Flex vs. Rigid Metal AC Duct - My preference may surprise you!
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- Опубліковано 8 вер 2024
- Building Science Geeks love to hate on flex duct in HVAC systems. They take pictures of terrible flex duct installations in crummy attics and link to this TAMU article which says that if you crush it, let it sag between joists, and force 3X the recommended CFM through flex duct, the result is a drop of pressure in the system ( oaktrust.libra... ).
Well, duh! Any product, when used incorrectly, will perform poorly.
I think the take away should be how small of a pressure drop there actually is even when the flex duct is crushed 45% AND sagged over joists for a long period using REASONABLE CFM rates - 1"H20 per 100' run. That is way better results than I would have predicted.
When installed correctly, flex duct moves air just as efficiently as rigid duct and is much easier and cheaper to install. Flex also has the advantage of being pre-insulated and eliminates joint work except at the terminations, reducing heat gain and leakage.
Yes, flex duct has to be installed correctly, but so does rigid ductwork.
Keep the flex duct liner tight, properly secured, the runs as short as possible, and eliminate as many tight bends as possible and you will have a great performing flex duct system.
In my opinion, the best bang for your HVAC dollar is to have a professionally designed system that is COMMISSIONED after install to ensure that the performance of the as built air conditioning system matches the designed requirements.
www.mtcopeland...
36 year Mechanical contractor here. We specialize in airflow and are generally hired to "fix" hot rooms or "not enough air" issues. The absolute most flex I recommend is 6' which is to say use hard rigid duct sized correctly for friction loss and static pressure then the very last 6' of duct may be flex installed without any sharp bends. The only place I would use full runs of flex would be oversized returns but you need to be careful if it is installed with sharp bends then it may collapse upon itself. Hard duct is much better for airflow, rodents, you can have it cleaned on a regular basis without worry of damage and it can be easily modified in a remodel. P.S. that duct up to the left in the video with the sharp bend makes me cringe.
I noticed the same and I don’t even work in HVAC. The sagging duct just above his head isn’t great either and you can also see a crease in the 90 down. Good news for you is that’s job security
Rigid has my vote too. But if you have an idiot working nearby check ductwork before they leave. 5 smashed rigid ducts I need fixing. South Tx rn is 105° Thanks a**hat!!
I had rodent problems with my flex duct. Never again.
😂😂😂😂on that duct that makes you cringe 😂😂😂
I was in the HVAC business my entire career, the best Practice is to rough in your trunk and attach a 12” class 1 flex to your takeoff and run out with rigid. This method eliminates expansion noise and keeps airflow as designed.
I use flex duct, but I add in metal elbows for 90 degree turns and usually add some metal going into the registers on the ends, so the flex is straight shots or gentle bends. The more important thing is to have the airflow calculations and cross-section sizes matched to the air handler total and the CFM needs for each room. There aren't very many bad HVAC materials, just misapplied and poorly installed ones.
Nice info. Can duct work go through the joist like he has it? Is that safe?
@@The1SimLash I haven't gone thorough I-joists with it. I've gone through I-joists with line sets between the condenser and evaporator, but not with ductwork.
@@The1SimLash If it's allowed in your area there's nothing "unsafe" about it
my only issue with interchanging them with 90s is at termination points, when moving from a 6 in flex to 6 in rigid, the friction rate is so different that you can screw with your desired exiting velocity with this transition. Sometimes you can account for this by going from a 7 in flex to a 7 rigid then 7-6 reducer then 6 inch rigid and then a 90 if you need.
@@The1SimLash - Notice the truss/beam supporting the span...
Flex duct warranty is 10 years. I've heard it will last between 10-25 years before the Mylar liner starts to deteriorate. I would never use flex duct anywhere that it is enclosed like inside a wall or vaulted ceiling cavity. You will have to tear out stuff to get to access to it if it needs to be replaced. Like others have said, I choose to run trunk lines in metal with step down reducers along the way and only use flex for the last 5-6' to connect to the boot/register. You have to be careful not to use Ty-wrap on the outer insulation jacket because it will crush down the insulation and cause condensation to happen on the inner duct liner. Ty-rap the inner liner only! Tape and mastic the insulation jacket on flex. Flex needs to be stretched and held tight for 1-2 minutes before installing. It's literally in the flex duct instruction manual that nobody seems to read. Lastly, duct cleaning to remove dust over the years is sometimes needed. The various tools used to do this can easily poke holes via the high pressure air stream coming out of the rotating nozzle on older flex duct. Metal has it's own installation issues but won't be ruined from cleaning or aging like flex duct.
My mobile home has flex in the crawl space. All chewed up by rats. We've been paying what adds up to thousands of extra $ over the last 5 years in utility bills since we moved here, but the rats have been really comfy and we found notes down there thanking us. We just got an inspection and figured it out, stupid me.
I’ve been an HVAC for 36 years flex duck for a long runs I don’t recommend ever at homes. I’ve seen too much friction loss causing low airflow throughout the house. If you want to use it for a short straight runs in a crawlspace you can. Rodents do like it as well. If anything goes wrong with that flex duct in finished ceilings you’ll have to rip all the sheet rock out to get to it. I’ve just seen too many issues over the years with too much flex on a job. If I was building my house I want rigid yes it’s gonna cost more but it will last forever and I don’t have to worry about friction loss. Builders are trying to make too much money on the HVAC part of the house and when the HVAC doesn’t work the house is a piece of shit. And I’ve seen that in multi million dollar houses where there’s really nothing can be done to correct the issues but cut the house up. You’re trying to save too much money on your houses HVAC in my opinion. Especially in this video where that flex duct is buried
This guy on video don’t know how to install metal duct or he would not be on here. I use flex only in crawlspace or attic. But my main trunk is metal wrapped on outside with r8 duct wrap or bubble wrap
You guys can't run duct as good as me and don't even know it!lol
Ben, how long is a "long run"?
