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Community and Q&A

HVAC ducts in crawl space

satinder | Posted in Energy Efficiency and Durability on

I am a homeowner who is very much involved in the building of my home. The house is on a 4 ft crawl space. The ductwork for the first floor HVAC is running in the crawl space. Since the ceiling of the crawl space is flat I presumed (my fault) that the HVAC contractor was going to run rigid sheet metal piping. I just discovered he has run flex piping to all the registers and for the returns. I think this is a very poor job since there is going to be so much resistance to the air flow. I would like an expert opinion on this because I have to question the HVAC contractor on his wisdom on doing this and I am going to meet with a lot of resistance. So I need to be able to show him some authentic proof that what he did was not appropriate. I need help fast. Appreciate all responses! Thank you SO much!

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  1. Expert Member
    NICK KEENAN | | #1

    If the system was engineered, and build according to the plans, it's fine.

    In a properly designed system, the designer will calculate the needed airflow to each room, the resistance from each duct, and the size of the fan needed to provide that airflow with that resistance. If the designer included the flexible ducts when sizing the fan, there is nothing wrong with that per se. It's often a good idea to make the last few feet of a run flexible to reduce noise transmission.

    That said -- is your crawl space insulated and part of the heated and cooled part of the house? Flexible duct is often used when ducts are run through unconditioned space because they are easier to insulate, you just buy insulated flexible duct. This is a suboptimal way to do it. However, it's not the installer's fault, he's making the best of a bad situation. If this is the case, post back and we'll talk about alternatives.

    1. satinder | | #2

      Thanks for your response. My crawl space has flood vents since I am in a flood area and therefore it is not insulated. It is vented.

  2. TangleWoodConstruct | | #3

    Is there anything stated in your contract as to what type of ducting is to be used? If so, that may or may not help your standing.

  3. Expert Member
    BILL WICHERS | | #4

    If the project was engineered for flex duct as DC mentioned, it SHOULD be fine IF it was installed properly. That means the flex duct should be installed as straight as possible with a minimum of bends, and the hangers (usually straps) should not be smashing the duct so much that it gets deformed. Many installers just randomly snake the flex duct around, and that's a worst case scenario for airflow.

    Don't ever assume anything with construction. Unless it's shown in the prints, or detailed in a callout somewhere (or a job spec document), it's pretty much up to the contractor, as long as they do it to code.

    If you want a hybrid approach, much of the benefit of a fully rigid ducted system can be head with a rigid plenum with short flex duct branchs to individual registers. This is less labor to install than a full rigid system, and should be cheaper.


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