This Model Code May Limit Flex Duct

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New home installation with sagging flex duct

Some people love flex duct. Some people hate it. Some of us are OK with it if it's done right. As I've documented here numerous times, many flex duct installations leave a lot to be desired. They sag. They're kinked. They're twisted around pipes. If there's something bad that can be done with flex duct, someone has done it. And the result of all those mangled flex duct installs is poor air flow, which creates comfort problems, uses more energy, and is one reason systems get oversized.

A proposal to limit flex duct

Now, some professionals in the industry are pushing for limits on the use of flex duct. The I-codes from the International Code Council seem to get most of the attention, but there's another set of model codes developed by the International Association of Plumbing and Mechanical Officials (IAPMO). The Uniform Mechanical Code (UMC) is their code for heating, cooling, ventilating, and refrigeration systems. As with the I-codes, it gets updated every three years.

Among the changes recommended for the 2018 UMC is this:

603.4.1 Length Limitation. Factory-made flexible air ducts and connectors shall be not more than 5 feet (1524 mm) in length and shall not be used in lieu of rigid elbows or fittings.

That's a pretty strong limitation. If passed, adopted, and enforced, I believe it would have a positive effect on air distribution for HVAC systems. And although that proposal is labeled "Length Limitation," it actually goes further than that by saying you have to use rigid elbows and fittings, too.

At one point, there was an exception for residential buildings to the proposal above. In one of the UMC meetings, however, that exception was struck from the proposal.

A few reasons to limit flex, including firefighter safety

Since I've been writing this blog, I've had a lot of bad things to say about how flex duct often gets installed. I haven't been one of the voices calling for it to be banned, though. I think it's useful product and can work just fine when it's installed properly. But the truth is that building officials probably will never do enough enforcement to make it right.

In the typical flex duct install, you might find these problems:

  • Duct liners not pulled tight. Even if it looks nice and straight on the outside, the inner liner, which carries the air, may not be tight. Even a little bit of longitudinal compression hurts air flow. See my article about the research on sagging flex ducts from Texas A&M.
  • Sharp turns that kill air flow. They're everywhere. They may not all be as bad as the one shown below, but they all hurt air flow.
  • Squeezing flex duct through tight spaces.
  • Flex duct intentionally left too long. Some installers do this to reduce noise from the air handler.

Sharp bend in flex duct

The two ways that air flow gets reduced in duct systems is through friction and turbulence. Flex ducts do worse at both of those than hard pipe. Because of the helical wire running the length of the duct, there's more resistance to air flow. And although it's possible to turn air with flex duct if you have enough space, it's always better to use rigid fittings for that job.

Speaking of that helical wire in flex duct, it can become an entrapment hazard for firefighters. When a building burns and the rest of the flex duct material gets destroyed, the helical wire forms a big tangle that can drop through ceilings. (Search for the word "flex" in this article to see what I'm talking about.) When firefighters enter an area with that wire in the space, it can trap them. Firefighters have been trapped in duct and other wires and have died as a result.

Does anyone use the Uniform Mechanical Code?

Yes, the I-codes get most of the attention but the UMC is certainly used. The biggest is probably the state of California, which has adopted it as the base of their state mechanical code. Other code jurisdictions at various levels have adopted the UMC as well. It's hard to know exactly who and how many. The IAPMO website doesn't seem to give that information anywhere, nor does any other source that I could find online.

But the UMC is certainly in use, so this flex duct proposal, if adopted, could have a real impact.

Got an opinion?

This proposal is out for public comment now. If you want to make your voice heard, go to IAPMO's website and leave a comment. You have until 5 pm PST on 3 January 2017 to do so. You can read the documents available for the 2018 UMC to find out more about it. Just be sure to select 2018 in the little drop-down menu for "Edition to display" near the top of the page.

I'll be submitting my comment this week.


Related Articles

The Science of Sag - Flex Duct and Air Flow

How to Install Flex Duct Properly

Keep Your Elbow Rigid — A Lesson in Flex Duct Installation

The 2 Primary Causes of Reduced Air Flow in Ducts


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Dec 19 2016 - 9:17am

I have to disagree with this. It's a sad day when an organization seeks to limit the use of a single product (with many potential advantages) instead of addressing the fact that the industry they are managing is overrun with bad contractors and installers who are either careless or poorly trained. Not to mention inspectors unwilling to do their part. Should they next limit rigid duct as well because it is to prone to leakage (often a far more damaging problem), the same contractors who can't run flex, can't seal ducts either. Shall we also limit the use of gas furnaces, they are far to prone to oversizing. I could go on, but hopefully I've made my point. Although the firefighter safety deserves to be addressed, I don't think this is the way to go about improving the HVAC industry.

