I'm interested in hearing opinions on this as well. Everyone so far has priced flex for my house, saying that 90% of new installs (new construction and remodels) in this area are using flex.

 

saying that 90% of new installs (new construction and remodels) in this area are using flex.

90% of the sheep followed the other sheep off the cliff.

 

Metal in general, if sized and installed correctly, is considerably less restrictive than flex.

Metal is also much noisier than flex.

A good approach is a hybrid between metal and flex. Metal plenum, metal trunks, flex branches to the supply boots, flex stretched as tight as possible. Flex runs to the boots around six feet or so, to dampen air noise without imposing much additional restriction to flow.

A total flex install, to maximize system performance, must be hung tightly with no kinks or excessive bends.

Both forms of ducting, especially metal, must be sealed well to prevent excessive loss of conditioned air to the attic, which will depressurize the house if allowed to leak unchecked. A depressurized house will invite hot, humid air to infiltrate from outdoors, increasing cooling costs. It will also penalize in the winter.

 

Most systems are run in flex today because it is the cheapest to install, and therefore part of the lowest bid.
It requires the least tools, the least expertise (to hack in), the least space on the truck, etc. Rats and mice can chew holes in flex, not metal.

Metal duct requires careful measurements, some amount of shop work for the "special" pieces, and more skilled labor. Properly insulated metal duct will not sweat and therefore will not rust out.

This is not to say you cannot get a good system out of flex duct, you can. But you are more likely to get a hack job.

A properly designed system will have the least friction (restriction). I use a hybrid design as mentioned above. Plenums, main trunk line, and runs greater than 5 feet are all metal. Runs to the individual boots of less than 5 feet are flex (cuts the noise). Usually some part of the return is flex.

You already realize proper design and installation are key. Take it from there.

 

Most systems are run in flex today because it is the cheapest to install, and therefore part of the lowest bid.
It requires the least tools, the least expertise (to hack in), the least space on the truck, etc. Rats and mice can chew holes in flex, not metal.

Why do rats chew on flex insulation and not the insulation wraped around flex? I know the metal is safe but isn't a rat problem, a problem with both? No insulation on metal due to rats and wouldn't it sweat and then rust a hole?

Metal duct requires careful measurements, some amount of shop work for the "special" pieces, and more skilled labor. Properly insulated metal duct will not sweat and therefore will not rust out.

This is not to say you cannot get a good system out of flex duct, you can. But you are more likely to get a hack job.

Is it harder to do a hack with metal? I'd assume the technical difficulties of laying metal would open it up to more error if in the wrong hands?

A properly designed system will have the least friction (restriction). I use a hybrid design as mentioned above. Plenums, main trunk line, and runs greater than 5 feet are all metal. Runs to the individual boots of less than 5 feet are flex (cuts the noise). Usually some part of the return is flex.

How much loss is occuring with properly laid flex vs. metal? Inquiring mind want to know. I know it occurs but how much is the concern.

You already realize proper design and installation are key. Take it from there.

Thanks for the replies. The metal is new to me.

 

i was going to post a reply which would have been pure speculation on my part so i decided to wait until a contractor posted. indeed, his thoughts echo what i assumed just based on the way flex looks compared to metal. it seemed to me from looking at pictures in the hall of shame that flex was used to get the job done quickly and from those pictures at least, poorly. a metal duct will give you better airflow and possibly less static pressure because of the flat rigid walls compared with the texture and most probably more twists and turns of flex. i assume that flex has a specific purpose that it is more suitable for.

take a look at the hall of shame, maybe search "flex" in that forum to see some of the bad things people do with flex. it seems a lot easier to mess up a duct system with flex than metal. then again, i read a post here of a guy with metal ducts that were a mess too.

 

From "Air Flow is Critical to HVACR System Performance", RSES Journal, April, 2008:

In 2002, a comprehensive review of air flow in an HVAC duct was commissioned by the Air Distribution Institute in cooperation with the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and other funding partners. The Energy Systems Laboratory (ESL) at Texas A&M University undertook this research to verify the accuracy of the "Manual D" friction-loss tables and assess whether the test methods used to gather friction-loss data was appropriately related to modern installation practices.

