Hi: Appreciate any comments as to the benefits of flex duct vs. sheet metal ducting.
Thanks
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Replies
I do believe that the flex duct can be a 10:1 restriction over ridged pipe. In other words....1 foot of flex has the same restriction as a 10 foot ridged pipe.
2 to 1, I believe, not 10 to one.Flex duct has its place. If the added restriction isn't a problem, then great! for IAQ, flex is usually fine. for A/C, maybe not.it doesn't look very good exposed though. for any exposed ducts we are going to metal.-------------------------------------
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I do almost all remodeling, so I get to tear things up and see the old.Most heat here is hydronic, but I do see the flex on exchangers or for hot air systems and for AC sometimes.It seems that very often the flex duct is crimped or collapsed someplace along the line - maybe a third of the time causing extreme limitations of flow.So it is imperative in my thinking to install carefully in such a way that it cannot be collapsed, but installers seem to have the attitude that the reason for using it is to make their jobs easy and fast, forget about long term effectiveness.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
"..but installers seem to have the attitude that the reason for using it is to make their jobs easy and fast, forget about long term effectiveness."
The difference between profit and professionalism. It seems to be a disease that's plaguing just about every industry in the country these days.New knowledge is priceless.
Used knowledge is even more valuable.
All of the mechanical engineers I know fairly insist on limiting the amount of flex duct used, always prefering smooth-walled ductwork instead. One I know tries to limit flex runs to under 36" in length, too.
Now, it occurs to me you may be asking a different question than the way it read. Are you, in fact, asking about using the high pressure (small diameter) supply lines versus sheetmetal ducting, perhaps?
Flex duct is widely used in our area - south carolina because there is no space left for sheetmetal.
I didn't realize that S Carolina was getting that crowded!;)
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Somewhere on the internet are pressure drop and flow diagrams.
I find that up sizing 1 size is sufficient. And it installs so much easier in tight places.
sheet metal is noisy.
quit banging around inside of it!;)
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
The only advantage I can think of is that flex can be routed all over the place and sheet metal can't (at least not easily)
That flexibility carries a penalty in reduced flow so it's a good idea to upsize the flex runs - and/or use it only when necessary.
Have you ever seen flex installed correctly in residential building?
To be installed correctly it should be stretched tight and supported every few feet with a 2" web strap. Runs should be limited to diffuser and/or return drops only with as short runs possible. The joints should be made with band clamps, not duct tape. Bends in the flex need to comply with the recommended radius for the duct size.
I work commercial mostly, and seldom see much of the proper installation done there. Following the manufactures installation instructions are not real high on the priority list when speed, production , and money are the issues. In residential, I have seen a lot more sh*** work than I have good work in rigid duct, almost zero in good flex duct work.
The HO and builders save money by using flex because of the "labor savings" factor, but to get adequate system performance, most HVAC contractor just over size the units a little more. Not much net savings in total cost and it goes negative when you factor in long term energy use.
IMO a poorly installed rigid duct system is still better than 95% of the flex duct systems I have seen in residential construction. Ya, have to beat a rigid duct pretty hard to collapse it, but flex can be smashed by any bufuss arse working around it or installing it :)
Dave
My folks had central AC installed sometime in the late 80's with a foam and foil based rigid duct. Are products like this still available? It was certainly of superior insulation value than any flex duct or blanket wrapped sheetmetal I see around today. The ends were tongue and groove and seams were taped I believe. The only problem I see was the potential crushing damage during installation. Has anyone used a similar product recently? I would be curious to know the cost, pros and cons for our new home construction.
http://www.specjm.com/airhandling.php?category_id=6
Duct board is still used. Fairly esay to DIY a project with it if you follow directions supplied with the board.
Dave
Dave -
Do you mean that tossing a coil of flex under the house or in the attic and connecting the ends isn't a proper installation? Gee, who would have thunk?? - lol
My first shop dust collection system was made up with 4" plastic flex hose and most of the "collection" was whatever settled on the outside of the hose. (As a mechanical PE I should have known better, but the hose was inexpensive and it went in fast. - lol)
I eventually replaced about 75% of the plastic flex hose with 4" sheet metal duct and was amazed at the improvement in performance.
Many years ago I built a house for a couple I knew very well. I trusted the HVAC contractor to design and install a premium system. He used flex duct runs to all diffuser from the main sheet metal supply and return lines. It all looked good to me, in my blessed ignorance, until the first cold months after occupancy. Two of the four bedrooms were uncomfortably cold, and no amount of raising the T-stat seemed to help.
Want to guess which side of the load bearing beam they were on?
