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What Size Furnace Do I Need?

  • September 2013
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When you’re in the market for a new furnace, the first and most important consideration will be the furnace size. Misjudging the size of the furnace you need can lead to higher energy bills, reduced indoor comfort and a shorter life for your new system. Learning how to size a furnace is not a do-it-yourself project and since you’ll need to use a licensed HVAC contractor to install the unit, it’s best to start with a professional who has the tools available to size your furnace accurately.

HVAC contractors use a detailed method called Manual J to accurately size heating systems, since sizing is so important. The software executes a thorough load calculation of your home, and it will determine what size furnace you need based your home’s size, energy efficiency, household occupancy and preferred temperatures.

The software lets the HVAC contractor adjust the factors that contribute to the heating load, so you can learn if increasing your insulation and sealing air leaks would allow you to install a smaller system, which will cost less initially and throughout its usable lifetime.

Installing too large a system results in short cycling, which increases wear and tear, along with insufficiently heated spaces indoors. Too small a system won’t warm your home as well during our coldest weather.

The output from Manual J will tell you how many BTUs (British thermal units) that you need. Besides improving your home’s energy efficiency, you can also choose a system that’s smaller based on its overall efficiency, known as AFUE (annual fuel utilization efficiency).

The AFUE is expressed as the percentage of fuel the furnace uses to create heat versus wasted up the chimney. AFUE ratings run from 80 percent, the least efficient, and go as high as 98 percent. High efficiency furnaces cost more initially but will pay for themselves in reduced heating costs, especially in our climate.

If you’d like to learn more about how to size a furnace, contact the pros at Meyer’s Company, Inc. We’ve provided trusted HVAC services for Griffith, Munster, Highland, St John, Scheriville and Gary since 1951.