The ESL research concluded that when following test protocols prescribed by ASHRAE, the "Manual D" friction-loss tables-and therefore the ductulators that correlate with "Manual D"-were accurate for both round metal duct and flex. However, when flex was tested under the "as built" conditions seen in modern installations, friction losses increased as much as 10 times more than predicted by "Manual D." A similar study conducted by Lawrence Berkeley National Laboratory found very similar results.

The prescribed method for testing air flow in flexible duct work requires that it is stretched tight and released on a flat surface. This results in the flex resting at approximately a 4% straight line compression. Air is then forced through the test duct section and sensors record pressure loss. The problem is that flex is rarely installed in a straight line and stretched as in the prescribed test method.

One common mistake installers make is not cutting off excess lengths of flex duct before making connections. This practice compresses the installed flex into a shorter length, much like an accordion. The amount of compression in flex duct is described as a percentage of the stretched length. For example, if a piece of flex measures 10 ft when fully stretched, but is 7-ft long when installed, it results in a 30% compression.

ESL researchers investigated the affects of straight line compression in flex duct and found that even small amounts of compression dramatically increase friction loss.
It has long been an accepted practice to up-size by one diameter when using flex instead of metal duct. ESL research found that when installed at as little as a 15% straight line compression, a 10-in. diameter flex duct is required to equal an 8-in. diameter metal duct.

Compression is just one installation variable that increases friction loss in flex. Besides excessive bending and crimping, installers often find it easier to simply use the piece they have rather than take the most direct route and cut off the excess. This is both a waste of material and needlessly adds additional length and friction loss to the duct system.

 

That was what I was looking for. Thanks and I appreciate everyones input. Since my attic is tight (not sealed tight but limited access), the smaller metal ducting may be my choice.

 

We design and install flex systems ,test the static of everyone,seldom an isue ,and when it is,it's usually a change made in the field or a design error.

Flex delivers the same air flow at the same static as metal,if deisned and installed correctly.Yes the flex will be larger sizes then round metal pipe.

 

This has been my experiance as well. Bottom line, flex OR metal you still need to understand MAN D and install either correctly. Metal is more resistant to varmits and critters especially in rural settings

 

You can install a sheet metal system as hacked up as a flex system.
The duct material is of little concern. Its the guys installing it that count.
if sheet metal isn't seal right, and insulated right, you get nice rust, possible mold, and low system performance.

A quality contractor can install either right.

 

That's what I was thinking....

 

The less room your installers have to work in, the better chance you have of unintentional mistakes being made. Not that any one would install something wrong on purpose. Flex may have less of a chance for leaks in that kind of install environment. Bottom line as previously mentioned, The integrity of the installer will make or break a job.

 

The most important thing about your duct system is for it to be SEALED correctly.
You also said, “Since my attic is tight (not sealed tight but limited access), the smaller metal ducting may be my choice.” The size of you duct is not going to change enough to justify choosing metal or flex because of the size of your attic. If your attic is really that small then flex will probably be the best choice because you only need to seal two ends and the insulation is already installed. With metal you have to seal it the entire length the duct and then insulate it the entire length of the duct.

 

Thanks Luckyair. That's why I read this forum. Makes perfect since.

Big concern was the contractor that gave me my 1st bid uses metal and brought his guy in to even see if he could get metal up there. I told them they could rip drywall ceiling if necessary. 2nd bidder suggested flex and gave me more of a lack of condensation reason for it. I was thinking he may have just not wanted to attempt the metal due to the confinement of the attic space.

I do think the sealing is the issue for me to use flex. Thanks!

 

Since I live in Texas I know what your concerns are with humidity. I personally would choose flex everytime over metal. The main problem I see with metal installation is not the duct itself. I have seen some beautiful works of art with sheetmetal. However, getting metal duct sealed is hard to get someone to actually accomplish. And then it has to be insulated. Flex is quieter, sealed better and better insulated. Any concerns about rodents will be same whether flex or metal.

 

Any Joe Blow can install a couple boxs of flex,make sure the flex guys price is half of the guy that is going to install metal system.Because if you go with the flex it takes half the skill and you are only getting half of good of a job!

 

That is complete and total BS. Anybody can install either.

