Bonus points for taking advantage of a balanced ventilation system’s ability to continuously extract air from stinky areas, e.g. bathrooms.

Even more bonus points for avoiding negative pressure due to conventional bathroom exhaust, which can defeat stack effect-based exhaust from non-power-vented combustion appliances, which are, for some reason, still legal.

(Seriously, WTF. There’s a straightforward design that could safely created a forced draft even with legacy leaky ductwork: put the fan on the exterior vent terminal, so the duct is under negative pressure. The wiring could be fished through the existing duct using class 2 / SELV wiring with high-temperature insulation. A pressure or airflow-sensing interlock in the appliance could prevent gas flow if the fan stops working. Sadly, I’ve never heard of a system remotely resembling this. The choices appear to be stack effect (category I or II) but basically crossing fingers and hoping the pressure works out) or positive-pressure sealed but not tested “category III” or “category IV” pipes and crossing fingers and hoping that the pipes are actually airtight.)