Replacing Flex Duct

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Old 03-28-11, 03:01 PM
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Replacing Flex Duct

I've got a bedroom in my house that is a shade warmer than the others in the warmer months. My daughter (the occupant of that room) has been relatively quiet about it up to now, but she is really starting to let me have it as the weather is heating up, so I've got to do something about it.

I've been up in the attic and the flex duct that runs from the plenum to the register in her room is 7 inches in diameter and about 30 feet long. It's gray and probably about 27-28 years old (judging by when the 2nd AC unit was added to the house). It's showing its age and has some holes in the outer layer and I can feel air seeping through in spots, so I'm thinking that replacing the flex duct will do the trick, however, I've got a few questions for the experts out there.

My prior research, IIRC, has told me that flex duct now comes in 8" diameter (no longer 7", or it just may be harder to find) and in runs of 25 feet. If either of these aren't true, please correct me, along with any advice on the 7" vs. 8" ducts. Otherwise, here are my questions:

1. 8" flex duct will obviously (to me) increase the amount of cool air that flows to that room, which I believe is a good thing. However, what modifications (if any) will I have to make at the plenum or the register to accommodate the larger duct? And, what effect (if any) will the larger duct have on the other rooms that are serviced by this same unit? I don't believe there are dampers (at the plenum) on the ducts from this unit.

2. Since I need 30 feet and the ducts come in runs of 25 feet, what is the proper way to splice two runs together? I don't see any type of splice on the current run (maybe they sold them in longer runs back then?) to see how it was done, so I need some advice here.

3. What recommendations can you make toward any special brand or type of flex duct? I know the foil exterior is the way to go to stop heat transfer from the attic to the inside of the duct, but are there any other recommendations?

Any advice will definitely be appreciated.
 
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Old 03-28-11, 07:46 PM
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You can still get 7in flex. Adding an 8 you will decrease from the rest of the rooms. You will have to replace the collar and the boot to use the 8 in duct.
 
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Old 03-29-11, 02:16 PM
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Airman's right. Stick with the 7" & get the highest R-value you can find. Splices can be made with 7" metal pipe. You may want to run a couple of sections of metal pipe (externally insulated) then connect your flex to that. It would be cheaper than buying 2 boxes of flex.
 
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Old 03-30-11, 06:18 PM
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Thanks airman and Grady. I just went up in my attic and refreshed my memory as to what exactly is up there. I had forgotten that a couple years ago, I had an A/C technician replace some flex duct to another room and there is a 5.5 foot section of 7" flex duct just sitting up there ready to be used. I believe that, along with a new box of 7" flex duct should get me to the 30 feet needed.

However, I do have a few more questions:

1. I'm recalling (maybe incorrectly) that the big box stores (HD and Lowes) don't carry 7" flex duct. Would any A/C supply house carry it, or where do you suggest I get it? Are there any recommendations on a brand or type of flex duct? I seem to remember a while back, a local radio show handyman recommending a certain brand of flex duct, although I don't remember what the reason for it was.

2. I couldn't see an R-value on the piece of flex duct that is up there. How do I determine the R-value? What is a good R-value for flex duct?

3. Finally, where do I obtain the 7" metal pipe for the splice? I'm guessing that since I found that 5.5 foot piece and thus, the duct work should cover the full 30 foot run, I won't need the external insulation for the splice. Is that correct?

Thanks again guys.
 
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Old 03-30-11, 06:48 PM
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Any HVAC supply house would have the flex & pipe as would most plumbing supply houses.
I suggest an absolute minimum of R-6 & prefer R-8.
I don't care about brand but do prefer a mylar jacket to one of vinyl or polyester & the silver seems to hold up better than the gray.
To make the splice use about a 2' section of pipe & slide the pipe 1/2 way into each piece of flex. Work slowly as not to tear the flex liner. Slide the insulation & outer jacket back enough to give you room to work & wrap the end of each liner with 3 full wraps of UL181 tape. Slide the insulation & jacket of each piece of flex up to meet the other. You can wrap this outer joint with the tape or use large zip ties made for the purpose.
 