@@davidb8865 over 6 ft
I've been doing duct for 30 yrs, flex or hard pipe, to me there's not much difference if done right
There is no way you can escape from the zombie apocalypse through an eight inch duct.
Best. Argument. Ever. Case closed. I'm only installing 24-in square duct from now on, making sure to have registers in all rooms that lead directly outside.
@@JordanSmithBuilds 💀💀💀💀🤣🤣🤣🤣👍
@@JordanSmithBuilds 36" you have to be A.D.A. compliant! And it allows room for the zombie slaying "devices"
@@MD-lm7fj after our government has obliterated our building codes England will start to look like casablanca without the sun lol
Hilarious! 😂
Matt Risinger is going to call you over this one.
Master Risinger has been glove slapped in the face. Now to regain his honor, he must challenge Master Smith to a duel.
I'm in the design process and watch a ton of Risinger videos. BUT, I like to hear what other professionals have to say. This guy has some good points. With that said, I live in a place that gets up to 118° in the summer and occasionally will see 20° in winter. Maybe, based on the farthest distance of 44 feet, I may go with a hybrid of the two. Metal for long straight runs and flex on the shorts. However, if the flex is pulled as straight and tight as possible, I've read that this minimizes friction and turbulence.
Meant to add that equivalent length plays a part in air flow as well.
Sir, let me tell you something - I am grateful for this video! I watched a lot of videos in the past and had a very hard time renovating my house and after seeing this I feel so much better and look forward to completing my projects. Thank you.
South Florida A/C guy here. My dad died and before I sold his house, I upgraded his AC unit. When I inspected the attic, I found his 28-year-old flex looking almost new. When I removed his air handler and was able to look up into his plenum, the inner liner looked great as well. Don't get me wrong I've replaced lots of deteriorated flex I'm just saying not all flex manufacturers are created equally. the brand you choose is also important.
All metal ducting is loud. You hear the blower and all the booings of the metal expanding.
Flex at the end of the run.
Flex doesn’t last forever, how will you replace it when it has outlived it’s useful life ?
Never install flex in a inaccessible space.
I guess you can cut the drywall out
We are in a double wide manufactured home. I need to connect the side with the furnace to the opposite side with 12 inch duct 25 foot long. There are obstructions to deal with. More than once animals have gotten under the home and ruined the flexible duct. So I've been trying to decide whether or not to switch to rigid duct and then insulate it. After watching your video, it seems so much cheaper to use flex duct and smarter to use the savings to make the crawl space less accessible to critters. Thanks for your video.
The house we just built was designed with a rigid main plenum w/ flex duct branching off to the rooms... Cheers, great video!
I like to use blended system, rigid mains, flex runs.
From a professional HVAC technician who is building his own house I'm not putting in any flex duct. A properly sized and installed sheet metal trunk system is the best way to go for even, quiet, and comfortable air distribution with long term performance. You're correct it needs commissioned for it to work properly, but it all starts around a good design and proper execution. Thus rarely happens and that's why people make the octopus nest for duct systems. Flex is quick to run and cheap to buy causing a poorly designed and executed system for as little money as possible.
The main reason for cheaper installs and poor work is because the HVAC is the most expensive mechanical bill for the house. Plumbing and electrical are hard to cut the cost down that much. HVAC is always the easiest way to cut a large amount of cost out of the equation.
In short.. just be good at your job. I'm in hvac.. I 👍
Did you line those metal ducts with insulation or add it to the outside?
@@aayotechnology I wrapped the outside for the duct running above my garage, but no insulation for the duct in the basement
I love the contra position. Instead of saying, like everyone else, spend 20k more to save $20 a year, you said, the extra cost is not worth the improvement in performance. America needs to hear this!
Mat risinger like to spend thousnd to save dollars juat to prove his point hahahha
I’ve seen jobs where there’s been way too much flex in the system doesn’t work. Flex no matter how you run it has friction. And I’ve seen friction loss caused equipment not to work not to cool A house. What I don’t like about this is that flex duck will be buried under a mountain of sheet rock and paint if anything goes wrong with it guys cut it all out to fix. If you have low airflow because of all that length of flex run there’s nothing you can do about it. I think he’s trying to save too much money on his installs probably have an illegal aliens installing flex all over the place. That’s how a lot of builders are and I’ve had her go out and tell the homeowners you’re screwed more than once in my career. And that’s also on Multi million dollar homes that the builder went cheap ridiculously cheap on the HVAC the most important part of the house
Feel thats the way solar power is
I am an hvac contractor. Flex is much more likely to suck. Poor airflow. This guy is full of shit. Once the squirrel chews it up..
@@jadfaris4119 good duct work isn’t cheap and cheap duct work isn’t good.
MAN that video is awesome! Tks for that! And I thought you had a chrome key behind you all that time, at the end you touched the duct and my mind was like “WOOOW what’s happening?!”
SMACNA standards address all these details in great details with scientific examples of various products. It recommends standards that are speced or specified on drawings by MEP engineers. Flex is to be used any the end of a run, tying in the duct to diffuser box.
Testing and balancing is done upon completion, to record how each vent is performing and is adjusted or balanced by adjusting dampers, so everything works as speced by architect.
