The existing ductwork should always be reviewed prior to completing a furnace retrofit. Most high efficiency furnaces need more cubic feet per minute (CFM) of air flow than existing furnace systems and the existing ductwork must be able to accommodate this extra air flow in order to function optimally. Furthermore, the ductwork for many existing systems may not even match the existing furnace system - an important consideration before proceeding with a retrofit. The contractor should always complete an external static pressure or ESP reading (measurement of duct friction) in order to determine whether the ductwork and fittings require modifications prior to installing the new furnace. Ductulator® calculations (see Figure 5) can be used to provide quick, general insight into the existing ductwork capacity. The ductwork system should be designed to have with an ESP reading in alignment with TECA or HRAI recommendations, once the new furnace is installed. Readings that fall outside this range indicate that the ductwork may need modifications; often duct air sealing and/or additional ductwork.

In situations where the existing ESP is very high and it is not possible to change or include any additional duct runs, the solution may be to select a high efficiency furnace that has a lower air flow, which is one BTU output category below the calculated heat load. In this case, an additional or supplementary heat source would then be required to meet the design temperature.