Hey it is "flex" right ? It'll squash from a 6" round to a 3x14" oval real easy won't it ? Ya, man, and we don't have no transition ducts on the truck anyhow. Go on Bubba, shove her through thar, and I'll pull er out tuther side.
Lessons learned the hard way are remember the best.
Trust and HVAC design/installs aren't mutually exclusive. However they are scare items in my experience.
4" flex for dust collection system? I'm surprised it didn't get sucked into the collection bag after it collapsed on itself :)
Dave
Not the 4" flex like is used in dryer vent lines. The DC stuff is fairly stiff and corregated - like drain line that bends.
Would you mind describing what a quality sheet metal installation would consist of in residential?For my own home I want sheet metal for the central HVAC system. Tied into that system is a whole-house HEPA air filtration unit and an HRV. Furnace is variable speed motor, natural gas fired. The fan stays on all the time to better power the air flow for the HRV and filtration units (if they were stand-alone, they'd both have smaller ductwork).It's the setup I have now and it works fine. There is some flexible duct near the registers, to reduce vibrational noise I guess.
whole-house HEPA air filtration
Well, that suggests that the ductwork ought to be sealed to that level, too. Which is going to likely be brush-on sealer, plus tape over that.
Otherwise, it can be a tad inapt to have sub-micron filtering on ducts with millimeter gaps in the joints (or, my personal least-liked practice, only taping the insulation blankets <grrr>).Occupational hazard of my occupation not being around (sorry Bubba)
bj,Get a copy of ACCA Manual D (just google it); Every thing you need to know about duct systems. Also, it will save Dave R. a lot of typing :-).
Thanks.
I am far from an expert on duct. I have installed a lot of it for commercial HVAC systems we have added in our power plants. We generally have everything sized and designed by one of our mechanical engineers or suppliers so I don't have to wrap my brain around all the calculations. I have done some shop drawings for the sheet metal shop to use for really simple installs that we self design, but wouldn't even consider doing it for anyone else.
When I installed my shop HVAC I opted to go with 26 ga. metal rather than the lighter 28 ga. used in most residential systems. It starts of quite than the lighter gauge, plus it is easier to work with.
One of the biggies that I see in residential installs is the lack of sealing of any of the joints. Another is poor or to little insulation on systems installed outside the heating cooling spaces. Drives me nuts when I hear the rational for such shoddy work from a licensed professional.
Dave
Dave,Whether or not you design a duct system, you know what a good installation looks like, and that's really where the rubber meets the road. A perfectly designed system will perform miserably if it's poorly installed, and the sad fact is that far to many installers are just incompetent. Unfortunately, the only effective recourse the customer has is to educate himself and demand performance testing.
One of the biggies that I see in residential installs is the lack of sealing of any of the joints.
We see that a bunch with the lower 2/3's of hvac installers. For something that's so easy to do, it's amazing that so many screw it up.
Beer was created so carpenters wouldn't rule the world.
Thanks, will do. Any "best practices" ideas beyond what's required?
bj,mrsludge (interesting nickname :-) ) raises several good points. An excellent resource is Home Energy Magazine. There's lots of good info in their archive section which is on-line for free, in particular, see issue September/October 1993:
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/93/9309contents.htmlSeveral years ago, when most of the HVAC industry still though duct tape was wonderful stuff, they did well designed test on various duct sealing methods, and proved just how bad duct tape was. Metal foil taped performed pretty well, but mastic is the best. BTW, don't just use your favorite caulk, use real duct sealing mastic (for which several ASTM & UL standards have been established). RCD is one brand that is quite well respected, there are many others these days. Just look on the label for some words about what standards are met.At this point, I think I need to point out that, as important as duct sealing is, combustion safety is far more important. Doing anything that alters the relative pressures in a house (this includes duct sealing) has the potential for causing serious, even deadly problems with combustion appliances, especially naturally vented gas water heaters. A combustion appliance zone safety test is a must, unfortunately, it requires some pricey equipment.
The Affordable Comfort web site has an on-line copy of the slides used for one of their seminars which are useful to look at, even if you don't get the lecture that went with them:
http://www.affordablecomfort.org/images/Events/23/Courses/697/DUR_3_CoxCombustion_Safety_Do_No_Harm.pdf
bj- if you post on THS at all, I think I responded to some of your system questions a while back (I'm 'thull' over there).
The best way to tell whether you have a quality duct installation (flex or sheet) is to get a duct blaster test done. Southface can do this or point you to someone who can. A tight system should leak at around 0.05 cfm/ft^2 (i.e. 5% based on the square footage of the house). Ideally, I'd want to make clear to your contractor that this is going to be done and that they're responsible for addressing any performance deficiencies. It needs to be contractual, b/c I think the codes in GA aren't going to protect you at all (based on content and lack of enforcement).