I like the presentation and I agree with you 100 %. I am in the process of installing a new central air system and you have influenced and convinced me about duct flex ductwork. Thanks Pal.
sucker.
I agree about everything you said except one thing. Longevity. I would have no problem with flex duct in a new build. Get 50-100 years down the house and there is no way that everything isn't going to have to get opened up to redo the work. I say plan for the long haul. That plastic in those flex ducts get brittle and deteriorate especially if they ever come into contact with condensation, or any pest in the home. When someone tries to tell me how to do something I always make them qualify their statement if its about aesthetics, functionality or longevity. Aesthetics is not the most important thing here assuming both jobs are done cleanly. Functionality is fine either way although there are arguments either way. On longevity I see it as no comparison. I say build things that might last for hundreds of years. All the building science goes out the window if you have to gut half a house to retrofit mechanics at a fairly early point in the life span of a house. Things go together best the first time and should stay that way as long as possible.
I’ve been back in attics that have 30 year old flex and rigid board, and there’s no difference, the worst is usually wrapped metal. I’m not talking about the old owl flex systems, but the Mylar systems. I’m in Ft Lauderdale where 130 deg attics. If I bought a 25 to 30 year old house, I would replace the ductwork anyway. I wouldn’t want other people’s mildew growing in my ductwork.
"or any pest in the home". Beware the rat & mice monsters. Even squirrels. Those tiny rodent teeth can tear up your peace of mind like crazy.
As a union sheet metal worker who mainly does commercial and sky scrapers you actually did make some very valid points. I went into this video expecting to disagree. I think for residential flex duct is completely fine. As long as the runs are not super long and installed properly. Unfortunately so many of these track homes don’t have good installers and Like you said don’t commission, so rooms are not balanced correctly. Therefore some rooms are super cold or hot while others don’t get much air flow. Another huge factor is return location and return size. Do your research on who does your system. It’s worth paying top dollar for a system that is so important.
Yea, yeah. I like to design the lowest static pressure possible to get uniform velocity and volume at every register. Everyone with a brain designs size for AC. I've used an old program from Carrier which works very well. You can adjust everything by the fan CFM to get everything the same static pressure & volume. I don't want to hear the air flowing from any of the registers.
I can't remember the name of that program. It's from 1983. Airflow properties are eternal.
They stole my computer.
The metal coiling that provides the shape of the duct internally. IS also what creates all the texture internally breaking laminar flow and requiring more CFM to overcome it.
The "spiral wire" inside the flex tube can be thought of as being very similar to the "rifling" found in most gun barrels." The air flow over the "rifling" of the flex duct is therefore "spinning" or "rotating" causing the air to become more "dense" and "concentrated" - just like happens within a storm cell in nature . Further, the air flow dynamics and efficiency is most likely"enhanced" to a greater degree than it is "degraded" or "corrupted."
You have any source to back to up?
@@impactodelsurenterprise2440 just thinking back to fluid dynamics in undergrad.
@@steven7650 I was replying to fatman
The inner plastic liner is going to fail over time as it become brittle and splits. Then your air is likely to get contaminated with Fiberglass, which you will be breathing in. You might get 8 to 20 years before the inner liner fails requiring replacement.
I know a facilities manager at a hospital that ran into that problem with some flex having particles entering the airstream. They now require hard duct as much as possible and only allowed flex where hard duct couldn't possible be installed due to clashes with other materials and in very short instances just to attach to a diffuser
Sorry sir, but that is absolutely false. Polyester film does not get brittle over time, UNLESS it is exposed to ultraviolet light (usually sunlight).
Mice love flex! If you live outside of the subdivision or in the beginning phase of one your going to get mice. I see lots of droppings in flex with custom exits for the mice. That has been what I've seen in remodeling homes. This is with mostly crawl space installation.
the biggest problem with flex in vented attic are those darn rodents. yes, thermal is an issue but not yet hazardous.
Playing devils advocate, couldn't the flex duct simply be replaced if mice become a problem?
@@TheRoadTaken yes, but who wants to come up with thousands of dollars to replace damaged ductwork.
@@tysleight flex duct in my area is not that expensive last I looked but maybe COVID has affected the pricing too.
@@TheRoadTaken mostly labor. Our area is years out, even unlicensed uninsured guys are booked out and can charge anything they want.
Had brand new metal duct installed in my home professionally. Everything had mastic and insulation with the silver supersonic tape on all the joints. Had it for many years not knowing that because of the high humidity location of our home, that insulation wasn't doing diddly squat other than becoming soaked. And I mean soaked - so badly that it began dripping and ruining drywall. Removed that soaked insulation, replaced it and at the end of that summer season, it was completely soaked again. Ripped it all out, replaced with flex and problem solved. Plus it's so much quieter and a conversation in one room is not transported to every room with a vent! Maybe hard pipe is great for low humidity locations or if it's inside the heated/cooled space. But lesson learned in the south!
Thats what i don't like about rigid, you get one little air leak in the insulation it starts making condensation soaking it which tears more places.
What area do you live in? I've never seen ducts have condensation in my area
@@ELIRAXPRT In super humid areas like the american south where the ceiling cavity is not sealed well or just in the attic you can have high humidity in there. At that point any improper insulation on the ducts will cause condensation to form on the rigid pipes. For many places in modern homes however, condensation from rigid duct can be a sign of improper AC function, rather than an insulation issue.
Might have been cheaper to leave the metal and spray foam the roof. Cheaper... And problem solved.
