How Do I Determine the Correct Gas Furnace Size for My Home?

Why Proper Sizing Matters

One of the most important aspects of selecting a heating system is proper sizing. A correctly sized furnace provides a high level of comfort while maintaining good efficiency. An oversized system will short cycle, meaning it turns on and off more frequently than it should. Short cycling limits comfort and reduces the operating life of the equipment. With gas furnaces you do not have to nail your requirement down to the last BTU. Furnaces are typically sold in 20,000 to 25,000 BTU increments, so you mainly need to land in the right ballpark for how many BTU your home requires.

Your Sizing Options

Manual J Load Calculation

A Manual J calculation is the professional standard and is required by many municipalities when permitting new construction. It accounts for square footage, insulation, windows, and local climate. No best guesses are allowed when it comes to a code correct installation.

Square Footage and Climate Zone Estimate

If you are replacing an existing furnace, find its BTU input and output ratings on the nomenclature plate behind the service door. Most older furnaces are about 80 percent efficient, so a 100,000 BTU input rating produces roughly 80,000 BTU of output. Then measure the heated square footage of your home and apply a rough rule for your climate.

Match an Existing System

If your current furnace was sized properly and has performed well for a heating season or two, replacing it with the same size is a reasonably safe choice. This first hand experience makes the match more reliable than a guess.

Professional or Contractor Input

A contractor can visit and evaluate the size needed based on your floor plan and local experience. Contractors do not always agree, so if estimates differ, ask what criteria each one used. When you buy your equipment from AC Direct you can lean on free technical support to confirm your sizing before you order.