What Are the Best Ways to Size a Gas Furnace System?

Why Sizing Accuracy Matters

Proper sizing is one of the most important parts of selecting a heating system. A correctly sized furnace delivers a high level of comfort while maintaining good efficiency. An oversized furnace short cycles, turning on and off more often than it should, which limits comfort and reduces the operating life of the equipment. The good news is that you do not have to nail your requirement down to the last BTU. Furnaces are sold in 20,000 to 25,000 BTU increments, so being off by 20 to 25 percent causes no real harm. If your heating requirement runs about 10 percent or more above a furnace's capacity, step up to the next size.

Your Sizing Options, Most to Least Accurate

Software Load Calculation

A sizing program that uses all of your home's construction data is by far the most effective method because it removes guesswork, opinion, and rules of thumb. Your system performs as the manufacturer intended and your power bills stay as low as possible. Software calculations are required by many municipalities when permitting new construction, because no best guesses are allowed for a code correct installation.

Free Sizing Calculator

A detailed free calculator can estimate the proper heating and cooling capacities for your home. Plan to spend about 30 minutes gathering dimensional and construction data before you start. It is not as exact as full load software, but it is still an excellent free way to size your equipment.

Contractor Evaluation

An experienced local contractor may know the size needed from your floor plan and past work in your area. Contractors do not always agree, so if estimates differ, ask what criteria each one used. If the evaluation relies on square footage alone, there is no assurance the recommended system will function properly. Industry studies have repeatedly shown that more than half of furnaces sold are dramatically oversized.

Match an Existing Furnace

If you are replacing a furnace, find its BTU input and output ratings on the nomenclature plate behind the front service panel. Most older furnaces are about 80 percent efficient, so a 100,000 BTU input rating produces roughly 80,000 BTU of output. If the old unit was sized correctly and performed well for a heating season or two, the same size replacement is a fairly safe bet.

Square Footage Estimate

A square footage and climate zone selector gives a quick approximation but assumes all homes in a zone share similar construction and insulation, so treat it as a starting point. When you buy from AC Direct, you can use free technical support to confirm your sizing before you order.