In light of this please tell me they're not still allowing the use of framing cavities for return air.

Dec 19 2016 - 10:19am

This proposal will require actual HVAC design so there's less of a justification to oversize the mechanicals. Also building codes today are more stringent as it pertains to exterior air leakage so leaky duct work will become less of an issue. Besides how much hard pipe are we talking about here? Maybe 25 ft in total for a typical home?

Tract builders will of course fight this and some states will probably never adopt it.

Dec 19 2016 - 11:19am

Andy, I think you are after the code change proposed probably by an inspector or city (don't know for sure) that address's an observed problem. Basically this should be resolved by the manufacturers and the contractors but you are pointing out that the only regulation is inspectors. Inspectors do not know everything and are probably not current in installation standards on many product so the problems appear when one inspector tries to enforce proper installation. This is a construction industry wide problem since very few material manufacturers provide a training manual for their product and depend on the government to insure proper application methods. All construction related people need to up their game.

Dec 19 2016 - 10:35pm

I don't mean to be throwing inspectors under the bus but they are the only required third party check we have. I'm saying the code should be looking at requiring better design and commissioning instead of trying to micro manage everything in between. If contractors are held to a performance standard the flex duct issues will start to sort themselves out. I've seen this happen with Energy Star version 3.

W. Blake
Dec 19 2016 - 10:26am

A hard duct system designed for a specific residence is definitively the most energy efficient solution.

However, insulated flex duct is very beneficial in many ways if properly installed especially when applying to existing conditions. Minimizing usage to comply with commercial standards resolves some problems but adds cost to the project.

One recommendation to minimize flex duct kinking (air restrictions) is to require duct saddles based on duct size.

Dec 19 2016 - 11:12am

Din,ding, splits end all this bureaucratic suffocation. bty...I bet the steel industry love the proposal.

Dec 29 2016 - 8:15pm

With modern design trends, The "window unit built into a wall" look probably won't fly with too many people.

Dec 19 2016 - 11:27am

I felt the need to clarify my comment above with something less sarcastic. I feel this would set a precedent. That the code, instead of setting minimum design criteria or requiring commissioning to show a system is working properly, would attempt to limit available options until we're left with only that which contractors and inspectors (hopefully) can't screw up. Is this really what we want?

Dec 19 2016 - 11:30am

Makes great sense - flexible ducts are "absolutely" not installed right. Kinked where they are tied to framing, stuffed into hard to get places, etc,etc

Dec 19 2016 - 1:43pm

I would much rather see a performance standard based on leakage, total static and air balance.

I tell contractors if their duct system meets my performance specs, I don't care how ugly it is. In reality, a poorly installed system won't make the grade.

This approach not only forces bad actors to clean up their act, but solves a myriad of design deficiencies. When it comes to regulating proper duct design, those Manual D submittals aren't worth the paper they're printed on.

Dec 28 2016 - 3:37pm

I was hoping someone would comment on performance. We should concentrate on the measured result, not how we got there.
I have seen well designed, high performing systems that rely on flex ducts. The bottom line is measure the results and force the industry to do a better job as a whole. The ducts are not the problem it's how they are used and poor "best practices" that are to blame.

Dec 19 2016 - 2:38pm

As you can tell by the pictures for this article, it doesn't take a journeyman to see the issues. If we as an industry check our installations to prevent problems, then someone will attempt to address it by code changes. Installation instructions are included in each box of flex duct, but not read. As mentioned, if installed properly, pulled tight, supported correctly, proper radius, sized properly using correct friction rate and limited restrictions, flex duct is an approved material without noticeable differences. 5' limitation is extreme though.

Dec 20 2016 - 12:51am

I would much rather use more measurement and verification than see more restrictive codes. That goes for all home performance. Its the wild west in many regions.

Dec 20 2016 - 3:53pm

Just because some do not install a product correctly is not a reason to deny others the ability to use it. Test at the National Labs in TN several years ago proved that a properly installed straight run of flex duct had within 2% of the airflow of hard duct. However, I agree that it should only be used straight where it can be pulled tight, turns should be hard fittings.

Dec 21 2016 - 12:27am

...and Chris VanRite will be discussing flex duct in Las Vegas at one of the free ASHRAE seminars being held at the AHR event. Allison will be taking on the hard duct challenges and yours truly covering the original unducted system hydronics [;@) Three presentations in one hour - it's a Ted Talk for techies...

