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Fan coils and return air from framing voids

user-2094615 | Posted in Mechanicals on

Dear Advisor,

I’m a home inspector working in the Chicago area and recently I was tasked with inspecting a three-bedroom condo on an upper floor of a newly constructed high-rise building downtown. The fan coil units are located inside alcoves behind hinged metal fire rated access doors. They are in an interior framing void space approximately 3 ft.² and about 10 feet tall. The return air grilles are located in the ceiling but there is no ductwork; when the fan coil air handler kicks on it draws air through the return air grille, swirls it through framing voids in the walls and ceilings (some of which are visible and some of which are not), draws it across the standard 1 inch “rock catcher” filter and then across the coil.

Now, common sense tells me that this is just plain idiotic and I recall reading one of Uncle Joe’s articles a while back which echoed my sentiments on the subject. In response to my report, the builder says it’s standard procedure and was approved by our fair city during the design process. I still find it completely idiotic. I’m looking for some type of standard or code which this installation contravenes. Your help is greatly appreciated.

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  1. user-2094615 | | #1

    I forgot to add that there are fiberglass batts in the wall studs within these alcove/framing voids.

  2. user-2094615 | | #2

    Here's a photo looking upward from inside the alcove in which the return air passes on it's way to the fan coil. The open sheet metal rectangular box on the right is the return air grille. It just dumps into the alcove.

  3. user-2094615 | | #3

    I mean here!

  4. GBA Editor
    Martin Holladay | | #4

    You're right -- using stud spaces as return air ducts is idiotic. Believe it or not, however, it is still legal under the 2012 IRC.

    The space you are talking about might still be in violation of the code. The relevant requirements can be found in Section 1601.1.1-7 of the 2012 IRC. The section notes several limitations, including this one:

    "Stud wall cavities and the spaces between solid floor joists to be used as air plenums shall comply with the following conditions: ... Stud wall cavities and joist-space plenums shall be isolated from adjacent concealed spaces by tight-fitting fireblocking in accordance with Section R602.8."

    Other limitations include: "Stud wall cavities shall not convey air from more than one floor level" and "Stud wall cavities in the outside walls of building envelope assemblies shall not be utilized as air plenums."

    So, this may or may not be a code violation. In any case, it's stupid.

  5. user-2094615 | | #5

    Thanks MH. One would think that there would be a sanction of some sort against recirculating fiberglass insulation fibers throughout the home via the FCU's but that old comic strip used to say, 'there oughta be a law'....I reckon in this instance there isn't one yet. Much obliged.

  6. charlie_sullivan | | #6

    Is that insulation on an exterior wall? If so, the "wind" washing of it by the ventilation air is reducing its effectiveness, drastically. As I understand it, that's not against code, but it might be against other energy standards if there is anything like that intended.

  7. user-2094615 | | #7

    No, it's sound attenuation on the interior partitions.

  8. iLikeDirt | | #8

    Even if it's legal, can't you still point out that it's stupid, reduces the efficiency of the system, and possibly harmful to health? Your clients are paying for your judgment as well to your knowledge of the relevant building code.

  9. user-2094615 | | #9

    Here is the pertinent excerpt from my report.

    The fan coil units in the subject condominium utilize wall and ceiling framing voids for return air intake/ducts. This creates a significant risk for contaminants being introduced into the conditioned air of the home. As a general principle, HVAC ducts should be designed and installed so that occupants have near-total control of airflow in the home. The current configuration of the return air "ducts" falls far short of this best practices concept. In addition to dust, dirt, and fiberglass insulation, (fiberglass has been deemed to be carcinogenic by the state of California) using framing voids as return air pathways will result in air being drawn into the HVAC conditioned air stream from concealed areas of which little or nothing is known. This is a poor practice in so far as indoor air quality and energy efficiency are concerned. In addition, the return air intakes are partially blocked by the supply ducts and may not allow adequate return airflow or may result in excessive wind noise during operation of the HVAC equipment. It is strongly recommended that the seller address these concerns in a logical and professional manner and that the mechanical contractor provide the buyer with documentation showing that the systems as designed conform to industry standards, the manufacturer's installation instructions, the city of Chicago energy conservation code, and all other applicable guidelines.

  10. JC72 | | #10

    I wonder what the prospective buyers will decide?

    I suspect it will go down something like this:

    Buyers, "Our report came back with a few issues that need correcting"

    Developer, "The City of Chicago signed off on the design. Do you think we would've built this project without proper permitting? Tell you what, how about if we add a HEPA filter? That's all we're willing to do."

    Buyer, "Not acceptable."

    Developer, "Look, if you have a problem that's fine, we have a long list of buyers waiting in the wings so our escrow company will refund your deposit. Have a good day".

    Buyers, "No, we'll buy w/out the improvements."

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