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What Size Furnace Should You Choose?

What size furnace will best keep your family comfortable at the lowest operating cost this winter depends upon both the furnace specifications and the unique characteristics of your house. The same unit that’s installed next door might be a bad choice for your household. In fact, the existing furnace in your own home isn’t even a reliable guide when upgrading to a new unit. A professional heating load calculation is the only way to accurately determine what size furnace best matches the thermal characteristics of your house.

The heating load refers to the amount of British thermal units (BTUs) of heat required to maintain a comfortable indoor temperature on the average cold night of the season. Nailing down the precise heating load isn’t a product of educated guesses or outdated rules-of-thumb. Accurately determining proper furnace size is a science these days, but this wasn’t always the case.

HVAC contractors upgrading existing equipment in homes commonly discover that the installed heating system is oversized for the requirements of the house. In this case, bigger’s not better. A furnace that delivers too much output squanders energy, producing BTUs that aren’t required. It short cycles, or turns off and on rapidly, causing abrupt temperature swings. Short-cycling furnaces also have reduced average service life and suffer component failure more frequently.

In the 21st century, heating loads are determined by a qualified heating and cooling professional using Manual J software, the industry-standard load calculation program produced by the Air Conditioning Contractors of America (ACCA). Manual J accepts data inputs about these characteristics of your home:

  • Solar exposure
  • Square footage
  • Number and type of windows
  • Type and amount of insulation
  • Weatherstripping and air-tightness of the structure
  • Number of occupants
  • Large appliances and lighting
  • Landscaping and terrain
  • Local climate averages

Once the data is gathered in a room-by-room survey of your home, Manual J will crunch an accurate heating load expressed in BTUs. Your heating contractor can then consult Manual S, a separate database containing the performance specifications of available furnaces to suggest the best make and model.

For a professional heating load calculation to determine what size furnace is right for your Fairfax area home, contact HVAC Unlimited Heating & Air Conditioning.

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