Dec 21 2016 - 10:03am

The performance metrics for distribution systems should include more than just air flow and leakage rate, it should include delivered BTUs, and noise criteria. While it is possible for installers to insulate all metal duct systems well, it is a rare occurrence, which measuring delivered energy confirms. Even in systems that are well insulated we air systems with 90F degree air, and more commonly 140F air, which leads to substantially different heat loss through the same insulation. As building get more architecturally complex, we need more tools, not less in our tool bags to achieve high performance outcomes. I vote that if we are concerned about outcomes that we check those, and let every one get there how ever they want. (Add what ever out comes you want to the list: saftey, IAQ, comfort)

Dec 25 2016 - 10:25am

We live in South Carolina and are plagued by poorly installed HVAC systems including bad flex duct installs. Multiple mini splits cost too much to be viable in all situations. I approached a builder to install the air handler in a mechanical room stolen from a walk in closet, utilize plenum trusses so that the ductwork would be inside the envelope and to use insulated hard ductwork. They would not entertain the thought because it was "different". I was willing to pay extra and told him that this type of change could save 20% on energy costs. Still not interested, even as a "green" construction company.

Dec 25 2016 - 11:26am

Municipal inspectors have no business inspecting period. To much corruption. Most 3rd party inspectors are legit and do their job. The biggest problems are poor installation, lack of proper training and contractors being beat up by the builder. Commercial is much better as cfm checks per room, based on manual d's are required typically to get paid. Most Residential is how fast and how cheap and what can we get by with verses doing it better. I prefer hard duct but there are situations where flex makes sense. Most use trunk and flex because it much faster on job site. The other issue is time from installation vs system startup. Insulation contractors are about the same and fast and cheap vs quality installation. They don't typically care about smashing, cutting or holes in your insulation. Energy Star version 3 and above has helped. Truely in the end it's really up to the contractor and builder to make sure it's load calculated and installed correctly. Both during installation, duct blast testing before insulation and Sheetrock. Then duct blasting at unit startup to test again for leakage and cfm distribution for each room. Once that in mandentory things won't change much. In the end best practice, common sense and testing to verify make most sense. One has to overcome the price per ton method and budget restraints first to design and install a good system first.

Dec 31 2016 - 4:14pm

thank you all for the info regarding flex duct, I'm GC building remodeling a house with a new addition in W. Seattle. Old house, limited joist space in old and weird workaround parameters to get all things remodel to fit - typical of a remodel. Im using a trunk and branch design in the old house and a "redial" design off the end of the extended plenum entering the addition. Thats a 16 x 20 x 60" box with 5 supply's off it(2 8" and 3 6") I like the idea of using hard 90's in the flex duct for better, less restricted flow and following the install tips for flex is quite helpful.
Its hard to justify the additional cost for rigid duct in the addition over flex so I went with flex.

Following the idea of not installing supply's in small rooms (ie: closets, laundry) Im wondering if a wall transfer grill or a ceiling jumper duct from a master bedroom to a master closet is a good solution to another supply.

Thanks for all the info, appreciate the help!


Jan 4 2017 - 8:26am

Flex duct is really an abomination. In addition to the installation and airflow problems mention in the article, flex degrades over time, can be torn by rodents and by poor installation, catches and hold dust (static prone plastic & in the folds) and is next to impossible to properly clean. is really, really cheap to install.

Mar 16 2017 - 2:35pm

Thank you Bill for saying it.

Feb 8 2017 - 3:09pm

Flex duct is a disaster. What most people don't realize is that flex duct cannot be cleaned - EVER! Mold is becoming THE most dangerous consequence of the sustainability movement. Once mold spores get into the duct system, the spores are spread around the home or building until addressed. Mold grows on the insides of duct work quite commonly. The food source is dust, condensation is often the source of moisture. Even well maintained systems will require the ducts to be cleaned a regular basis (every 8-10 years)to maintain healthy air quality. Flex ducts can only be replaced. Air quality is critical for occupant health. Since flex ducts cannot be cleaned it is ridiculous to allow them to be installed anywhere replacement will be difficult or impossible.

I am finding the use of HVAC systems during drywall work to be an increasingly common problem also. This results in ducts full of toxic silica for eventual occupants to breathe on a daily basis. When home and building owners start to realize the potential liability and danger, there will be even more incentive to change. In any case, this change to limiting flex duct cannot come soon enough!

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