Furnace Sizing Calculator

The Furnace Sizing Calculator is a tool designed to help determine the appropriate heating capacity for a furnace based on the building's heating load and the furnace's efficiency (AFUE). Proper furnace sizing is critical for comfort and energy efficiency. An undersized furnace will struggle to heat the space adequately, while an oversized furnace can lead to short cycling, reduced efficiency, and uneven temperatures. This calculator typically requires the total heating load (in BTU/hr, often from a Manual J calculation), the furnace's AFUE rating, and sometimes a general climate zone consideration.

Understanding Furnace Efficiency (AFUE): AFUE (Annual Fuel Utilization Efficiency) measures how efficiently a furnace converts fuel into usable heat over an entire heating season. For instance, an 80% AFUE furnace converts 80% of its fuel to heat, losing 20%. High-efficiency condensing furnaces can achieve AFUE ratings of 90% to over 98% by extracting more heat from combustion gases. Higher AFUE means lower fuel use and operating costs, though initial equipment costs might be higher.