Before discussing the specific procedures needed for high quality furnace installation retrofits, it is important to discuss the connections between the heating system, the building enclosure (also called the envelope or 'skin'), and other electrical and mechanical systems. Any change to one element in the building will have an impact on the furnace. For example, additional attic, wall or crawlspace insulation and/or new windows and air sealing can lower building heat loss; a lower capacity furnace may now be able to accommodate your building's heating demand.

At any given moment, heat is flowing through the building enclosure or being generated within it. The following graphic introduces some of the typical heat flows that exist at any given moment within a building. Changes that affect any one of these heat flows can influence building heating demand and, subsequently, the selection of your new furnace.