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Am I being too picky about flex duct? What am I not understanding?

holleecee | Posted in General Questions on

When shopping for a new heating system, several companies recommended tying into the current duct work and going with a heat pump instead of one-to-one minisplits. It’s a 1200 square foot house built in 1922. Currently there are fissures in both of the return lines that were likely installed sometime in the 50s or 60s and two of the register lines are disconnected from the boots leading to our main floor and instead conditioning our basement space. (This isn’t intentional.) I’m calling reputable companies that are all giving me conflicting results.

One company recommended a 3-ton system and said they could patch up the fissure. I did the cheater manual J calculations based on my energy use and this was completely overkill so I’m insisting on a 2-ton system.

Another told me that we didn’t have enough registers or the proper sized duct work to handle a 2-ton set up let alone 3-ton. I appreciated this information and we’re contemplating where we might add a register but meanwhile…

I’ve also been told that “no one does rigid ductwork.” And when I asked about manual D, I was told “I’m old school and go by square footage.” Oh.

When I asked the last guy I talked to if he would design the duct work using Manual D, he said, “Yeah, yeah.  We can add manual diffusers so you can close the vents.”

I’ve read up on Allison Baile’s duct work guidelines here on GBA and in other locations ( and when I asked the company (with all of the reputable technical credentials) that I trusted the most so far about using rigid fittings to navigate bends and turns, I was told flat out no.

Attachments don’t seem to be working right now but I’ll eventually attach pictures in the comments of the current system that is “fine” according to the 5 different folks that have been out for an assessment so far. One of the lines is 10 inch flex duct coming from the trunk line at a roughly 90-degree bend then attached to a boot reducing it to 6 inch that has a few different angles that connects to 10 feet of 6 inch uninsulated rigid duct.

Am I overthinking this for our small old house? Should I just get my own Manual D assessment and DIY? I really don’t think I’ve got those skills but I’m questioning the $8000 price tag for duct work that is essentially a repeat of the same system.  I was thinking that all these bad angles would overwork my fan and cause weird cycling for the future heat pump. Am I wrong?

*I’ve attempted on two different computers and my phone but still can’t get jpegs, pdfs, pngs to upload even at 1MB.  Hopefully they will load later today to help demonstrate the situation.

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  1. user-5946022 | | #1

    You have perfectly described the problem. The industry just does not want to install a proper system. There are companies out that that will do it, but they are few and far between. I'd look for passive house builders in your area and figure out which HVAC companies they use...

  2. Chris_in_NC | | #2

    This is the usual problem when business is good. There are plenty of customers who want the normal cheap-and-easy installation types, and your local HVAC people aren't interested in deviating from that because they don't have to. They'd rather you find some other company that will spend the extra time trying to calculate time and materials for something out of the ordinary, etc...
    As another example, we have more than a few local electricians with multi-truck crews that no longer do anything but service calls (replacing switches/outlets/GFIs, replacing ceiling fans and fixtures, etc) and won't quote new circuits or panel replacements, etc. They've done the math and know where the profit is, as long as the volume is there.

    1. holleecee | | #3

      Fascinating. I actually had that experience trying to find an electrician when I tried to get a quote on a new panel.

  3. Patrick_OSullivan | | #4

    Without specific pictures and layout info, it can be tough to appropriately contextualize feedback. (Sorry you are having trouble posting photos. I think we all have technical frustrations with this site from time to time.)

    The responses you've gotten are, in my experience, typical for residential installers who do their own "load calculation", "equipment selection", and "duct design". I put all this in "scare quotes" because these companies are typically not doing any of those things, and are, as one directly told you, deciding what to install based on a rule of thumb.

    One option is to hire a company to do your load calcs, equipment selection, and duct design. Then you take those specifications and ask installers to bid on installing to the specification, with little wiggle room. Residential only installers will still disappoint you in this area, because it is not their norm. I recommend finding a company that does commercial work (where they are used to following a plan/design) that also does residential.

    A company like this will not balk at using rigid fittings. As an example, the company that did the install of one of the systems in my own home showed up with a fitting that didn't quite work as they expected. The remedy? They called their duct shop with updated dimensions and a custom part was delivered later in the day.

    Did they do a perfect job? Of course not. Where there was flex duct (e.g. from trunks to register boots), I had to remind them about straightening the runs, removing slack, etc.

    Unfortunately residential construction without a competent GC/intermediary is a minefield of issues like this.

  4. Expert Member
    BILL WICHERS | | #5

    Flex duct saves lots of labor, but also offers much higher resistance to airflow, making for a less efficient system. Flex duct is often poorly installed too (wonky and/or tight bends, squished at overly tight hangers, etc.), which adds to the problem.