I agree with you, it all comes down to planning and execution
Depends on the climate you live in. In zones 1 and 2, flex is more appropriate. Metal is 25x more conducive to temperature than the plastic liner, so is very prone to condensation on the supply side, in a Florida attic with high humidity and A/C is on 90% of year.
The group of flex ducts like the one that was showcased in this video would deteriorate over a relatively short period of time. Well, all polymers deteriorate. But, the lifespan of ductwork to be around 30 years (depending on the conditions) or so is not adequate. The point about low performance increment is inadequate because people choose one product over the other for reasons not only performance, but also service life and maintainability as well as cost.
So which do you recommend?
@@aayotechnology It depends on specific user requirements and needs. If the goal is to provide a makeshift housing, flexduct is extremely cost effective and easy to install. Many other uses for flex duct other than that. However, if you add longevity and maintenance into the desirable properties of your structures, along with other parameters (area size, etc, climate, type of structure, etc), rigid duct can be a better alternative. Usually such videos imply "best practices", but there is no solution that fits it all in reality.
Someone should take a high quality plumbing camera and show what the elbows look like internally in an install, live, no demo's or lab conditions, real world conditions. I have a feeling that would gain a lot of UA-cam traction.
I'm just here for the 80's theme music
what about the green screen schtuff? the videos are like a modern soup with an 80s bouillon cube
@@GreatDivideSven exactly. Here for it all!
You are installing flex duct because you can make more money as a contractor on reduced time and material costs.
• Centrifugal blowers do not push air as a positive displacement pump does, they circulate off of the pressure differential created through proper design.
• Noise in ductwork comes from velocities that are too high which would be from undersized duct or a collapsed flex duct.
• Installing flex duct in structures that will be covered in drywall will cost the owner a lot of money in the future as flex does not have the lifecycle expectancy of hard duct.
• ASHRAE and ACCA have standards in place which recommend best practices for the greatest efficient and life cycle of the equipment.
• Flex duct has a higher friction rate as compared to Ridgid duct which means you will pay more money in kw to move the same amount of air through flex rather than Ridgid.
• Installing a high quality system usually involves training which is governed by standards. What training and standards are you commissioning your work against?
If you had the money to build a brand new house, would you install flex or Ridgid and why? Give customers the best, give them what you would give yourself and if you are going to reference “not that much” or “not that big of a difference”, than at least have the decency to give actual numbers as a true technician would.
Rigid is the correct spelling. Other points well taken.
"Save where you can, spend where you must."
I’m building my own home hiring contractors myself. I’m glad I came across this video. I hear people say use the the ridged. I’m going with the flex. Thank you for the tip.
do not use flex, it is crap.
What kind of reasoning to you have to back it up? What problem did you have with it?
@@josephferron7672 Just use your logic. How air acts, long term effectiveness. Only advantage of flex duct is that it is easier and cheaper. In every other aspect rigid duct is superior. You are building your own home. Do it the right way. This guy is a contractor and he is biased. Because he wants to get it done easy way.
Only go with flex if it is absolutely necessary. If rigid ducts can be run in your house, flex shouldn’t be an option. I never seen these flimsy flex duct in Canada. Must be a Murican way.
@@Faruk651 I greatly appreciate your response. I’m going to have 2 units. One for downstairs and one upstairs. My runs will not be no more than 30 feet. Installing in crawl space and upstairs will be all in the attic. No ductwork will be in walls. I also live in southeastern US.
I just completed a project re-doing the flex duct in my attic. Original builder did a piss-poor job, leaving metal supply and distribution plenums uninsulated and the flex was installed very sloppy and was only an R4.2. I built my own distribution boxes with proper design and insulation and replaced all the duct with R8 flex. I used hard elbows where it turns to connect to the ceiling registers and for turns coming off the distribution boxes. I made sure to stretch the flex tight to eliminate as much of the accordion effect on the flex walls to reduce airflow resistance as much as possible.
Did you notice a big difference in performance after the upgrade?
@@gnarnal in terms of air velocity, not too much. There are some issues with the design of the main take-off at the furnacethat I feel are impacting air flow. But the heat last winter was definitely much warmer than it had been. The upstairs was getting even hotter than before. So I’m that respect, it did improve. Much less heat transfer between the attic space and the ducts.
In the southeast I very much prefer rigid for 1 simple but persistent reason. Rodents! The can’t get through rigid. I do use a short section of flex at the air handler for noise reduction and ease of install but rigids pros outweigh the cons in this area for that reason. Yes I know if the house is built correctly the likelihood of rodents getting in is less but they will get in at some point and it can be a huge pain trying to get them out. Oh and unfortunately very few houses are built with this in mind.
Jordan agree to disagree. If you building typical USA stick houses flex duct is the best way to go. But if you building industrial buildings, Steel, or Concrete buildings than for sure ridged is much better on the main run's and only Flex at the end. Also on Mobile homes I go ridged for reason so animals can't destroy and also it's easier to maintain and service. So it really comes down to what building you have.
I replaced all my flex with hard metal. Why? Rodents eat through the flex duct. Got sick if dealing with it and ripped it as out. No more problems. My house has crawl space not a full basement which likely.make a difference.
Installation location definitely makes a difference!
@@JordanSmithBuilds is there a way to prevent this? I plan on installing this in a crawl space area.
I like doing the main plenum and then for long runs i like doing half the run in ridged, then I do the rest of the run in flex. I think it's a great way to save money and still utilize the best part of using flex duct aka (making a weird offset easier).
you sold me, I bought a house, the previous owner installed a quagmire of ducts, most rooms dont have any vents, I just need one maybe two long ducts to complete the rooms in the house, this looks like the easiest and most practical way of doing it.
