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improper sized ducting

Jebadia | Posted in Energy, Heating & Insulation on November 27, 2008 06:24am

Hello all,

I have an old house with a newer high efficiency furnace. They didn’t change out the ducting from the old gravity feed furnace. I have some ducting that is like 20″ dia. with about a 15’run. The smallest is about 10″ dia.

I am thinking that even flex dust would be better than this. The basement stays relatively cold and I have to be loosing allot of heat just heating the ducting. Let alone trying to push the air through this ducting.

I am considering( because of my limited budget) using insulated flex duct. Elbows for corners and stretched fairly tight. building a new plenum of course. I am also open to suggestions.

Thanks,
Jeb

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  1. DanH | Nov 27, 2008 07:00pm | #1

    If the basement is cold you aren't losing that much heat through the ducts.

    Your main savings in redoing the ductwork would be to gain space in the basement.

    The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel
    1. tab1 | Nov 28, 2008 05:39pm | #4

      <you aren't losing that much heat through the ducts.>I would have thought the opposite--or maybe it depends on how much 'that much' is. <g>My heating and cooling guy recently did a whole house analysis for me using his new updated energy program which allows for very specific construction details. He found that 8 inch metal ducts in an unheated dirt floor basement of a small house, were losing about 15-20% of the heat. We're ripping them all out and replacing with insulated flex duct.YMMV

      1. user-253667 | Nov 28, 2008 06:44pm | #7

        I think I would be looking for a new heating pro.

        Why not insulate the existing ducts.

        Flex duct does not blow, mainly it sucks at moving air efficiently.

        1. tab1 | Nov 28, 2008 08:21pm | #10

          <Flex duct does not blow, mainly it sucks at moving air efficiently.>Rather broad statement. What I read tends to indicate that flex INSTALLED CORRECTLY and correctly sized moves air nearly as efficiently as steel duct. Some sources state flex is the most common type of duct being installed in the U.S. Does that make it the right thing to do, or the best? No, but.....Lots of reading here:
          http://www.mmmfg.com/Default.aspx?sPageName=services_pdfAnd remember, that's from a steel duct manufacturing website. <g>In my case the ducts are already oversized, so apparently no need to make the flex larger. This HVAC guy is VERY science oriented, ALWAYS does load calculations, measures static pressures, etc. I believe he's in this for the long haul, and not out to make a quick buck on me, or anyone else.Insulating the ducts I have might well cost more than installing flex and in addition to ending up with ducts not as well insulated as the flex, they'd still be oversized.Although heat loss from ducts in basements that are reasonably well sealed--which this one is not, yet--may not end up 'outside', that heat is not being directed to the house occupants. It's primarily heating the basement, which is not the goal. I tend to put more trust in a professional heating and cooling computer program that is designed to measure these parameters, that I do in seat of the pants guesses.As to Jebadia's question: Were it me, I'd get a professional to do the calculations (probably for free), and go from there.

          1. user-253667 | Nov 29, 2008 12:18am | #11

            You  are correct. Probably too broad a stroke but something about flex seems to bring out the worst in many of the people installing it.

            Any product, properly applied as per best practices or manuf. will serve its purpose.

            I'll still take metal insulated duct first,second, and last though ;)

          2. Clewless1 | Nov 30, 2008 06:31am | #18

            Yes, but the purpose of flex duct is NOT to be used as a replacement for rigid. It is the occasional situation where rigid doesn't work or is very difficult. In commercial construction, it allows connection to a diffuser while still allowing the register to be readily relocated down the road (and limited to SHORT runs of a few feet). Even then it tends to be over used.

            Well installed, flex will still result in loss of static pressure that you don't want to happen.

            Flex is the lazy mans way of not doing good work. Just say NO ... you already did. Now everyone all together ... "no flex duct".

          3. user-253667 | Nov 30, 2008 07:34am | #19

            Okay, you are absolutely and utterly right. just trying to be amenable and pleasant.

            Better to reroute, extend and then insulate properly real ductwork,as opposed to using  flex every day of the week. Certainly no flex for me or my clients.

            I like someone else's comparison to vinyl siding though.

            I do neither. IMVHO flex duct and vinyl are utter rubbish, intended to cheapen and degrade the great tradesmen out there.

            In trouble again, I suspect. Anyone for radiant?

            Edited 11/30/2008 12:37 am ET by s crough

          4. namenotinuse | Nov 29, 2008 12:39am | #12

            i still see nothing wrong with oversize ducts, and i also don't see how insulating the ducts you have would cost more than removing existing rigid metal ducts and properly reinstalling flex ducts.

            most of the cost of sealing and insulating the existing ducts is in the form of labor. the kind of labor that doesnt require facy computer programs or testing equipment. in fact in can be easily self performed as i outlined, and with a couple of conscientious helpers/laborers to help it could be accomplished in a weekend.

            it isn't rocket science, you just have to be motivated to do a quality job. you may get dirty and itchy, but proper protective clothing minimizes this.

          5. seeyou | Nov 29, 2008 01:11am | #13

            you just have to be motivated to do a quality job.

