can anybody offer some feedback on the pros and cons of flex-duct vs. traditional metal ductwork?
I know the obvious would be cost of install and materials, but how about performance and efficiency?
Thanks
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Replies
The loss is terrible. It should be used like toilet paper...
Only when you need it. To do a job with it only says that the contractor has no clue about duct work.
I know a lot of installers like to use it because it is easy to run. This is not a dig on them lots have to compete to get jobs so time is money. That said metal ducts have a smoother wall on the inside so less friction and less power needed to move air. They have to be installed correctly with mastic at the seams, joints and ribbed elbows.
I would consider metal ducts for the cold air return side also. This includes between studs. In the long run operating cost will be less because less air loss in the system.
I remember seeing information on various web sites including the energy star site about efficient duct systems. I prefer metal. Jay
We do a lot of remodeling and after seeing what accumulates in ducts I'd want a system that has the potential to hold as little crud as possible and would be straight forward to clean if needed. As much as I like flex for some purposes, if you are going to have a furnace duct it should be metal as much as possible and keep the velocity consistant without wide low velocity areas where dirt will settle out.
It gives me the willys just thinking about it. ;)
Beer was created so carpenters wouldn't rule the world.
in a perfect world... air like water does not like to turn... i use to spend hours on a flow bench working on air flow... so i can see some advantages of the longer sweeps you can make with flex.. does it overcome the the loss from the not so smooth walls? I'm not sure... just as i'm not that sure that the not so smooth walls killing your air flow... air in motion is a strange thing... at ridges and obstructions it will create "eddies" like in a river ... small eddies cal almost act like bearings for the larger mass of air... think of the dimples on a golf ball... if perfectly round was better.. the balls would be perfectly round...
most hard duct is insultated on the inside which I can't see as be'n all that great... (i use it all the time) but... it has to hold dirt and it's not smooth and it's usually square... air doesn't like to flow in corners...
in my own use i use a mixture of hard and flex duct... where i can run a trunk it's hard... when i have to do some snake'n it's flex... i think i get less sound transmission from flex...
then you get another question... volume? or pressure? just like in spray'n paints... to you want higher pressure and smaller duct or do you want a larger volume and low pressure? in a system geared to high volume I would venture that duct type has little or no effect but levels of insulation do... i think your chances for leaks are much greater in a hard piped system than in a flex piped system with fewer fittings and joints... i also would say that flex installed well would equal the performance of most hard piped systems...
economics? in most situations i don't see the payback in a hard piped system which has to be at least 3x the cost of flex... if not more... i've pulled flex that has been in place 10yrs and it's 100% reusable clean inside and still intact... most hard duct i pull i always find places where it has leaked, been crushed, joints that have come apart ect...
I don't think they allow "dct board" around here any longer... but i could see where with a higher grade product it could be a good choice... i never used it but the idea is pretty good...
alot of products can "get the job done" and they all have their place... like most here will tell you... many times the time and care going into the install is more important than the materials used...
p
I have a cold air return in the attic that is flex, 18 years old it needs to be replaced the outer layer has become like a garbage bag left out in the sun for several months, it is turning to dust. The supplies are all steel and fine. All i know is some one is going to be very busy in 25-50 year replacing miles of the stuff. Steel will be good for a 100 years or more no problems, My next house will be all steel, though it is getting harder to find a good sheet metal guys
Wallyo
I've replaced 40 year old galv steel with holes rusted in it. All materials have their inherent problems and will behave differently under varied conditions. BTW, we did replace the rusted metal duct with new metal. Hard duct is probably the best but most won't pay the difference for it. I still don't believe in duct board. I do agree with what Ponytl said about possibly avoiding some duct loss with flex.....but possibly having duct loss due to flex. So it may be a wash. I'm no HVAC guy so all I'm giving is my 2 cents worth.John
J.R. Lazaro Builders, Inc.
Indianapolis, In.
I've replaced 40 year old galv steel with holes rusted in itJeez, I hope they did something about the water in/on the ducts that caused the rust. Galvanized steel ductwork, properly installed, should last the life of the structure. If steel can't stand up to the 'stress' of transporting air, then we should rethink the materials used, how 'bout PVC ducts?
In my opinion, pvc would make an excellent duct material. I don't know if anybody uses it or not and if there is any downside to it. Static may be an issue.
I don't rember the particulars of the job too much other than it was a tight crawl space but I believe it was dry and the furnace didn't have a humidifier attached. That was the only time I have seen it happen but I just wanted to raise the point that ALL materials have their issues. Our HVAC guy typically uses a metal rectangular trunk line with flex branches and does a very nice job. IMHO metal branches would be better in terms of longevity but in trying to be competitive, it often gets left by the wayside.John
J.R. Lazaro Builders, Inc.
Indianapolis, In.
"In my opinion, pvc would make an excellent duct material. I don't know if anybody uses it or not and if there is any downside to it. Static may be an issue."PVC duct is used for corrosive exhaust ducting and it comes in sizes up to 24" diameter. It is very expensive ($30/ft for 12") and that tends to limit its uses.
I worked in remodeling hospitals in So Cal and they had strict guidelines for the use of flexible duct. Only the last 6-8 feet were allowed down to the register.
Undersizing, badly routed, excessively long, ductwork creates back pressure that adds up. Especially on long runs. I've seen large houses with 3-4 furnaces because of this issue. They have small units with flex running everywhere. Do you think they care about the energy use?
Given a choice I'd probably take metal with sealant around the joints and insulation on the exterior. I've also used flex w/ good results. To minimize drag I usually pull the liner as tight as I can. Any bends I use elbows secured to the framing. That way the flex is only doing straight runs and when pulled tight the walls are relatively smooth.
Rule of thumb, I never go more than 8' with flex.
Don't think PVC would take the heat and might give off some fumes. You can use fiberglas duct board and make your own.