A well designed residential heating and air conditioning system plays a critical role in providing comfort and energy efficiency for today's new and existing homes. The process for acquiring such a system follows three basic steps: the selection of the HVAC equipment, the proper sizing of the air distribution duct work, and proper installation and maintenance of the system. Selecting the HVAC equipment starts with determining whether the system requires only a furnace, or the building needs to be an air conditioned as well. At that point, the different choices in components can be weighed based on a comparison of efficiencies and upfront costs versus energy savings over the lifetime of the equipment. Additional considerations such as air conditioning filter efficiencies, humidity control, and the inclusion of ventilation duct can also be addressed at this time. If the HVAC equipment will only include a furnace for residential heating applications, then the HVAC designer will size the system to include heating duct only. Local duct work fabricators will manufacture the duct system to the exact sizing specifications of the system designer. Similarly, if the system includes a condensing unit component for air conditioning, the local duct work manufacturer will insulate and seal the duct work differently to address the potential of moisture control issues. Ventilation duct sizing is based primarily on whether the ventilation system is active or passive. Many active ventilation devices incorporate their own fan, and the duct sizing is reduced to accommodate the low level of air flow. However, in a passive system where fresh air is brought in and circulated through the HVAC equipment, the ventilation duct is typically larger and becomes an active component of the entire system.