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Old 03-30-11, 07:01 PM
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I'd never recommend splicing flex duct! It's also against code in my area
 
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Old 03-30-11, 07:26 PM
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Originally Posted by airman.1994 View Post
I'd never recommend splicing flex duct! It's also against code in my area
Thanks airman, but since I'm not an A/C tech and have never bought flex duct (I've only seen it in the big box stores in runs of 25 feet max and I've also seen it online in runs of 25 feet max), where do you find flex duct in runs longer than 25 feet? Do HVAC supply houses sell it in longer runs? Please help a novice out.
 
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Old 03-30-11, 08:19 PM
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I don't know that I've ever seen flex sold in over 25' lengths. Personally, I wouldn't make a flex run that long but I've seen 'em. There's a lot of friction loss in flex due to the helix construction. If all you want to do is replace an existing run, do it. If you want to upgrade, that's a whole different ball of wax.
 
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Old 03-30-11, 10:44 PM
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Thanks Grady. I wish I could say that is the longest flex duct run in my attic, but it's not even close. I've got what must be a 15" flex duct that runs oh, I'd say anywhere from 40 to 45 feet and goes into a junction box at about 30 feet, where it splits 3 ways. That goes to our master bedroom with a tray ceiling (9 foot in the center) and bathroom. Our master bedroom cools fine in the cooler months and in the evening in the hotter months, but during the day in the summer, it gets very hot in there. I know I need a complete redesign of my bedroom A/C system, and have sought it out in the past, but what I've run into is a lot of "maybes" and "I think(s)" with a huge price tag attached. So, I've been unwilling to do it. I guess I haven't found the right A/C technician that both knows what they're doing and has the equipment to do it properly.

So, yes, I'm just looking at a replacement right now of that one flex duct run, hoping that it improves it enough to make my daughter comfortable and get her off my back. If I can get flex duct in a larger run, and it's not cost prohibitive, I'd prefer that, as it would be less labor intensive (i.e. no splice), so, I need airman to let me know if it's a possibility.

The bedroom A/C unit in my house was a retrofit back in 1983 when a previous owner added on to the house and installed a 2nd A/C unit and zoned the house. It seems that a poor job was done overall, but from my untrained eye, there may not have been much choice in the placement of the unit, because the attic access is not very good (I have to almost crawl to the unit). The A/C unit servicing the front of the house is fine as it was installed when the house was built in 1970, with metal ducts and very good service access.
 
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Old 03-31-11, 08:22 PM
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I'd run twenty feet of 7 inch hard pipe over two her room then flex from there
 
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Old 03-31-11, 08:27 PM
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Originally Posted by airman.1994 View Post
I'd run twenty feet of 7 inch hard pipe over two her room then flex from there
Yep. Sounds like a plan to me.
 
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Old 04-11-11, 05:33 PM
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Originally Posted by airman.1994 View Post
I'd run twenty feet of 7 inch hard pipe over two her room then flex from there
airman and Grady, I donít want to sound ungrateful, because I truly appreciate the time and advice youíve given me so far, but that advice is causing me more grief, when the fact is, I came here in hopes of relieving at least some of my grief. I also realize that you guys are giving me the best advice with the hard duct recommendation and that my attic and the current set up of my rear A/C system, along with my lack of experience are what is truly causing me the grief, but it is what it is, and I have to deal with that. Thus, I would greatly appreciate a little more information and input from either or both of you, or anyone else for that matter.

First, let me tell you guys that Iíve had numerous A/C contractors in my attic to get some recommendations on how to improve the efficiency of my rear A/C system. Not a single one of them have suggested hard duct, and only a few of them have recommended replacing the flex ducts. I truly believe that itís not because they donít think either would improve the situation; itís that nobody in their right mind will want to get up there and deal with the lack of work room and head room it would take to run new ducts.

