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Is attic ductwork a bad idea?

mwgaines | Posted in Energy, Heating & Insulation on December 29, 2006 01:16am

I’m remodeling a 1400 sq. ft. ranch-type home and plan to install my own heat pump and ductwork. I have no experience with residential HVAC, but I’m a hardcore DIY’er and confident that I can do the job with some sage advice.

The house I’m remodeling is a basic rectangle (26′ x 56′). I’ll be installing a 2.5 or 3 ton total electric heat pump. I’m leaning towards a split system, but open to suggestions on that too.

My primary concern at this point is the ductwork. This house has a rather tight crawlspace, so I’d prefer to install the ductwork in the attic where there’s sufficient room to get around. However, I’m uncertain about the efficiency of routing the ducts through that area. The attic gets really hot in the summer. Would that be problematic during normal AC operation? And what about the heated air in the winter? Since warm air tends to rise, I’m wondering if the ceiling registers won’t serve as a massive “thermal leak” whenever the blower isn’t running.

I’d sincerely appreciate your feedback on this.

Thanks,

Michael

Darlington, SC

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Replies

  1. Framer | Dec 29, 2006 01:20am | #1

    Putting HVAC units and duct work in attics is done every day where I'm from. You will have no problems with it if it's done right.

    Joe Carola
  2. DoRight | Dec 29, 2006 01:56am | #2

    mwg:  I am with you.  Attic ductworks makes no sense to me.  The heat in your house does heat your (unconditioned) attic through heat loses, so why compound it by running hot ducts in the attic to just that much easily lose heat to the space.  Sure it is done every day an so are things like spending more money than you make, and boating without lifevests, and . . . .

    AC?  How can running cooled air through a hot attic be efficient?  I could answer that question . . . but . . .

  3. DanH | Dec 29, 2006 02:07am | #3

    I would never do it here (in Minnesota), but I think it makes reasonable sense in SC. You do want to have the ductwork well-insulated, both to conserve energy and to prevent condensation on the outside during cooling season.

    And the "outside" half of the heatpump should be outside the attic if at all possible. (If not, try hard to arrange so that it can draw outside air and discharge outside, vs just sitting there in the attic recycling its own exhaust.)

    How you position the registers (high vs low in the walls) is an entirely separate issue -- has very little to do with where the distribution ducts are.

    Half of the harm that is done in this world is due to people who want to feel important. They don't mean to do harm but the harm does not interest them. --T.S. Eliot
  4. DaveRicheson | Dec 29, 2006 02:18am | #4

    I am sure you have seen this before, but go to http://www.buildingscience.com

    What you are planning is on Joe's top ten list of no no's, but given your limited options it may be your only recourse. In that case a split system is better, since it  places your condenser/heat pump outside of the attic space, as Dan suggested.

    How much room do you have in your crawl space?

     

    Dave

    1. User avater
      BillHartmann | Dec 29, 2006 04:08am | #5

      Actually that is not in the top 10 list.What is is VENTED attics..
      .
      A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

  5. User avater
    BillHartmann | Dec 29, 2006 04:13am | #6

    Look at using a SEALED conditioned attic.

    That brings duct work into conditioned space.

    Here is one example.

    If you the search and also the Technical Resources you will find more information.

    .
    .
    A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
  6. curley | Dec 29, 2006 04:40am | #7

    I live in NW indiana. Building a 28X 32 addition (single floor) and hired  the best HVAC guy in town (didn't care about price) He ran the heat ducts in the crawl space and the return ducts in the attic. My point........I blew cellose in the attic. While the duct work was flexable insulated stuff, I blew cellose over all the ducts (couple inches?)

    Don't know if you're suppose to do that but thats how I delt with it.

    Was reading TOH Magazine last night Tom Silva recomended  14" of cellose in an Illinois house. If  the duct sat on the bottom cord of the truss, then it would be totally covered by 2"



    Edited 12/28/2006 8:44 pm ET by curley

  7. JoeArchitect | Dec 29, 2006 05:03pm | #8

    Running ducts in attics is done all the time here in the midwest. The ducts have to be insulated if they are running in an unconditioned space. The attic should also be vented.

    1. User avater
      BillHartmann | Dec 29, 2006 05:57pm | #9

      "The attic should also be vented."Why?.
      .
      A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

      1. DanH | Dec 29, 2006 06:14pm | #10

        "Should either be well-vented or included in the 'conditioned space'" is probably a better way to state it. Ductwork in the attic can compound wintertime condensation problems due to an unvented (and "non-conditioned") attic, since some air leakage is inevitable. And a hot, unvented attic will add to AC costs, since it's difficult to insulate ductwork as well as a ceiling can be insulated.Getting back to the original question, when you consider various common alternatives for design and construction problems there are rarely "good" vs "bad" techniques. Rather, every technique has advantages and disadvantages, and a tradeoff must be made based on an understanding of those advantages/disadvantages, how they interact with other features and construction techniques used in the structure, and how they serve to satisfy goals of appearance, function, and cost.

        Half of the harm that is done in this world is due to people who want to feel important. They don't mean to do harm but the harm does not interest them. --T.S. Eliot

        1. m2akita | Dec 29, 2006 07:11pm | #11

          "........when you consider various common alternatives for design and construction problems there are rarely "good" vs "bad" techniques. Rather, every technique has advantages and disadvantages, and a tradeoff must be made based on an understanding of those advantages/disadvantages, how they interact with other features and construction techniques used in the structure, and how they serve to satisfy goals of appearance, function, and cost."

          DanH,

          You just got 7 votes from me for the statement hall of fame for your above statement.  Beautifully said.

          Live by the sword, die by the sword....choose your sword wisely.

        2. JoeArchitect | Dec 29, 2006 09:28pm | #12

          DanH, I like your advantages and disadvntages view. I have to keep this phrase ih mind.

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