Furnaces are rated according to their efficiency as expressed in terms of AFUE, and also according to their “size,” or heating output capacity, as expressed in BTU/h. When the subject, “how big a furnace do I need” comes up with your local Carrier HVAC expert, it is usually in reference to its BTU rating. BTU stands for British thermal unit and represents the amount of heat needed to raise one pound of water 1⁰ Fahrenheit. Furnace size or capacity is expressed in BTU/h, or the amount of British thermal units the furnace can generate per hour.
Choosing the correct size furnace for your home is important because an undersized furnace won’t generate enough heating capacity to keep your home warm. It will run constantly to keep up with demand. An oversized furnace will frequently turn on and off because it is supplying too much heat, which creates uncomfortable, up-and-down temperature swings and reduced efficiency. Your local Carrier dealer is experienced at calculating furnace size and will do what’s called a “load calculation” to determine the size most appropriate for your home. Typical residential gas furnaces will be available in various sizes between 44,000 BTU/h and up to 120,000 BTU/h.