To do this, the duct joints are going to have to be well sealed- all joints mechanically fastened, followed by mastic and/or tape. I'm not totally clear on what some of the pros do combining foil tape and mastic. I'm not a pro, and my observation is that the UL foil tape alone may not hold after being subjected to a lot of hot-cold cycles (1 yr in our case). So it seems that putting mastic over foil wouldn't work great if the foil turns loose.
I ended up having to go back and fix our ductwork after it failed the duct blaster test miserably due to hack HVAC subcontractor (working for hack GC that ended up in a lawsuit). I DIY'd this mainly b/c we were out of money otherwise and I didn't want to pay to heat/cool the attic.
I'm a huge fan of Hardcast 1402 butyl mastic foil tape. It's expensive at $28/100', but it's tenaciously sticky, even where I was brushing off accumulated dust to close off the insulation jacket I'd cut to work on fittings. The butyl backing is the same type of stuff that's widely used for flashing windows/doors/openings these days.
Some detailing I noticed that I had to address:
-Sealing duct boots to ceiling; you can argue if these really leak under normal operation, but they will during a duct blaster test, which covers registers with sticky film before pressurizing the ductwork. I used caulk where I could (new drywall) and expanding foam elsewhere; ideally, they might have gaskets on the ceiling flange. The boots themselves need either mastic inside or tape on the outside of their joints.
-Sealing misc openings at air handler; there are a few test ports and switches that will leak at the unit; I just used regular foil tape for these so it could be removed easily if needed.
-Sealing between the air handler, filter rack, and plenums. It's just a matter of attention to detail to get these boxes all sealed to one another in addition to mechanical connections (screws)
-Attention to detail on the sheet metal duct- every longitudinal seam and every connection should be sealed. Our hack was less concerned on the return air side and where they were obviously rushing at the end of the job. On fittings, the adjustable miters should be sealed in addition to the end connections.
-Flex duct connections; despite specs to the contrary, our hack put in a lot more flex than we said was OK. Where pieces of flex join together, the regular foil tape seems to have done a lousy job of holding the connections.
Here's a document on the various tests (blower door and duct blaster):
http://www.southface.org/web/resources&services/publications/factsheets/22blowdoor.pdf
Yes, thull. Whatever you posted is in my ever-growing files. I always thought that mastic was the best sealant, so I'm likewise surprised to see mention of using it in conjunction with foil tape. I see what you mean about the failure of the tape. I love that aluminum tape, though; it adheres real well.Part of our build includes a duct blaster test; has to meet Amer. Lung Assn's Health House criteria. So I'll verify your numbers and Southfaces's against ALA's.Thanks, once more.
Flex duct won't last long, probably 20 years or so. Leaks like crazy as it gets jostled around during installation.
Properly installed flex results in a quieter and less expensive install for the same performance level.
Metal duct is used to maximize flow through tight locations and for large volumes in strange sizes such as when maximizing headroom with an overhead duct.
Duct, flex or otherwise is sized for the flow required so I don't see a disadvantage to the increased size if it fits a location since it's still going to be cheaper than ridgid duct.
Many will poo poo flex duct, especially 4" as used in bath vents, but I talked our HVAC guys into a run from a basement clear up to a 2nd floor attic, nearly 45' to the roof. They claimed it wouldn't pull anything, but I simply asked them to humor me. No, the 100 cfm fantec wouldn't pull 100 cfm's through 45' of flex, but it pulled at least 65% of the others with probably six 90 degree bends, which is plenty for the application when used on a timer.
Best of luck
Beer was created so carpenters wouldn't rule the world.
The AHJ here will allow 4" insulated flex to about 10 feet for bath fans. Longer than that and he wants to see 5", and then not longer than 25 feet.
Our mechanical inspector doesn't care what we do with fans that are considered optional. If it's not an optional item then we have to follow a path that gives a pretty close guestimate of the true cfm output and exchanges per hour.
It's funny how some get fixated on duct size and pay no attention to the fan attached to the duct.
Beer was created so carpenters wouldn't rule the world.
There's been a lot of good info shoved your way, but one thing hasn't been mentioned about flexible ducting.
I've worked on several older rural homes with retrofitted central heat and those with flexible duct were great habitat for mice, rats and packrats and, in one case, fox squirrels.
I don't do HVAC but have a good sub who I call to correct these kind of situations.
But my point is that rodents have an amazing ability to gain entrance to a lot of homes and cabins new and old and if they come across an assessable run of flex ducting, they'll move in immediately and create all kinds of problems.
IMO, flex ducting should go the way of pre-1970 aluminum house trailers.