Flex Duct Vs. Metal/Insulated

 
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Old 09-05-2008, 07:00 AM   #1
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Flex Duct Vs. Metal/Insulated


I have a friend who is a HVAC contractor, is retired from the business but retains his license for situations like these.

He's layed out my HVAC for a home of mine and he has me sold on the design with exception of one thing.

A lot of flex duct.

3 years ago, I bought a house that the home owner changed out the furnace. HE had flex used off the plenum, bending all over. In short, almost no heat made it though the flex.

My HVAC friend says it's fine to use, only on strait runs off of a main trunk and it has to be pulled tight.

It's way easier to use in this application as the home is pretty much on grade.

My question is this. What is a estimated % that flex is going to restrict compared to metal? It would be 6" Flex and 8" returns. No flex runs are over 18' and most are under 12'.
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Old 09-05-2008, 09:50 AM   #2
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Re: Flex Duct Vs. Metal/Insulated


Flex must be oversized at least one duct size. Even when doing this the flex duct creates much more resistance. Air flow in a steel duct tends to travel smoothly in a coil-like motion. Air flow in a flex duct tends to tumble, probably because of the rib like intrusion every inch or so. Because of this action, the longer the run the greater the difference in performance with the steel duct being the better performer. This assumes a proper installation.

Oftentimes flex duct is not properly installed. Joints tend to fail more often, generally an installation error. Bends are especially resistant to air flow in flex.

It seems that the savings from a faster and cheaper installation don't translate into real money in your pocket. The increased cost of running the system longer more than eats up any installation savings.

Of course all this ignores duct cleaning which can completely destroy a flex duct.

In my jurisdiction the problems were great enough that flex duct was outlawed in residential as of 01/01/08.

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Old 09-05-2008, 10:00 AM   #3
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Re: Flex Duct Vs. Metal/Insulated


Thom said it all !!!
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Old 09-05-2008, 11:09 AM   #4
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Re: Flex Duct Vs. Metal/Insulated


Thanks, I felt the same way.

He kind of sold me on the idea because he said, the resistance is more, but the insulation keeps the heat rise higher than non-insulated galvanized.

The job is almost complete and I have flex ran down most of the strait runs, no bends in them just strait shots down the floor joist to the register boots. I was sure to pull them tight, no sags or changes in directions.

I've read that round galvanized is actually the best as it "cyclones" though like you had mentioned and flex and square duct work it has to "push" though.
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Old 09-05-2008, 11:45 AM   #5
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Re: Flex Duct Vs. Metal/Insulated


He sold you so he could have an easier installation.

you say the runs are in floor joist bays. Assuming this is between floors or in heated space, the insulation doesn't mean much. The real value of insulation withing heated spaces is to eliminate condensation when running refrigerated air conditioning. Insulating a steel duct will do that.

A not uncommon issue with the flex is that the tension put on the duct to stretch it works to pull the duct off the boot. Over time and with the assistance of a pressurized duct and vibration, they can separate. If properly installed, with screw heads holding the wire properly, this probably won't happen. If the fastener is just the zip-tie, it may. Even with the screws, the plastic inner duct can tear and pull away.

Flex duct was originally designed for installation above suspended ceilings. It was for short runs between the trunk line and the register that was dropped into the suspended ceiling grid. The flex duct would allow accurate placement of the register, reduce vibration on the grid, and insure that there was not excessive load transferred to the grid. This is a good application for flex. It can be accessed and replaced as needed.
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Old 09-05-2008, 07:13 PM   #6
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Re: Flex Duct Vs. Metal/Insulated


first of all you should never just upsize a run because it is flex, and second you should never install a forced air system without a load calculation. If you properly size your ductwork, then flex is not an issue, yes they should be ran as straight as possible and pulled tight, but I have personally tested airflow with a flowhood with and without flex using the same heat run for consistency and the air flow remaind the same, flex will increase the static but if properly sized will reduce overall noise of the system. and as for cleaning that is a SCAM. it is proven that the dirt and debris that are in the duct work under normal operation will not move around after initial start up, also maintaining a clean filter will also eliminate the use of a duct destroying cleaning system. I recommend a proper load calculation, and you can work fine with flex in the system for branch runs, just not the main trunk.
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Old 09-07-2008, 11:24 AM   #7
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Re: Flex Duct Vs. Metal/Insulated


Quote:
Originally Posted by thom View Post
Flex must be oversized at least one duct size. Even when doing this the flex duct creates much more resistance. Air flow in a steel duct tends to travel smoothly in a coil-like motion. Air flow in a flex duct tends to tumble, probably because of the rib like intrusion every inch or so. Because of this action, the longer the run the greater the difference in performance with the steel duct being the better performer. This assumes a proper installation.

Oftentimes flex duct is not properly installed. Joints tend to fail more often, generally an installation error. Bends are especially resistant to air flow in flex.