    There are plenty of installers out there who will still install rigid ductwork, but they're likely all geared towards the larger commercial projects. Standard commercial practice is a rigid duct plenum, with short runs of flex duct to go from that plenum to individual diffusers in a drop ceiling. In this situation, the flex duct (which tends to be large 8+ inch stuff) helps to quiet down the system, and also allows for connection to the somewhat randomly located diffusers that have to fit into the ceiling grid openings.

    I'd try calling around to some commercial mechanical contractors and ask if any of them would be willing to do a residential project. Many/most will probably just say "no" (lots of good reasons for that, BTW), but a few might be OK with it -- but expect to pay a premium for their services. You'll get a better performing system, but you'll pay extra for it.

    Pretty much all contractors are swamped with work these days, so they can pick and choose what jobs they want to do. That makes it difficult to get anyone to do anything extra.


  5. walta100 | | #6

    When you are searing for contractors to bid look for ones that are named “XXX heating and sheet metal”

    They are likely to be long established companies named before 1970.


  6. holleecee | | #7

    Thanks for the search term suggestions! I was stuck on the term "HVAC ductwork" but just found the sheet metal workers local union page and a listing of residential/commercial HVAC installers. Feeling hopeful!

  7. acrobaticnurse_Eli | | #8

    I don't know if there is anyone on the list near you, or if they would be in your price range, but I came across this directory of building science oriented HVAC professionals, along with an offer for doing duct design/calculations:

    This is the direct link:

    Energy Vanguard can also be consulted here:

    Depending on how much you need done and how handy you feel, it could be an option to do some ductwork yourself, but if you can find a good professional to do it and can afford their service it might be worth having them do it. This video covers a bit of advanced DIY ductwork:

  8. 4khz | | #9

    I met with about twenty hvac contractors before finding one that I was comfortable with.

    Half of the contractors did not believe (and did not bid) that my proposed 3/4 t + 3/4 t + 1 ton ducted mini splits would be sufficient for a 2,800 sf two level.

    Most contractors that did bid proposed flex duct although I had requested rigid. They rebid with rigid and the prices came in too high.

    Finally found a contractor willing to work with me - and they addressed all of my concerns during the project. Fair price (here in hudson valley, NY) @ $32k

    Finding the right contractor took a lot of time and complicated my job/ sequencing. I read here on GBA that it was tough. I didn’t realize it would be THAT tough !

    Btw - I did my own manual j and duct design. My contractor ended up doing their own via a third party. It was not done correctly. Contractor reverted to mine. I did it using the betterbuiltnw online program. I didn’t know what I was doing, but after 40 hours or so, I felt confident that is was fairly accurate.

    What a learning experience !

  9. LukeInClimateZone7 | | #10

    Airflow airflow airflow
    There's nothing inherently wrong with flex duct
    Sure, for a given diameter, flex duct will have a higher friction rate than rigid.
    Fine. So long as the system achieves proper airflow then what's the actual problem?
    To me, this seems to be confusing means and methods with outcomes. If we can achieve proper airflow then I don't care if the ducting is PVC, ABS, flex duct, or building cavities.

    Mo'st of my projects avoid flex duct because it's too expensive relative to sheet metal and duct board.

    1. Patrick_OSullivan | | #13

      > So long as the system achieves proper airflow then what's the actual problem?

      Needing larger equipment to meet static pressure requirements to overcome frictional losses. Those frictional losses are made manifest with increased noise (because higher velocity air is needed to overcome them) and overall wasted energy.

      The extreme example is a high velocity system like Unico or SpacePak. Useful for retrofits when there is no other option, but they are noisy and less efficient.

      I've had both and I will never go back. My ducted Fujitsu mid-static unit is phenomenal.

      1. LukeInClimateZone7 | | #14

        Im not suggesting upgrading the equipment to meet higher static. Design the system with flex duct and hit the same static.
        Big ducts, small fans!!

  10. paul_wiedefeld | | #11

    Just a clarification: a mini split is a heat pump. A heat pump can be ducted, non-ducted, or both, but the system is the same: a blower blows air over a coil of refrigerant. You can find ducted “minisplits” from every manufacturer. Sometimes, lazy contractors will call a wall mounted unit a minisplit and a ducted unit a heat pump, but that’s incorrect.

  11. walta100 | | #12

    Paul when a unit gets labeled “Mini split” I think it is an Asian designed heat pump with variable refrigeration flow. But it is very muddy waters as US manufactures are happy to slap their brand name on equipment designed and made in Asia and Asian manufactures are assembling equipment in the US now.

    The mini part is not true in terms of BTU capacity but yes, the Asian outdoor equipment has a narrower foot print.

    The split part is no any different than any US made heat pump.

    The variable refrigeration flow is not unique to Asia every US manufactures top line HP has variable refrigeration flow.


    1. paul_wiedefeld | | #15

      So it has no meaning! Those same Asian manufacturers also make heat pumps that look more “traditional” too. Just a term to cause confusion.

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