Only problem is you should only use flex in places you can access like attics, or crawlspace.
You shouldn't bury flex in a wall or ceiling.
-Sheet metal worker for 27 years and counting.
This guy calls a 40 ft run of flex is a short run. That's rediculous.
@@geraldgilpin1966 agree.
1/2 box max and be prepared to T.A.B.
Well put! Will be opting for flexible ductwork in our house. You look exactly like one of my nephews. Felt like I was watching him talk. Thanks for posting this!
Is there such a thing as rigid plastic ductwork? I’ve seen some rust in mine but don’t want flex ductwork because of mice.
i hear the higher efficiency systems wear out faster and end up costing you more money over time than the older systems. They are also much harder to get replacement parts for.
Like you say, a lot hinges on the installation, if you are getting adequate airflow, cool. That corrugation inside the flex duct CAN NOT flow as much as a identical sized rigid duct. Immaterial if flex is going to be able to supply an adequate amount of volume but the difference is real & easily quantifiable.
Jordan isn't wrong, but not 100% right either. Even most rigid duct guys use at least some flex - usually just before registers to quiet blower noise/vibration that is transmitted thru rigid. That said, flex simply isn't durable. Difficult to clean old flex without damaging it (especially at bends). Running flex that is covered by ceilings/walls is a problem for me - fixing/repairing/modifying hidden flex requires a lot of money - maybe that's something a future owner will end up paying for (households move every 7 years on average). And it's subject to mice/rats eating thru the plastic.
I'd use rigid in a trunk and branch system. Rigid for straight runs, flex for branches. And ideally, ducts are inside conditioned crawlspace or attic so you have easy access. As for leakage in rigid, the few places that may not get sealed only leaks small amounts. And if you know what to look for when rigid is installed, pretty easy to check with a flashlight and extendible mirror for those tight spots.
The biggest thing I see with rigid is too thin mastic coverage - after some years it can crack due to expansion/contraction or simply drying out.
All has pros/cons. Really boils down to quality of design and installation, not so much material choices.
I would assume if you have gravity on your side these work just fine. Might not be the best option if you are pushing uphill.
I have an interesting problem here in NC - all insulated flex duct in the vented crawlspace...the main trunk coming out of the air handler drips with water from the humidity. Even though it's insulated! It's usually 80%+ humidity down there, but I can't do anything about that unless I seal the entire crawlspace - it's just as humid outside!
Encapsulate the Crawlspace, that humidity will drop overnight!
I would personally use metal elbows when using flex. Having flex bend at a 90 is terrible
California is considering all rigid except the last 6 ft. Code here is a blower test so you won't get away with leaking ducts. So with metal ducting and better airflow you can have a smaller blower or at least set at a lower speed which means less energy used. But as an installer I love flex duct. Put the furnace in a central location pull all flex duct so no sags or bends and you can have a good system.
What about dust accumulation in the duct? I was under the impression that it was much higher with flex duct due to the less smooth interior surface and that it is also more difficult to clean. Thoughts anyone?
Dr. Lsitburek also mentions this in his builder's guide. Similar to the comment about vermin infiltrating the ductwork and leaving evidence in the ductwork also.
*Flex can be more efficient than rigid.* Here's why: Flex can be routed often in a direct line to the registers with small bends under and around obstacles, whereas rigid will usually go at 90° angles, adding length to the overall system. Additionally, friction losses are usually measured in equivalent length, and just one fitting adds 10 to 100 feet of equivalent length. Between both of those factors, you're going to get *more friction on a rigid duct* than flex taking the shortest line with zero fittings. If there is a friction loss problem, it's also not a matter of what type of duct so much as what size you specified. If there's a friction problem, just go up one size. That's it. Also, a round duct will have less friction than a rectangular duct of equal section area because a circle is the absolute minimum surface per section area. So where you lose in flex due to the ribs, you gain due to geometry (assuming you're using rectangular rigid. If you compare rectangular rigid to flex, it's almost identical in friction. For instance, an 8" flex duct has a section area of 50 sq. in., and friction loss of 0.12 inches (100' @ 200CFM). A 5x10 duct is also 50 sq. in., and has a loss of 0.10 inches. A 0.02 inch difference is not perceptible, especially over 100 feet. Now add fittings to the rigid, and suddenly it's much worse than flex.
The absolutely best flow you can get would be rigid round with very long press-formed sweeps going in a direct line to the registers as possible, and no one is doing that.
So installed correctly, flex is ideal in most cases.
I thought for sure that you were in front of a green screen. Mind blown at the lighting you have set up in there
I had flex ducts and very low airflow. I had them installed with metal elbows and airflow was greatly increased.
I think this is the part that matters. In some systems, flow velocity matters and must be managed accordingly. Rigid is more predictable in this regard.
I did the same. Metal elbows often correct flow and balance issues.
Yeah it's been shown that 90s especially hard 90s in flex are so detrimental to air velocity by having such a stupid high equivalent length that you pretty much have to use rigid 90s when you need to make a turn.
This is the type of video that needs data to have credibility.
3 things. How do you clean out duct work ? Also dampers on that pipe? And is there air flow restriction since the pipe is not smooth on the inside like ridged
Putting ducts into unconditioned attic is stupid, regardless of which duct type is used
True. Everyone without a basement or crawl space should just run it down their hallway.
@@tjrooger1092 I would never run ducting thru unconditioned space, said no one ever.
I have high static pressure on a new 3.5 ton heat pump. I know for sure the flex duct is the issue. I have two 12" coming out of the supply in opposite directions. I know I need an 18" for a 3.5 ton heat pump. I did a diagram layout of the duct in the attic. What would happen if I just replace the two 12" to 18"?
