A furnace of the proper size is crucial for the efficient heating of your home. A furnace that is too small won't be able to heat the area effectively, and a furnace that is too large will short-cycle, causing it to turn off and on more often, which will use unnecessary energy and cost you money. A simple method of sizing a gas furnace is not a replacement for a professional consultation, but it will provide a helpful guide as you begin the process of purchasing a new furnace.
Determine the amount of output BTU (British Thermal Units) you need to heat the area; output BTU is a measure of the amount of heat the furnace produces. As a general rule, start with about 30 to 35 output BTUs per square foot; this can vary, however, depending on climate and how well insulated the area is.
Find the output BTU of the furnace you're considering. Generally, furnaces will have a list of product specifications. The trick is that many will list input BTU instead of output BTU to make it seem like they are better than they really are. Luckily, they are also required to list efficiency. So, if a furnace doesn't list its output BTU, locate its efficiency rating and its input BTU.
Multiply the furnace's input BTU by its efficiency. For example, if you were looking at a furnace specification list, and its listed efficiency is 85 percent and its input BTU is 100,000, you would multiply 100,000 by .85 to get 85,000. This is the amount of output BTU the furnace has.
Compare the furnace's output BTU to the amount of BTU you need for the area. It is fine to have a difference of 10 to 15 percent, but any more than that and you will either be getting too large a furnace or too small a furnace.