When a heating contractor receives a set of plans from a homeowner or builder, the first thing he does is perform a heatloss calculation. He will calculate the ability to retain heat of every window, door, ceiling, floor, and outside wall in every room of the house.
This calculation will determine how much heat each room needs. Once he knows how much heat is required, in BTU per hour, he will select a warm air furnace which produces an amount closest to that quantity.
The furnace he chooses will have specifications which will inform him of the CFM he will need to distribute. CFM meaning the cubic feet per minute of air that the blower will deliver.
He will divide this proportionally between the rooms and then design the ductwork needed to properly deliver the correct amount of air to each room.
If the system will use baseboard or radiant heat, the heating contractor will follow the same procedure with the different delivery method. He will determine how many feet of hot water baseboard or radiant loops to install in each room.