Ignoring the downstairs, because everything is between the walls. The problem was even worse with an undersized return. The original and only return was a 24x18. The total static pressure on the unit was .9 W.C. I was able to drop the TESP to .66 by making the return 30x24. Since there is so much static pressure and the unit TESP is a .2 and I need an 18" for a 3.5 ton for 1400 CFM. How big does the supply plenum should be and what shape? Should the supply plenum split to 3 18" going out to each of the 3 distribution boxes? Do the other three distribution boxes also need to be larger?
Looking at the triangle shaped supply plenum I have very little room up for a rectangle plenum. I think a 3 feet ductboard supply plenum should work. There is a foot from the Air handler to the ceiling and I think 2 feet from the attic floor to the roof. Now I am looking at two types of designs. Design #1 keeps the 18" ducts straight by having a single 18" coming out of the supply and then a distribution box splitting it to three 18". The sides would be coming out straight to the to side boxes and one going to the third box with a slight curve. Design #2 splits to three right at the supply plenum and goes to each box individually, but all three will have a slight curve. I will not be using any metal plenums..... it will be all flex duct and ductboard boxes, because of budget. I am not trying to make a perfect system, but trying to remove the restriction from the 12" undersized ducts installed and replacing the flex duct that was patched with mastic with new ones.
Another question about the installation..... I have the flex duct running up and strapped to the roof for support. Since the supply plenum is low to the floor due to the roof..... Can the flex duct be run on the attic floor? Can you tell me which layout makes more sense and less restrictive? Thank you.
Layouts:
Current
i.ibb.co/Ld9kmPM/Flex-Duct-Layout-and-Size.jpg
Option #1
i.ibb.co/GHyS00g/2022-07-05-0002.jpg
Option #2
i.ibb.co/PNN8mSB/2022-07-05-0001.jpg
Air Handler Specs
ibb.co/17VvF4B
Video of attic
ua-cam.com/video/iInIN89q8GU/v-deo.html
In indian river county florida every single house has a wall banner unit in the master bedroom because the design of the air conditioner prohibits equal cooling in all rooms. Nobody is addressing this.
Size matters.....there are sizing charts available to assure that you are using the equivalency to rigid duct. Best practice installation is up the the installer. If installation is done wrong, It will not work properly. I have use flex for years, with no issues.
i have a newer home in VERY HOT Southeast Louisiana. It's all FLEX. But i have just 1 room (our bedroom) that is hotter by 2-3 degrees. It's the longest run from the plenum, and it has one larger duct splitting in 2 registers in the bedroom. My main question is, can you wrap more insulation over flex duct? And if so, what should i look for? Figured maybe the long runs in the attic may be messing with the air temp inside the ducts.
So long as you size it correctly, you don’t pinch it or bend it too far, flex is better in almost every situation. When you size it you have to use a lower velocity also. So where you might get away with a 6 inch in metal pipe, you probably need a 7inch in flex. If you can run a board trunk line with flex take offs. In new construction, I would only use flex unless it’s a custom house with a correct budget. I would never use metal pipe, with wrapped insulation. There are very few people that will wrap it correctly and you will have condensation leaks, also people will set stuff on it and it will compress the insulation and leak where flex will move more and you’re less likely to leave something on it. You won’t use the difference in cost of the duct work on any extra electricity. There are so many other things that will cost you more money than using flex over rigid duct, especially when it’s sized correctly.
If you want to save money, just build a smaller house, air seal it well, and stick lots and lots of fluffy stuff in the walls and roof. Rigid vs. flex vs. VRF is arguing over pennies. There are much bigger thermodynamic fish to fry. Especially the way most US houses are built, too big, under air-sealed, under-insulated, and wore, over glazed with crap quality windows.
ZergZfTw Amen!
Who is the size police that determines if a house is too big, too small, or just right?
Done right, I would take the flex duct .. thank for the video.
As someone who's had an extreme experience, flex duct is easily damaged by rodants and for that reason alone, I wouldn't use it if i had a choice.
I'd like to know what the cost is up front to properly design the duct system. Because, when you add that to the HVAC budget, it doesn't seem like it would be much higher to go with the VRF as you discussed in yesterday's video.
I don't know. Maybe I'm lucky to have an HVAC guy that installs my mitsu mini splits to where I end up being into them for 1/2 of what the normal cost is. I don't do conditioned attics, therefore I will refuse to put a ducted system up there. So VRF has been the best choice for my houses. I do a single 18K BTU unit (20.5 SEER) for around $3000 installed.
is vrf the unit that replaces hvac units more commonly used? i am a lil confused so pls help...i thought at first ervs replaces the hvac unit but then found out that is only for fresh air supply
@@bamc3835 Watch Jordan's video from 2 days ago. VRF is 'variable refrigerant flow' which is the way a ductless mini split heat pump operates.
I don't disagree that flex duct, when installed properly (big disclaimer here) is almost as good as rigid. However I think a blended approach is best for flex. Use hard pipe for most of your run and then finish off with 4-6 feet of flex for ease of final installation to the boot and for sound deadening. I NEVER install more than one box length (25') in one run. Generally I am okay with it in runs up to 15-20 feet if they are straight runs that can be easily supported for the length of the run. When you said you keep it short at 40' I almost lost my drink. Around here (Ohio) it is hard to find someone who installs it properly and correctly supports the duct, manages the sweep of turns, and fully stretches the duct out. That is another issue you can have with flex, if it is not stretched out fully the coil inside will cause the air to "roll" through the duct which will lead to reduced air velocity and airflow at the register.
