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Should there be dampers in return air ducts for zoned systems?

dlmckain | Posted in Mechanicals on

The HVAC team just finished installing a zone control panel to split the house into upstairs and downstairs zones. The house was originally built in 1950 with a single heat register and no A/C. It was retrofitted with forced air heat and A/C but the installers did not provide return air ducts for the upstairs. All was well in the winter but we experienced horrible temperature imbalance during the summer when the A/C was on (70F on first floor, 80F upstairs in the bedrooms).

Anyhow, the HVAC team only installed zone dampers in the supply ducts and I was wondering whether they should have put them in.

There are many different scenarios I’ve thought about but the ones that are relevant are when one zone is calling for heating/cooling while the other is not and the stairwell acts as a big “supply” duct where the return air registers in the inactive zone are drawing air from the active zone via the stairwell.

Would it really make a significant difference in the efficiency of the system to put in return air dampers?

It was shocking how poorly educated the HVAC installers were. They are great at bending sheet metal and hooking up traditional equipment but throw a zoned system with multi-staged heating, a variable speed blower, and a humidifier at them and they get confused. They also seem to not understand the importance of sealing ducts.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    David,
    I have never heard of anyone ever installing a damper in a return-air duct. I would advise against the idea.

    The problem you describe is common. Here is an article on the problem: Keeping Cool in a Two-Story House.

    Here are some general observations:

    1. The severity of the problem can be reduced by (a) sealing air leaks in your thermal envelope, (b) improving the R-value of the insulation in your thermal envelope, and (c) making sure that your forced-air system is properly designed (with duct sizes determined by Manual D) and installed (with all duct seams carefully sealed).

    2. The problem can't be eliminated entirely if you have a single heat source and a single cooling source. The standard Texas solution is to install two HVAC systems -- one for downstairs and one for upstairs. The standard GBA solution is to install two ductless minisplits -- one for downstairs (this unit does most of the home's heating) and one for upstairs (this unit does most of the home's cooling).

    3. The easiest solution in your case might be to simply install a ductless minisplit upstairs and use it for cooling.

  2. dlmckain | | #2

    Thanks Martin

    I had read your article "Keeping Cool in a Two-Story House" when I was looking at alternatives for ducting air to/from the second floor. The HVAC team, at the least, did do Manual D analysis to arrive at proper duct sizing and determined that the furnace and A/C were sized correctly. Envelope sealing is underway as we're installing housewrap and additional insulation while we re-side the house and replace all the old double hung single pane windows. The addition we put on the back of the house (and substantial portions of the second floor) are framed with 2x6's and will have at least R-19 insulation + R-6 sheathing so we are covering that.

    Until we go through a couple of seasons (West Virginia, three way cold months over winter, two hot/humid months over summer) I won't know whether the lack of return air dampers makes a difference. It all might work out fine but I wanted to at least pose the question in case there was a significant downside to only having dampers in the supply ducts.

    At least my wife will be happy that the upstairs climate is more controllable now.

  3. Dana1 | | #3

    It's worth bumping up to R21 fiberglass or R23 rock wool from low-density R19s. It's a subtle but real difference, bigger in some ways than the labeled R value might imply, if still small in terms of the penciled out "whole-wall R" that factors in the thermal bridging of the framing.

    R6 sheathing is adequate for dew point control for 2x6 framing in the US climate zone 4 parts of WV, but in Zone 5 it needs to be higher (a minimum of R7.5). If you're in zone 5 but can't bump higher on the sheathing-R, install a smart vapor retarder such as Certainteed MemBrain or Intello Plus under the gypsum. If you're not pulling down the wallboard and are in a dew-point control situation, paint the exterior side of the gypsum board with a "vapor barrier latex".

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