1. Size by Room Size the duct system room by room. (Note: Oversizing the central unit is a no-no, but up-sizing your ducts by 1 inch or 2 inches can reduce friction.)

Balancing Act

  • 1. Size by Room Size the duct system room by room. (Note: Oversizing the central unit is a no-no, but up-sizing your ducts by 1 inch or 2 inches can reduce friction.)

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    1. Size by Room Size the duct system room by room. (Note: Oversizing the central unit is a no-no, but up-sizing your ducts by 1 inch or 2 inches can reduce friction.)

    Harry Whitver

    1. Size by Room Size the duct system room by room. (Note: Oversizing the central unit is a no-no, but up-sizing your ducts by 1 inch or 2 inches can reduce friction.)

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    2. Provide Return Paths Provide easy ways for air to flow back to the central return grille: under-cut the doors, or install return vents in the walls.

    Harry Whitver

    2. Provide Return Paths Provide easy ways for air to flow back to the central return grille: under-cut the doors, or install return vents in the walls.

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    3. Measure the Flows Commission the system: Measure all register airflows and room air pressures after drywall, with all doors closed.

    Harry Whitver

    3. Measure the Flows “Commission” the system: Measure all register airflows and room air pressures after drywall, with all doors closed.

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    4. Tune the System Adjust as needed to put the system into balance. Consider using registers with technician-serviceable dampers to fine-tune the flows.

    Harry Whitver

    4. Tune the System Adjust as needed to put the system into balance. Consider using registers with technician-serviceable dampers to fine-tune the flows.

A common mistake in heating and cooling design is oversizing—building in more cooling or heating capacity than the house needs, and causing the system to run inefficiently. Of course, under-sizing the system has its own set of drawbacks: nobody wants a house that they can’t heat or cool when the weather outside is at its worst. So “rightsizing” the HVAC, using modern software and following the industry standard Manual J, is one of the signs of a truly professional HVAC contractor.

But even if the system as a whole is perfectly matched to the total load, there’s another design mistake to look out for: a mismatch between the supply and the return sides of the system. This can make individual rooms too hot or too cold, and it can cause leakage of conditioned air to the attic or the outdoors—and yes, there’s an efficiency penalty, too. So here’s some advice on the final step in HVAC design: balancing the system.