With that in mind, hereís what Iíve got (and Iím only going to address the one duct that caused me to come here in the 1st place), along with a few questions (in bold) that follow:

1. This duct comes off the top of the plenum and immediately makes a 90 degree turn. Iím sure there is some airflow loss right there, but even worse, I canít even get to it, because of 2 other ducts coming off the plenum and various support boards coming off the roof. Iíd have to remove the other 2 flex ducts from the plenum and then literally crawl and balance on the beams to get to the duct in question.

2. The duct then goes about 8-10 feet, over and under those other ducts, and is hung up to the rafters with some sort of wire cable (more loss of airflow, Iím sure).

3. Then, the duct makes another 90 degree turn to head in almost a straight line to the register, approximately 20 feet away. That long run is where the duct is the most accessible, but there is still not ideal space for putting together and wrapping hard duct, but I think it can be done.

4. Finally, at the register, there appears to be a 90 degree hard duct transition that attaches the flex duct to the register.

5. I have located 5 foot sections of 7Ē 30 gauge hard duct at the local Lowes at an affordable price. Iíve also found 3 foot sections of 7Ē 26 gauge hard duct online that is around the same cost as the 5-footers at Lowes. I havenít checked with any local supply houses, because I just donít know who to contact, but I plan to do that as well. Besides the obvious (i.e. thickness), what are the benefits of the 26 gauge vs. the 30 gauge, and do you recommend one over the other (with cost and effort putting it together being factors)?

6. Would you recommend against running flex from the plenum and then hard duct to the register? Because of the access issues, this seems more doable than the other way, which is how I understood your last post.

7. I have specific questions about putting the hard duct run together. I know that you use sheet metal screws, mastic, and the UL181 tape at the joints, but how is the mastic applied? Is it just over the joint after the screws, or do you also put the mastic on the inside of the joint before the screws, as well? Finally, on the insulated duct wrap, how is that closed off and secured around the hard duct? With some UL181 tape, or what? And, how tight does it need to be?
 
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Old 04-11-11, 06:10 PM
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26 vs. 30 ga.: Not a big deal with 7" pipe. 30 is easier to cut but not as rigid. Durability is not likely an issue but being thicker, obviously 26 is going to last longer.

Yes, I would recommend against starting at the plenum with flex. If you are going to use flex it should be at the register end for air flow restriction & sound deadening purposes.

Mastic is usually applied with a brush. Once the pipe is put together & screwed, then apply the mastic. No need to put it on the male ends prior to assembly. The duct wrap should be installed snugly but not so tight as to compress it. The overlap should be secured with outward clinching staples & then sealed with UL181 tape which is squeegied (sp?) down. An easy but somewhat expensive way to insulate metal pipe is to pull flex over it. That way you don't have seams to deal with.
 
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Old 04-11-11, 06:24 PM
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Originally Posted by Grady View Post
Yes, I would recommend against starting at the plenum with flex. If you are going to use flex it should be at the register end for air flow restriction & sound deadening purposes.
Thank you, thank you, thank you, Grady. All but the above were extremely helpful. I just don't see how I can start at the plenum with hard duct. There is just little to no room. I'll go up there and give it another looksie, but I'm not optimistic. While I understand it would not be ideal, would a flex run at each end with a long hard duct run in the middle be a better alternative than just a flex duct run of 30 feet?

Originally Posted by Grady View Post
An easy but somewhat expensive way to insulate metal pipe is to pull flex over it. That way you don't have seams to deal with.
Just a thought, but what size flex duct would I need to pull over the 7" hard duct? Would I go same size (i.e. 7") or would I need slightly larger (i.e. 8")?
 
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Old 04-11-11, 06:55 PM
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I fully understand your problem in starting with metal at the plenum. Sometimes it's impractical, bordering on impossible. Been there, done that. If you feel you must use flex, keep it as short as possible but not so short as to create kinks then go to the metal. If you want to terminate with a 3 foot or so of flex that's okay too.

Use the same size flex as is the metal. It will be snug but can be worked onto the metal. I have seen people pull out the wire helix liner to make it easier but you have to be careful not to bunch up or rip the fiberglass.
 