I agree. It's hard to find people who do anything correctly without training. Most of my subs I have to be very explicit on what I expect and monitor the process for the first few jobs. Unless we are using a balometer to verify flow rates, I stand by my argument that using rigid instead of flex is less of an issue than poor design and commissioning of the system. I take no issue with installing rigid and flex, if you have a wide open attic to work with. Thanks for the comment.
how do u find a guy that does great install?
@@bamc3835 your pray to the HVAC gods and hope for mercy
I don't know how long flex duct lasts in conditioned spaces but if your putting flex duct in a hot attic in Florida, especially heat pump systems, the life expectancy is considered 10-15 years. Compare that to the life expectancy of rigid metal ducting. studies have been done and it was found you do have friction loss due to the corrugated inside of flex duct compared to the smooth interior of rigid metal. These cookie cutter home builders also hire cheap help to install the flex duct duct and they do not stretch it properly and you wind up with more restriction of air flow
Seems to be a lot of videos about F1 installs when Lexus will do. Thanks for the video
🧐
F1 installs when toyota corolla will do and require less maintenance to boot
WHAT ABOUT DUCT CLEANING!!
I have worked in HVAC/R and plumbing and heating for 15 years and have used my fair share of flex duct it most certainly has its place and application. I recently did duct cleaning for the first time and the rotary brush used to clean ducts would easily destroy that interior wire coil and get wrapped up inside the duct. My concern is that now that duct has a hole in it buried somewhere inside a finished home years after the install was completed and now has a damaged heat run that no longer delivers air. There ARE more commercial flex duct options that are made of a much more durable interior liner, in Canada it’s called SLP flex.
Thank you so much! I always worried about the flex duct in my attic. My furnace is in my basement, with the living area on the first floor with one powerful outlet. The top floor has four outlets for the two bedrooms and the two bathrooms. I found that even if I completely close the living area outlet, there is almost no increase in airflow to the top floor. The furnace is a very tiny 14” wide, 45,000 btu, natural gas unit. There is no air conditioning and there’s so little airflow to the top floor, that it would be useless. There’s only some rectangular metal duct inside the wall to bring the air to the attic. It’s a house with an association, so I can’t just stick an package unit on the pitched roof. We’ve already covered the south facing roof in solar panels and north face the street.
I use an inefficient portable hose outlet air conditioner on really hot days.
A big swamp cooler would be awesome, because it’s extremely dry when it’s hot outside. It would only take a basic 120 volt outlet to power the water pump and the circulating pump.
Thank you so much for your opinion, if you have time for some very basic thoughts.
Take a look at the pioes going up stairs and make sure it isn't crushed, blocked or have a closed damper in the system. Also look at the floor registers, if they are undersized or if they have run small lines to the upstairs rooms, you may have to upgrade them. While you are at it, beg, borrow or steel a radiant thermometer to endure you have insulation where it is supposed to be - ditto leaking ducts.
As a designer and salesman for hvac I have builders specifically request rigid and then follow up with the question “ can you pass a duct blast?” The answer is yes regardless. But will also reject the discussion of flex as a logical solution to their current problem of duct leakage with current contractors? Riddle me that.
U should do some pro/con videos with Matt Risinger on these topics. Would be informative and ripe for some humorous satire between u two:)
Flex like you said, installed properly is as good as rigid with less worry of leakage and cost saving on labor which in most cases can put you into a higher efficiency system
That is absolutely not true. Flex ducting installed correctly will never be better than rigid ducting that is installed correctly.
I’ve been doing AC in Ft Lauderdale since 1982 and my house has a flex system. It’s quiet, it’s cleaner and the insulation is better. My duct man knows the right way to do flex systems. That being said, it might be the easiest system to screw up by being lazy.
What about rodents, also deterioration of the plastic covering the insulation. I will use as much rigid as possible, until some kind of new flex is invented to prevent rodent access.
The airflow characteristic of flex cannot match ridgid.
Yeah if it's engineered and installed properly. Good luck finding that in a residential setting.
@@20somthingrealestate
If what is installed properly?
@@sherifnabil9663 Well we're comparing with rigid. So rigid
You are aware of everything, so if you get it done with only flex, congratulations you,ve done great job , period However, it is not necessary to work for every case.
I wonder how well it will hold up after a hundred years compared to rigid. We know the rigid will still be just fine. And don't forget the hybrid approach of running rigid trunk lines and flex branches.
in 100 years we won't be using central systems. I am shocked people are still installing this crap.
I agree, time will tell, but given that my ductwork is in conditioned spaces away from traffic and UV, I am optimistic. I prefer a "home run" solution where each branch ties directly into the plenum. It makes a bigger plenum, but with dampers at the start of each run, it makes adjusting the flow through each run super simple. It really depends on the design of the house and AC location.
Jordan Smith will you, or rather can you, do a more in depth video on running ducts? This has always been puzzling to me on how to size the ducts - how large should you start your duct or plenum and do you step down the diameter, and if you should then at what point and to what size? And now you mention dampers and my head is spinning. I would love to know more. I know there are so many variables but is there a "general" rule of thumb for some of it?
@@robertrusso5525 There are rules of thumb like 600 square feet per ton. Sophisticated HVAC contractors will do a Manual J, that is, calculate the total heating or cooling load. Choose the right size equipment to satisfy that load, and then design a duct system that distributes a portion of the total air volume of the air handler proportional to the load for each room, following installation, the system would be balanced to make sure that actual airflow matches the design.
