The heating capacity of a furnace is measured in thousands of BTU (British Thermal Units). The amount of fuel energy consumed when running is called ‘input BTU’, while the actual heating capacity is called ‘output BTU’. The difference between the two is the efficiency. Common sense tells us that the more rooms we have to heat, the more powerful our furnace will need to be. But there’s much more to the science of furnace sizing than that. The calculation involves many factors, including doors, windows, location, construction material, insulation levels and more. Every home is different. A heat load calculation performed by a professional is the only way to determine you homes heat loss. The idea that bigger is better is a thing of the past. In order to obtain optimal comfort and efficiencies, your furnace should be sized properly for your home.