Furnace Sizing Calculations

Reasons

• Determining the correct capacity of a furnace affects both costs and heating efficiency. Furnaces that are too small will not have the power to heat an entire home during a cold spell. They may require closing vents and access to certain rooms to sufficiently heat others. To avoid this problem, home furnaces are generally sized too large. However, a furnace that is too large not only costs more money to run but cycles off and on too frequently, putting more wear and tear on components and requiring unnecessarily bigger ducts to accommodate the increased airflow.

Numbers

• Two numbers affect furnace sizing calculations. The first is BTUs, which stands for British Thermal Units. This unit of fuel and heat measurement indicates the amount of input energy a furnace needs to function. For example, a unit may be rated at 100,000 BTU. The second is the Annual Fuel Utilization Efficiency (AFUE). This percentage represents the amount of fuel that is converted to heat. A furnace with a 90 percent AFUE converts 90 percent of its input to heat. To determine the output BTU, which is the amount a home needs, multiply the input BTU by the AFUE. For example, a 100,000-BTU furnace with 90 AFUE produces 90,000 output BTUs while one with 98 AFUE produces 98,000 output BTUs.

Zones

• AC4Life, a heating supplier, recommends using BTUs per square foot, which varies by climate zone, for furnace sizing calculations. In the mildest climate Zone 1, which includes Texas and Florida, that number is 30 to 35 BTUs per square foot. The lower figure is for well-insulated structures while the higher is for poorly insulated homes. Zone 2 includes most of the Carolinas and California and uses 40 to 45. The middle Zone 3, which has Kentucky and Missouri, uses 40 to 45. Zone 4, which covers much of Iowa, Pennsylvania and Ohio, uses 45 to 50, and the harshest climate Zone 5, which includes North Dakota, Maine and much of New York, is 50 to 60 BTUs per square foot.

Calculation

• To calculate the total BTUs needed in the home, subtract the square footage of unheated areas, such as closets and pantries, from the total square feet of the house. Then multiply the result by the number for your zone. For example, assume 1,000 square feet of space that you want to heat. In Zone 1, that space requires a furnace with 30,000 to 35,000 of output BTUs while in Zone 5, the furnace capacity jumps to between 50,000 and 60,000 per square foot.