It seems that the savings from a faster and cheaper installation don't translate into real money in your pocket. The increased cost of running the system longer more than eats up any installation savings.

Of course all this ignores duct cleaning which can completely destroy a flex duct.

In my jurisdiction the problems were great enough that flex duct was outlawed in residential as of 01/01/08.
They outlawed flex duct (grade 1 flex w/ r-8 insulation?) Wow. I don't dipute the friction loss w/ flex duct, but not only is it easier to install, it's insulated. How often does one see insulated metal? At least here in the Boise area. I have seen many leaky metal ducts in crawl-spaces as well. Also, I think, spider webs are more likely to form in metal versus flex. I don't know why, but, I have taken metal pipe apart to find a lot of crud which I don't think would live in flex duct. I am not hating on metal pipe, but flex duct is more practical in many applications.
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Old 09-07-2008, 12:42 PM   #8
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Re: Flex Duct Vs. Metal/Insulated


I guess you live in a place with a different application. We don't generally build on crawl spaces or basements. The ducts are hidden by sheetrock. Replacing ducts requires pulling out ceilings.

Steel ducts must be sealed with a duct sealant. They should not leak. Then the ducts are insulated. This has been standard procedure here forever. Insulating a steel duct can provide the same level of insulation as insulating a flex duct.

And yes, they outlawed the flex in inaccessible locations. If you install it in a commercial building above a suspended ceiling where it can be changed or repaired when needed that's fine.
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Old 09-08-2008, 12:42 AM   #9
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Re: Flex Duct Vs. Metal/Insulated


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I guess you live in a place with a different application. We don't generally build on crawl spaces or basements. The ducts are hidden by sheetrock. Replacing ducts requires pulling out ceilings.

Steel ducts must be sealed with a duct sealant. They should not leak. Then the ducts are insulated. This has been standard procedure here forever. Insulating a steel duct can provide the same level of insulation as insulating a flex duct.

And yes, they outlawed the flex in inaccessible locations. If you install it in a commercial building above a suspended ceiling where it can be changed or repaired when needed that's fine.
Old school duct in general was sealed pretty well with duct tape, and hung with baling wire. Lot's of decent work at the time, but they never any mastic. Also, the ducts were not insulated. Yes, there is money to be made from insulating that duct, but I think it would behoove the customer to replace the metal branch lines with flex and insulate the main trunks.

In many crawl spaces, one is just glad that they are able to get duct from point a to b. not to mention insulating the duct. Don't you think the benefit of having insulated flex duct outweighs the lower restriction from round metal pipe?
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Old 09-26-2008, 10:11 PM   #10
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Re: Flex Duct Vs. Metal/Insulated


At the RSES Conference in Indianapolis one educational session was the "True Cost of Flex Duct". Here is the site of M&M Manufacturing that has downloads of flex duct information.
http://www.mmmfg.com/Default.aspx?sP...e=services_pdf
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Old 09-27-2008, 05:50 PM   #11
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Re: Flex Duct Vs. Metal/Insulated


We never use flex duct for anything. It looks like crap and restricts air flow and creates turbulence. It's noisy and easy to damage. It has gained great popularity amont those that wish to get the job done fast instead of right.

Crap, crap, crappity crap.
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Old 10-01-2008, 02:20 AM   #12
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Re: Flex Duct Vs. Metal/Insulated


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Originally Posted by nhmaster3015 View Post
We never use flex duct for anything. It looks like crap and restricts air flow and creates turbulence. It's noisy and easy to damage. It has gained great popularity amont those that wish to get the job done fast instead of right.

Crap, crap, crappity crap.
I don't know. I respect your opinion, and I dig on the fact that you hate to see the trades get dumbed down with flex duct, pex pipe, and ccst gas pipe. However, (at least where I live) almost everybody uses flex duct for residential installations. Honestly, most homeowners are more cocerned with how much the job will cost as opposed to correct static pressure, turbulance,...etc. Old school was not required to insulate the ductwork. Today, insulating the ducts is mandatory, and the extra labor/cost is just too cost prohibitive for most.
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Old 10-01-2008, 11:28 AM   #13
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Re: Flex Duct Vs. Metal/Insulated


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Today, insulating the ducts is mandatory, and the extra labor/cost is just too cost prohibitive for most.
That's false economy. In the real world tests have shown repeatedly that the savings from installing flex are more than offset with the losses in air flow efficiency. Assuming the buyer has a mortgage, the savings in mortgage payments from flex is less than the additional cost of energy from using flex.

Then there's the lifetime thing and the installation failures thing.
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Old 10-01-2008, 06:52 PM   #14
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Re: Flex Duct Vs. Metal/Insulated


[quote=thom;505753]That's false economy. In the real world tests have shown repeatedly that the savings from installing flex are more than offset with the losses in air flow efficiency. Assuming the buyer has a mortgage, the savings in mortgage payments from flex is less than the additional cost of energy from using flex.