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Old 04-18-11, 12:42 PM
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Originally Posted by Grady View Post
I fully understand your problem in starting with metal at the plenum. Sometimes it's impractical, bordering on impossible. Been there, done that. If you feel you must use flex, keep it as short as possible but not so short as to create kinks then go to the metal. If you want to terminate with a 3 foot or so of flex that's okay too.

Use the same size flex as is the metal. It will be snug but can be worked onto the metal. I have seen people pull out the wire helix liner to make it easier but you have to be careful not to bunch up or rip the fiberglass.
Thanks Grady. That's the type of advice I was hoping for. Now, for grins , I went up in my attic for another looksie (to check for "room to move") and I took my camera with me, so I could share with others. See the attached photos for an idea of what I will be dealing with to change out one run of flex duct. In case the photos don't accurately reflect it for others, I cannot stand up at all in this area of my attic.

Any ideas or feedback will be greatly appreciated.















 
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Old 04-18-11, 03:58 PM
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That flex is awful. Flex should be strapped and pulled tight. That's one reason u have no airflow.
 
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Old 04-18-11, 06:44 PM
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Originally Posted by airman.1994 View Post
That flex is awful. Flex should be strapped and pulled tight. That's one reason u have no airflow.
Agreed. Also flex should never be allowed to rest on other flex runs. To do so often crushes both runs. Something you pointed out in the second photo is the flex being suspended with wire. This too is a MAJOR no-no. When flex is suspended it must be done with webbed material similar to lawn chair material of at least 3" wide & run so as to support at least 2 wire wraps in the duct. This webbed material is usually black & comes on a roll.
 
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Old 04-19-11, 01:01 PM
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Originally Posted by Grady View Post
Originally Posted by airman.1994 View Post
That flex is awful. Flex should be strapped and pulled tight. That's one reason u have no airflow.
Agreed. Also flex should never be allowed to rest on other flex runs. To do so often crushes both runs. Something you pointed out in the second photo is the flex being suspended with wire. This too is a MAJOR no-no. When flex is suspended it must be done with webbed material similar to lawn chair material of at least 3" wide & run so as to support at least 2 wire wraps in the duct. This webbed material is usually black & comes on a roll.
Thanks again guys for the valuable information. I've been in this house since 1992 and the only flex duct that's been installed since I've been here is the foil one, which is neither strapped or pulled tight, and runs up over a rafter (i.e. kinked) and back down to a register right in front of the furnace. That register is actually a short distance from the plenum, but because of its close proximity to the furnace, the flex duct length is probably triple what it could be if run straight to the register. No complaints from the occupant of that room, however, and by feel, the airflow seems fine. The pictures go to show you both, if anything at all, that although there's been plenty of opportunity for almost 20 years, no A/C contractor has wanted to make my ducts right on this unit.

The question still remains though: With the limited access and space, will I be able to run hard duct (to replace the old gray flex) around the plenum area? I know you guys are seeing it in one-dimensional photos, but from what you see, is it something you could see yourself tackling with success? I am pretty handy and very determined to do things the right way (in case you haven't figured that out yet . . . lol), but I'm not stupid and don't want to try to tackle something that is next to impossible.

I can almost envision doing it with several 90 degree turns before getting to the long straight run. Plus, correct me if I'm wrong, but it would seem more appropriate for the hard duct to run under the flex duct, so the flex duct could be properly strapped up above it. Is that a proper way of looking at it?

Also, can the hard duct lay on the attic floor across the rafters, or does it too need to be strapped up?

I will definitely check into the strapping material.
 
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Old 04-19-11, 07:48 PM
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My attic isn't quite that tight but close. Is there any way to run a trunk duct & then have the individual branches come off of it as opposed to what appears to be a system with all or most of the branches coming off the plenum? From what I see, I would have to believe you'd have to install the branches & work the trunk back to the furnace as you backed out. Hard duct can rest on the floor or ceiling joists without worry as long as it is well insulated. Wherever flex crosses the hard duct the flex needs to be supported.
I hate to say this for fear of sounding like a salesman but at 20+ years old, the equipment itself has to be getting long in the tooth. It wouldn't cost anything to have a few contractors come in & quote you on a new system, duct work & all.
 
