For part 2, the gang starts out discussing the operation of gravity furnaces, and then discuss forced air heat vs gravity heat. They also discuss ductwork design and the design of heating and cooling systems.  Next, the gang discuss changing out the air in a home, and how older heating systems used to unintentionally change out the air, and what an 80% efficient furnace means, vs a 97% efficient furnace. That brings up the topic of combustion safety, combustion air, and makeup air. That discussion naturally transitions into combustion testing of gas appliances, as well as draft testing at natural-draft water heaters.