It takes skill to install either correctly.
If a guy installing sheetmetal has no pride in his workmanship, it will be a junk install!!!

 

they both work fine when installed right, make sure they leave plenty of room to get around the ductwork. flex and ductboard are easy to damage.

 

Leaving room around the ducting will be an issue. House wasn't built w/ AC. It was added later. I'll post attic pics soon.

 

I agree a metal system can be hacked in also I am saying it should cost alot less, Flex and ductboard systems were developed because of cheaper cost and alot less skill to install, alot of contractors give pricing of a system done with metal and then steer you toward flex and bend the customers over.In my opinion there is no comparision from a quality sheetmetal job sealed and sized right. it is like a corvette to a chevette. Flex duct was developed for people with no sheetmetal skills, flex has its place but it is also overused do to unskilled sheetmetal workers and service men doing a mechanics job.On an designed house the specs say 5 feet or less of flex and no 90 degree bends thats coming from engineer firms so what does that tell ya.

 

....it is like a corvette to a chevette.

Are you saying the chevette is better because of the metal body vs. fiberglass corvette?

Sounds like each has pros and cons. The flex sounds better in my attic. I'm sure casting solid copper ducting and foaming the entire attic would be the ultimate but the cost wouldn't be justified. Carbon Fiber? The biggest error I see when reading this forum is improper sizing and sealing of ducting. I would venture to guess that in a residential setting that flex would work. In a commercial setting, the higher CFM flow would make metal the optimum.

Is the cardboard plenum and dryer vent duct I have now able to carry 1200 CFM. It does have gorilla tape thank goodness. ( I actually did find a run with dryer vent duct on an interior wall!)

 

That the engineer doesn't know the difference between flex duct, and a flex connector.

If you want skill. Practice making radius els in duct board. Its also harder to support properly.

Tin is easier to support. Anybody can bang it together. Who can run 3 screws into a piece of round pipe. Slop some pookey on it. Should cost less then duct board and flex.

Your talking sour grapes because the sheet metal manufacturers are pricing it out of line.
Since they can make more selling it over sea, its just getting higher and higher.

Since you don't like it, or use it. You shouldn't be determining how to price it.

 

That is the best one ever I am going to post that one in the shop Monday,I'll put it to you this way , Duct board is what we train or apprentices on to make REAL fittings. A monkey could make a radius heel elbow out of ductboard and TAPE! Try making an ogee fitting with your duct board. Also listening to you like so many other serviceknockers you must be supply house sheetmetal workers, Cap and tap. Thats the way most of them roll look a commrcial spec book sometime see what REAL sheetmetal men do for a living.

 

ROFL... Knew that would get you.

I started out as an apprentice installer and did my share of sheet metal. Didn't know ductboard existed for the first 4 years I was in the trade.

But I won't take away from any tradesman, in respect to weather they use tin or ductboard. There is no reason to. Nor does it make any sense.

But, I'll jerk your chain about it all day.

And for the record, I haven't made a drive, let alone an ell in tin, since 85.
One of the last things I made in tin, was a shanty cap.

 

I bet OCC can make some serious metal ductwork...

 

It would take them a whole show to figure out which piece to put the pittsburg on.

 

Sr's temper and patience would not last that long...

 

This is an interesting website advocating metal duct. www.steelduct.org

 

I would be suprised if a organization formed by people with interest in steel formed a organization to promote flex or duct board. duh.

 

Beenthere, does that logic also apply to NATE, ACCA, RSES ?

 

Show me where they recomend against ductboard.
Show me where they have vested interest in one over the other.

 

Don't associate a commercial system that gas higher CFM meaning more pressure.
Most commercial is low pressure the same as residetial.

 

Why is it metal in commercial then? Fire code?

 

Engineers!
And not all commercial is tin.

Theres more then one 15 and 20 ton RTU with ductboard after the meal drops.

 

I consider anything under 2" low pressure.

A single truck off a 40 ton air handle would be too much of a PITA with duct board. Too much internal bracing has to be added to support it at those sizes. In that application metal is easier to install.

Would I install it on a 30 ton? Don't know, never been asked to.

 

I honestly dont have a problem with flex as long as it is installed correctly.