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Old 04-19-11, 09:39 PM
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Originally Posted by Grady View Post
My attic isn't quite that tight but close. Is there any way to run a trunk duct & then have the individual branches come off of it as opposed to what appears to be a system with all or most of the branches coming off the plenum? From what I see, I would have to believe you'd have to install the branches & work the trunk back to the furnace as you backed out.
Not sure I understand what you are describing, but let me take a stab at it. Are you saying that I should run a large single trunk duct off the plenum, closing off the remaining holes on the plenum, and then branch off that large trunk duct to the registers? If so, that would be way beyond my expertise. Just closing up the existing holes would concern me. However, I will keep that in mind for when I finally do get an A/C contractor that will go the extra mile to make that system right.

Also, I should probably go up into my attic from the other side (there is another access from the master bedroom, but it is not as easy to use [i.e. need a ladder]) and shoot some photos. That would give you a more complete picture of the plenum. There is a bit more room coming from that side, but it's a long haul (about 35 feet give or take) from the attic access to the plenum.

Originally Posted by Grady View Post
I hate to say this for fear of sounding like a salesman but at 20+ years old, the equipment itself has to be getting long in the tooth. It wouldn't cost anything to have a few contractors come in & quote you on a new system, duct work & all.
LOL Don't worry about sounding like a salesman. Truth be told, the only things that are 20+ years old on that unit are the furnace and the duct work (excluding the foil duct). American Standard condenser unit was installed in 2002 after a hailstorm damaged the old Coleman unit. The current unit has a 10-year warranty. The evaporator coil was replaced in August 2006. I just looked at the invoice and it says "very tight work area," which gave me a chuckle. I couldn't locate the invoice for the foil flex duct, but I believe it was done 2-3 years after the coil, so 2008 or 2009.
 
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Old 04-19-11, 09:56 PM
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I'd say u have no warranty manufactures do not warranty there equipment on mis matched systems.
 
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Old 04-20-11, 12:03 AM
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Originally Posted by airman.1994 View Post
I'd say u have no warranty manufactures do not warranty there equipment on mis matched systems.
A bit off topic, and I'm not sure why you're going there, airman, but you are wrong in this instance, as I've already made 2 warranty claims and American Standard paid them both, the last one being the replacement of the compressor in January 2010.
 
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Old 04-20-11, 12:31 AM
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I have found a new brand for home juicer... The Breville BJE510XL is such a wonderful juicer in the world.. I hope all of you can experience also like what I've experienced.
 
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Old 04-22-11, 12:54 PM
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Wow Kim, I think I need that juicer for when I'm up in my steaming hot attic replacing my flex ducts. Thanks a bunch.

Grady, below are a couple of photos from the other side of the plenum. These were taken from the attic access back in the master bedroom. The plenum is about 30 feet away, but I used a zoom lens to bring it closer to view. Knowing what I know now, after the advice I've received here, my flex ducts on this unit are a complete and utter mess.





I think I've mentioned previously that the master bedroom gets unbearably warm in the late afternoon during the hotter times of the year. I think I see at least one reason why (i.e. the ducts). A good fan, the fact that it cools down pretty well after the sun goes down, and the fact that I haven't been able to find a contractor to remedy the problem up to now, has caused me to just live with it. However, I'm ready to do something about it now. I'm going to renew my search for an A/C contractor. Can you give me any advice on how to find one both WILLING and ABLE to do the work and do it properly? And, do you think I should insist on hard ducts, maybe with a short run of flex at each register?
 
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Old 04-22-11, 05:33 PM
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Like trying to find any kind of contractor, I suggest talking to friends, neighbors, relatives, co-workers, etc. about whom they've had do similar work & how well satisfied they were or were not. I would much prefer hard duct, mastic sealed & externally insulated, with a short run of flex if so desired to the register.
 
 

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