A furnace is sized according to the amount of heat it can produce.
This is known as a BTU rating. A furnace will have two BTU ratings. One for INPUT and one for OUTPUT. The BTU input rating is the gross amount of BTU produced.
The BTU output rating is the amount of usable BTU. In other words, Input is how much heat produced and Output is how much of that heat can be delivered to your house.
The difference between Input and output is the efficiency rating.
A 100,000 BTU input furnace with an 80 percent efficiency rating will make 80,000
BTU of heat available for your house. (Eighty percent of 100,000 = 80,000)
A 100,000 BTU input furnace with a 95 percent efficiency rating will make 95,000
BTU available for your house.
Both furnaces will use the same amount of energy, one will simply waste more.
To properly size a furnace, a heat loss calculation is performed on the building.
This determines how many BTU per hour is needed to condition the space. A heat loss
calculation is specific to your area and building. Common sense tells you that the same house in Maine will require a different amount of BTU’s than a similar one in Florida.