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PATH Case Study

HVAC Systems: Size Matters


Continued from Page 1

"Renovating this historic home doubled its size from 2,300 sq. ft. to 4,700 sq. ft, yet the energy bills remained virtually the same. That's huge."

"A right-sized system combined with proper air sealing creates a healthier house. Together, the two will maintain a comfortable humidity level, reduce the occurrences of allergy-causing mold and help keep dust and pollen outdoors. The health ramifications are significant."

"This remodeling project resulted in a very healthy, air-tight home. Many green building programs set a target of changing one-third of the air from indoors to outdoors, or 0.33 air changes per hour. I got it down to 0.27 changes per hour in this home. That's a difference the homeowner will feel."


When it comes to sizing the HVAC system, bigger is not always better. This notion defies conventional rules of thumb. It can take some convincing to get contractors on board.

"Air conditioners cool a house first, then dehumidify it," says Seville. "In a humid climate like Atlanta's, this function is particularly important. If an HVAC unit is too large, it will short cycle. That is, it will turn on, cool the house down, and turn off again before removing much humidity from the air. Frequent starting and stopping increases energy consumption, makes the home uncomfortable, and contributes to mold and indoor air quality problems."

Sizing Essentials

Most of the information needed for sizing cooling loads in new homes can be taken directly from house plans. Essential information includes solar gain, which is a function of window area; orientation of the house; window type and glazing (such as low-e, low solar heat gain coefficient, gas-filled); shading from landscaping and building overhangs; and shingle and siding type and color. Load calculations should be done on a room-by-room basis so that ductwork can be sized accordingly. Use ACCA’s Manual D, Residential Duct Systems as your guide.

Read the Tech Spec on ducts in conditioned space.

"So builders, remodelers, and their HVAC contractors need to do more than rely on rules of thumb that suggest a standard number of square foot of living space per ton of air conditioning. Instead, they should match each HVAC unit to specific characteristics, including the air-infiltration rate, of each house."

"Many HVAC contractors don't understand that the system has to be right-sized to dehumidify well. They are afraid of under-sizing the system because they don't want their customers to complain about being hot in the summer. But a house simply won't be dehumidified properly if the HVAC is oversized. We've seen situations where the owners have to keep a house at 65 degrees in the summer to keep it dry."

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Content updated on 9/1/2006

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