Planned or recent home retrofits and occupancy can have a significant effect on replacement furnace sizing. New heating and ventilation equipment or building enclosure changes can decrease (or sometimes increase) the heating requirements of a building: this can result in incorrect furnace sizing if not addressed. Contractors should discuss recent or future building changes with the homeowner to ensure that any and all modifications will be accounted for in the heat load calculations.

Some possible impacts of building modifications include:

• Additional basement/crawlspace, attic and/or wall insulation (with higher R-value) decreases building heat loss and reduces building heating requirements. A lower capacity furnace may now be acceptable.
• A new bathroom or kitchen exhaust fan, or other ventilation changes, may remove higher volumes of conditioned air from the home than the previous equipment, increasing the building heating demand at a given moment.
• High performance windows and doors may reduce heat loss (with lower U-value), but can also allow greater solar gains into a home, depending on the glass coating and solar heat gain coefficient. This can have an enormous affect on building heating requirements.
• New heating equipment like gas fireplaces, wood stoves, room heaters or heat recovery ventilators might affect the load on the furnace, albeit the furnace is often needed as a backup.
• Building airtightness improvements with new windows and/or air sealing⁷ can reduce the loss of conditioned air as well as reduce the introduction of cold air through air leaks. A more airtight building will require less space heating.

The services of a Certified Energy Advisor⁸ can be very helpful prior to or during this stage for both
identifying improvements as well as conducting a pressurized airtightness test and calculating the
whole-house heating demand (discussed in a coming page).