            Which is one of the biggest downfalls of flex duct. It's the vinyl siding of the HVAC world. If done right it works OK. http://www.quittintime.com/      View Image        

      2. DanH | Nov 28, 2008 07:24pm | #8

        Thing is, so long as the basement is reasonably well sealed and not open to the outside, any heat deposited into the basement from ductwork serves to offset heat that otherwise would reach the basement through the floors. It's not like the heat that conducts through the ductwork is being blown outside.You would, of course, want to seal any air leaks in the ductwork, but that's a fairly simple task.
        The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel

      3. namenotinuse | Nov 28, 2008 07:28pm | #9

        why can't you just insulate the metal ducts? they sell insulation for just that purpose, it is not very thick and about 9 inches wide or so.

        what you do is take the big rolls that are about 3 feet in diameter and make smaller rolls that are tightly compressed. about 6-7 inches in diameter. then stab a 16 penny nail in the roll to keep it from unraveling.

        you then use these small rolls to wrap the ducts and use the nails to pin them in place. it is very labor intensive, but it is cheap. you can double wrap for extra insulation, and maybe even add some do it yourself spray foam for the tight spots. there is also a spray adhesive made for this purpose, to help you stick pieces where it doesent want to stay.

        flex duct is great, but it does reduce/restrict airflow from the same size metal duct (as long as the metal duct doesent have lots of unneccesary 90 degree turns and bends in it)

        so if you are ripping out metal duct you should replace with larger flex for the same airflow. a good system can be made with either flex or rigid or a combination. if yours works ok but is uninsulated i would just insulate. 

  2. DanH | Nov 27, 2008 07:02pm | #2

    But likely the original setup had no return ductwork to speak of -- has that been added, or is the furnace just drawing air from the basement? Probably the return ductwork needs more work than the supply ductwork.

    The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel
    1. Jebadia | Nov 27, 2008 07:11pm | #3

      The returns are 2 risers in the stairwell one on top and one on the main level. There is about a 8-10" x 24" in the floor on the main level that runs on 2 sides of one wall. So yes the returns could use some work. The supplys are on interior walls not exterior. The returns are close to the exterior. It is a turn of the century brick house no insulation in the walls as they are about 8"s thick of brick.Jeb

  3. Clewless1 | Nov 28, 2008 06:32pm | #5

    Oversized ducts mean lower air velocity ... that isn't nec. a bad thing. More heat loss ... but if your return air is into the basement, about a wash. The other guy says you will gain space ... right ... might be wanted if you redo the basement.

    Whatever you do ... say NO!! to flex duct!!! Flex duct is overused and oversold. You want to do it right, use rigid duct. You want efficiency ... then don't use flex duct ... I'm not referring to heat loss efficiency, but that of moving air.

    Don't push a panic button too quickly ... doesn't sound like you really have a [major] problem.

    1. user-253667 | Nov 28, 2008 06:40pm | #6

      I could not agree more.

      Flex duct is not even suitable for dryer vents.

  4. User avater
    rjw | Nov 29, 2008 01:54am | #14

    So far nobody has mentioned the differences between location of supplies and returns in old gravity set-ups versus 'newer' forced air systems.

    with the old gravity systems, the supplies are/were as short and straight up from the furnace as possible -warmed air doesn't much 'like' moving horizontally - the returns tend to be from along exterior walls, although that's not as much a matter of necessity as he routing of the supplies.

    The exact opposite with forced air: the supplies branch out and enter rooms along exterior walls (warming them and increasing the perceived warmth of the room -- cold walls make you feel colder than warm walls even where air temps are the same.)

    Any plan you come up with should account for the fact we can move heated air pretty much where we want it - and aim for the exterior walls.

    And having someone trained in designing ducts and calculating air flows etc will help immensely.


    "Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."

    Howard Thurman

    1. seeyou | Nov 29, 2008 02:01am | #15

      Good point. I just assumed the new ductwork would extend to where it should be. Bad assumption.http://www.quittintime.com/      View Image        

    2. Jebadia | Nov 29, 2008 04:47pm | #16

      Thanks for all the input and please keep it coming. The runs are all pretty short. The longer the run the bigger the ducting. My feeds are all on interior walls and I kind of like the old big defusers. Maybe I will have to sacrifice those but my options because of the brick structure would most likely require using floor registers which I really dont like. I will have to call a professional but I like to do my homework prier to. I want to make sure that they know what they are talking about.As far as the install I am capable. If I knew how to do the calculations I might be able to design something suitable myself.Presently I have a limited budget which is why I was considering going with the flex duct for now. Anything to keep the heat producing better than the last season. I actually wrapped the duct with fiberglass and it doesn't seem to have made much of an effect.I just find it hard to believe that a high efficiency furnace can be set uo properly with old duct work.Jeb

      1. DanH | Nov 29, 2008 05:00pm | #17

        The main problem with the old ductwork is that it isn't distributing the heat evenly. This wasn't as much of a problem with the old furnace because a) a lot of heat escaped from the furnace in the basement and warmed the floors, b) the air velocity (and hence balance between registers) was different, and c) you didn't expenct as much from the old furnace.Replacing the basement ductwork without changing/adding register locations is not likely to significantly improve comfort or efficiency.
        The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel

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