How many houses built today will be standing in 100 years?
Can I find best good quality flexible ducts from homedepot or lowes? What brand and specs do you recommend for HVAC ducts in attic of 2-story home?
A lot of comments regarding cleaning ductwork.
If the proper filter is placed in line with the evaporator and all the returns are filtered as well, what is to clean? Just change the filters on a regular basis.
Very few if any HVAC contractors, test the duct work or offer a third-party verification of duct work performance. Most would not pass.
Most HVAC contractors have their least experienced labor slamming duct work into the job.
They couldn't explain Manuals D, J or S if their life depended on it. These calculations should be documented and left at every install.
The flex duct you are installing has a major flaw in that it is extremely vulnerable to rodent infestation. My personal experience in the South with both rigid & flex duct and field mice is that they can easily chew through the flex duct and then nest within it ripping insulation apart to build their nests within the duct. This causes blockages of the air flow as well as introducing nasty mouse filth / odors and potential health issues to the home's occupants. Rigid duct work avoids this hazard and though mice may still chew on the insulation wrap they cannot penetrate the metal duct itself. The industry has answered this challenge with a newer aluminum accordion semi-rigid flex duct (similar to clothes dryer flex duct) which allows installation without the need for any galvanized / aluminum elbows and hence reduces labor install costs.
The round rigid duct generates about half the turbulence of flex though most systems will allow for this I’m still a rigid user although I will agree that flex duct does antinuate wait sound much better.
Ductwork in unconditioned space is to be avoided at all cost, well almost.
40 years in the frigid north.
I say straight runs rigid, insulate that and 15ft branch offs flex run, buffer from unit to main to avoid noise
loved the video content. def going to watch more JS videos
It seems odd to have such large openings in the joist (TJI) webs from the ducting. Am I wrong?
Those floor joists are designed to be able to have large openings cut in them. Just not the top and bottom plates.
@@brianlorge906 That hole size is probably the max allowed depending on how far away it is from end supports(wall). I believe its max 8 inch diameter hole on a 12 inch deep TJI joist if supports are minimum 8 feet away. I personaly wouldn't do that to be safe. It just looks weird and sketchy. Thanks.
You are absolutely correct. The building plan will give you the maximum size hole that should be cut in pre-fab joists, and it's alot smaller that shown in that video.
Matt won't be too happy hearing this
In air craft ribbits are flush with the skin of the plain had it had the crinkle of the inside of the inside of the flex duct it may not fly do to air flow / efficiency use when needed only and quit the bull ship
Static pressure curve, velocity, decible level, money is not every thing if a load calculation tell u ,u need x amount of cfm to satisfy load requirements, bottom line quality installation is the key.use a magnehelic to verify your flows,
By California Mech Code, I can only use 5 feet of flex duct for commercial applications. That's probably the main reason why I don't use flex for main trunks. I'm guessing long run of flex duct is allowed for residential homes?
Use a little of both. FYI 40 foot flex is two long.
I can tell you one reason to go with rigid ducts, mice. I have seen a flex duct that looked like Swiss cheese.
Is that a beam behind you that has the square / rectangular holes and orange color on the ends? Or is that just a fur down for something else like a tray ceiling
Flex ducts used under mobile homes are a BIG MISTAKE. I live in a mobile home with many flex ducts underneath. I had a problem with COCKROACHES.... The big ones here in Florida... kept coming into my home. I finally figured out that a RAT had chewed a hole in the flex-duct which lays directly on the ground. There is not much room between the bottom of the mobile home and the ground. So ... after the rats chew a hole in the flex-duct, then the cockroaches come crawling into the house through the floor vents!!!! I did a TEMPORARY fix by putting a fine netting material under every duct to keep bugs from coming in. Soon we will upgrade away from DUCTED vents to a ductless mini-split Mitsubishi air conditioner.
Cold air return side should be rigid metal, supply trunk line too imo. Now the supply lines flex? Sure maybe. You can work with flex easier, so I get what this guy is saying. This is all due to duct cleaning fyi. Can’t clean flex well, it’s delicate. Rigid metal can take the aggressive duct cleaning abuse way better. Returns get dirty hundreds of times more than supplies, due to typical systems having furnace media filters protecting the supplies, and less HVAC systems have filters on the return grilles to protect from dust.
The only time i would disagree is when i see some cheap looking flex that looks like its just a matter of time the inner plastic on the inside will degrade or punctured when cleaned after years of operation. Then you have insulation mixing in the air.... i don’t know lifespan of the flex though, if it does degrade at all especially in conditioned spaces. if your building that 50-100 year house or the ‘better than most’ house is probably how i’d make my decision.
DESIGNED systems installed correctly will always beat a "thrown together unsealed install.
Using some strategically placed elbows is nice in certain areas. The duct boot selection where it comes through the ceiling is also important. Starting from a basic, cheap system, what is the first thing to upgrade? (It's not the flex duct)
Flex duct fails in Florida heat. I wish my mother built this place with rigid.
It is all about the planning and design of the system as a whole. Combine a half assed plan with a crappy install and you get a crappy system, like mine..... One negative is that you can not clean a flex ducting easily. You can rip it with the brushes...
Crazy possibly, Wrong, yes. His opinion doesn't change the facts. You can do this yourself and use a manometer to measure the flow. I will personally do a video on this and show the results.
Flex is OK but ideally you should keep it 4-6 feet at the end of the run maximum, the rest should be hard piped.
I've installed miles of both flex and hard pipe. Only on hacked jobs are they more than 6 feet, or if it's not physically possible to install hard duct.