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Does a properly installed zoned forced air HVAC system required dampers in both the supplies and the returns?

David Adams | Posted in Mechanicals on

Our installer has so far only installed dampers in the supply ducts. Wouldn’t this cause some pressure imbalances in the off zone rooms when the blower is running?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    David,
    Designing a zoned forced air system is tricky. If too few zones call for heat, the furnace can be damaged because there isn't enough air flow to remove the heat produced by the furnace. Similarly, in summer, if too few zones call for cooling, the indoor coil can freeze for want of airflow.

    That's why most zoned forced-air systems include "dump zones" -- often a garage or a basement -- where excess heat is dumped in winter, and cool air is blown during the summer. Not exactly energy-efficient.

    In short, I advise against installing a zoned forced-air system.

  2. David Adams | | #2

    Martin,
    Thanks for your response. We have added a second story addition. The new construction has much better insulation than the original home. We have two zones one for the new second level and original square footage. Each zone is over 1200 square feet. We have always had problems with temperature differences between the main floor and basement. Adding the second story addition without zoning we felt would have made the temperature differential problem even worse. That said, my original questions is still do dampers need to be installed in the return ducts for the zoning to function properly?

  3. GBA Editor
    Martin Holladay | | #3

    David,
    My understanding is that, when a contractor creates a zoned forced-air system, motorized dampers are installed on the supply ducts, but not the return ducts, as you point out.

    Furnaces are designed for certain minimum air flows, which is one reason that the return ducts are not restricted by motorized dampers. These systems tend to involve technical compromises, and sometimes a work-around like a dump zone is necessary to prevent damage to the equipment. Be sure you understand your contractor's proposal before agreeing to the installation.

  4. Aj Builder, Upstate NY Zone 6a | | #4

    Another alternative that works well.

    Bypass duct with weighted damper. Damper opens to keep flow up through system. No dump. Also in my last two zone system, each zone is sized large enough for system, yet still ok when both zones call for use at the same time.

    No controls/dampers in return.

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