal ...
In Direct Vent installations, combustion air is drawn into a sealed firebox from outside the house through coaxial intake/ exhaust pipe. This eliminates depressurization, resulting in a warmer, healthier home environment.