Heat loss / heat gain calculations are the basis by which a new furnace is selected. In effect, all sources of heat loss (building enclosure, ventilation, etc.) are combined with sources of heat gain (occupants, appliances, heat recovery, etc.) in order to estimate the needed output of the furnace.

TECA and HRAI have explicit methodologies with worksheets and software for heat load calculations
that should be used for furnace sizing to ensure that heat load estimates are completed appropriately.