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Flexible vs. Rigid Ducts

stolzberg | Posted in General Questions on


We’re putting a ducted heat pump system into a 1900 farmhouse in  northwest Massachusetts.  Contractor is proposing Mitsubishi system with one air handler in basement for first floor and one air handler in attic for second floor and attic. I asked about rigid ducting and was told it would be $3000-$4000 more on top of $36,000 for HVAC system using flex ducting.  Is rigid ducting worth that additional cost?  Would it make sense to split the difference and ask for trunks in rigid with short connections to vents using flex?  What I’ve read on GBA is that flex can be OK as long as it’s installed properly and there should be a lot of straight runs through the basement and attic. How does one make sure it gets installed properly, such as fully extending the duct and not letting it compress inside the insulation.  Thanks for any input.

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  1. Patrick_OSullivan | | #1

    > Contractor is proposing Mitsubishi system with one air handler in basement for first floor and one air handler in attic for second floor and attic.

    How did the contractor arrive at the system design? Did they complete Manual J, S, and D calculations to arrive at their proposal? If not, you have no standard to which to hold their proposal.

    > Is rigid ducting worth that additional cost?

    Ask to see the Manual D duct design showing only flex. It's possible to design a system using only flex, but it'd be a whole lot less efficient than one with mostly rigid.

    > Would it make sense to split the difference and ask for trunks in rigid with short connections to vents using flex?

    Flex is common, expected, and even endorsed by many respected folks between a rigid trunk take off and register boot, provided it's not choked, etc.

  2. JC72 | | #2

    There's a builder by the name of Matt Risinger (Austin, TX) who has used rigid for main lines and flex take offs. IIRC he will sometimes do this in order to reduce noise transmission.

    Flex is fine when installed correctly.

  3. Expert Member
    BILL WICHERS | | #3

    It is very common in commerical buildings to run a big rigid main duct and branch off of that with flex duct to individual ceiling registers. If you open up a drop ceiling in nearly any large commercial building, that's they type of installation you're likely to see. Doing this cuts the total length of the flex duct down dramatically and gets you something of the best of both worlds -- less flow restriction on the main duct where it's most important, and the ease of installation of flex duct for the fussier areas where individual registers/vents are getting installed.

    Flex duct needs to be installed properly, which means minimal bends, not crushed, etc. Rigid ducting essentially enforces it's own installation rules, since it can't really be squished and bent around obstructions the flex duct can.

    See if you contractor can do a hybrid approach, which will gain you some operational efficiency. I'd also make sure you get multiple bids -- that $36k number sounds high to me, but it's hard to say for sure without knowing more details of your project. If your project is going to use a lot of labor because of the way the structure is built, that could be where that money is going.


  4. stolzberg | | #4

    The house is an un-insulated two story 1900 balloon frame farmhouse with real 2x4 wall studs and 2x6 roof rafters. We're planning to leave exterior (cement shingles over wood shingles over board sheathing), remove plaster and lathe on exterior walls, insulate bays, then continuous layer of rigid insulation as air barrier, and add 2x3 service cavity walls (filled with fiberglass batts). Hoping for about R28 in walls and R49 in attic ceiling. I don't know yet if Manual J, S, D calcs were done. Our contractor got the HVAC quote from a sub: he sent me a Wrightsoft load short form with 30,000 BTU heating load and 25000 cooling load for 2000 SF and 1200 CFM (see attached). Basement is unfinished stone wall foundation with cement slab; we're planning to spray foam rim joists and maybe down above grade part of wall. Air handler for first floor and heat pump hot water heater will go down there. We're planning to bring attic into envelope and put air handler for second floor up there. FYI: I'm a total DIYer here, learning everything I can from GBA and other sources.

  5. charlie_sullivan | | #5

    The design document you sent does say the calculations were done per Manual J, but it doesn't actually show what inputs they used for R-values, window areas and U-factors, etc. So it's hard to know how accurate it is. If you look at a sample full report from that software, there are more pages with more information:

    Even for that size, it seems very much overpriced, but I've heard that MA has pretty high prices, so maybe I'm not calibrated very well. It's around double the prices I was seeing for a similar size system in New Jersey recently, although that was without any new ductwork, and was a single central air handler rather then the two here.

    As for flex vs. not, I'd want to see some pictures of their recent installs for flex, so see whether they are doing them well. Or find someone who can do the job including rigid duct throughout for less money.

    As for the rest of your job plans, insulation at the roof is tricky so it might be worth dicussing plans for that in more detail here. Spray foam in the basement makes sense but I advise 1. Specify HFO-blown foam to avoid the high global warming impact of HFC blown foam, and 2. Be really careful to find a good contractor to do that. Even though failures are rare they are disasters when they do occur.

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