Installing a New HVAC System? Tips for Properly Sizing the Furnace

Posted On: February 02, 2014

When you’re getting ready to replace your home’s central heating system, it’s critical that you take the steps for sizing the furnace accurately. HVAC contractors use special load calculation software to start this process. The Air Conditioning Contractors of America’s Manual J is considered the industry standard when it comes to determining proper furnace size. How accurately the furnace is sized impacts your energy bills, comfort, and the longevity of the system.

A furnace that’s too small will run continuously and may not be adequate when the weather is exceptionally cold. One that’s too large will run in brief cycles, reducing your comfort and driving up energy bills. When HVAC equipment first starts, it uses the most energy. When your furnace is constantly turning on and off, your utility bills will be higher. Those starts are also hard on the equipment, hastening wear and tear.

Going by the size of your existing equipment can be misleading, especially if you or another homeowner have improved the home’s energy efficiency. Meanwhile, a contractor who bases the size solely on the cubic footage of your home isn’t considering other critical factors that go into a heating load calculation. In addition to your home’s size and layout, factors include:

  • Its insulation levels;
  • The size, number and quality of your windows;
  • Air infiltration rates;
  • Preferred temperatures;
  • Heat-producing appliances you use;
  • Size of household and age of family members; and
  • Landscaping factors.

Once these data have been input, the Manual J software calculates your home’s heating load, an essential step in the sizing process. Ductwork design and size are calculated with Manual J’s sister software, Manual D.

Another issue when you’re selecting the furnace’s size is the system’s efficiency. A combustion furnace with a higher annual fuel utilization efficiency (AFUE) rating will heat your home better than one with a lower rating. The minimum AFUE available is 78 percent, which means that the furnace wastes 22 percent of the fuel that goes into it. A furnace with an AFUE of 90 percent wastes just 10 percent, which means that more heat goes into your home than up the chimney.

To learn more about sizing the furnace, contact Ernst Heating & Cooling, providing top-notch HVAC services for Troy, Edwardsville and Hamel area homeowners for more than 60 years.

Our goal is to help educate our customers in Hamel, Alton, Glen Carbon, Highland, Greenville, and Troy, Illinois and surrounding areas about energy and home comfort issues (specific to HVAC systems).  For more information about furnaces and other HVAC topics, download our free Home Comfort Resource guide. 

Image courtesy of Shutterstock

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