Then there's the lifetime thing and the installation failures thing.[/quot

Do you mean somebody built two houses that were identical, one was run with flex, the other with insulated metal. The energy savings from the metal setup was so great that it paid for itself within ten years? If so, great. I am for energy savings, and want to provide what is best for the customer. But, it's a mighty bold claim to be makin Mr.


I don't dispute that air flow is better through metal pipe, but flex isn't so bad.
  • The flex should last a lifetime as well (plastic flex-liner, fiberglass insulation, netting, and another heavy grade of plastic)
  • The reason flex tears is due to pulling the duct against sharp edges. Otherwise, barring something out of the ordinary, flex should never tear in the middle of the run. It's pretty durable.
  • There can be problems with installation issues, but there can be problems with metal installations as well.
  • Flex is preferable in shallow crawlspaces and attics.

To me, the best set-up for the dollar is metal trunk-lines, and flex branches. Just keep things strapped tight and make the runs as straight as possible. And make sure nothing leaks.
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Old 10-01-2008, 07:42 PM   #15
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Re: Flex Duct Vs. Metal/Insulated


Testing duct is not nearly as complex as you make it. In fact, building two houses would allow for other factors that would make the tests less accurate.

It is pretty easy to measure the resistance to air flow through a duct and through a bend. The difference in resistance can be used to compute the additional cost to run the system. Pretty simple stuff. Yes, it has been done.

Flex duct installations tend to be sloppier than rigid duct because they can. This results in even more difference in energy consumption.

Using flex for an ell exaggerates the differences even more.

As for splitting in the middle of a run, where did I say that? It does have sharp edges against it. Every boot, every take-off, every tee, etc.

Flex duct is a great product for short runs from a trunk line to a lay-in register in suspended ceilings. It's always accessible. For houses, no.
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Old 10-01-2008, 08:22 PM   #16
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Re: Flex Duct Vs. Metal/Insulated


Quote:
Originally Posted by thom View Post
Testing duct is not nearly as complex as you make it. In fact, building two houses would allow for other factors that would make the tests less accurate.

It is pretty easy to measure the resistance to air flow through a duct and through a bend. The difference in resistance can be used to compute the additional cost to run the system. Pretty simple stuff. Yes, it has been done.

Flex duct installations tend to be sloppier than rigid duct because they can. This results in even more difference in energy consumption.

Using flex for an ell exaggerates the differences even more.

As for splitting in the middle of a run, where did I say that? It does have sharp edges against it. Every boot, every take-off, every tee, etc.

Flex duct is a great product for short runs from a trunk line to a lay-in register in suspended ceilings. It's always accessible. For houses, no.
You are the customer. You want metal pipe insulated, it's my job to do what you want, not what I believe is best. I don't think flex is that bad, it's just my opinion. I certainly appreciate opinions that are different
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Old 10-02-2008, 09:27 AM   #17
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Re: Flex Duct Vs. Metal/Insulated


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Originally Posted by MechAcc View Post
At the RSES Conference in Indianapolis one educational session was the "True Cost of Flex Duct". Here is the site of M&M Manufacturing that has downloads of flex duct information.
http://www.mmmfg.com/Default.aspx?sP...e=services_pdf
That was some "interesting" stuff. Good, bad or indifferent, where I live, flex is the norm and one is likely to lose a lot of jobs if they insist on doing the work in metal (extra labor). What I got out of the articles is...
make sure that the ducts are pulled tight, and keep the sagging to a minimum. Otherwise, the homeowner is wasting $$$.

I really appreciate how folks post various litterature on sites like this.
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Old 10-02-2008, 06:38 PM   #18
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Re: Flex Duct Vs. Metal/Insulated


Quote:
Originally Posted by flashheatingand View Post
That was some "interesting" stuff. Good, bad or indifferent, where I live, flex is the norm and one is likely to lose a lot of jobs if they insist on doing the work in metal (extra labor). What I got out of the articles is...
make sure that the ducts are pulled tight, and keep the sagging to a minimum. Otherwise, the homeowner is wasting $$$.

I really appreciate how folks post various litterature on sites like this.



Thanks always willing to pass info along. If you are not a member of RSES join your local chapter. Lots of good information and literature available at monthly education meetings.
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Old 10-05-2008, 02:00 PM   #19
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Re: Flex Duct Vs. Metal/Insulated


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Thanks always willing to pass info along. If you are not a member of RSES join your local chapter. Lots of good information and literature available at monthly education meetings.
30 year member here. Damn, where'd the years go
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Old 10-05-2008, 02:08 PM   #20
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Re: Flex Duct Vs. Metal/Insulated


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30 year member here. Damn, where'd the years go
By the looks of it......
sleeping in a coffin?

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