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Old 06-06-2010, 01:29 AM   #16
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OMG It's Dr. Octopus. Where's Spiderman when you need him?

why use flex for all that?

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Old 06-06-2010, 05:57 AM   #17
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Going from a rectangular/square plenum to flex back to a rectangular duct adds over 70 foot of equivalent length to the duct system. Which then means the duct should be made bigger, to handle the added restriction. So doing that isn't really a good method. Best to use the type trunk line right from the plenum, instead of transitioning back and forth from one to the other.

Flex can be use as the trunk line. And then wye's used for the branches.
Your trunk line done in flex would need to be 16" flex.
I'd advise going to 7" for the 2 bedrooms.

The current system, has lots of restriction, and forces enough air out to them, since they are the first supplies. The new duct system will have them as the last supplies, and need to be made less restrictive. Going to 7" will do that.

How long will the 8" to the new room be?

On the return. When the flex ties into the return plenum. It should be tied in that its as far back on the plenum as it can be. without being tapped in at the end cap of the plenum. Turning vanes in the plenum will greatly help to reduce the turbulence in the plenum. Allowing easy air flow in it.
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Old 06-06-2010, 09:12 AM   #18
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Morning
I like the idea of using flex for the return. I would like to make the air handler as accessible as possible and it seems flex would give me some options I would not have with duct board.

I think the 8in flex to the new room will be in the 15 to 20 foot range, nothing is 100% yet. Is 8in flex suitable for a 12X15 room with 90 square feet of vinyl, insulated,tinted, low E windows and R30 in the ceiling? Windows get little direct sun. I am going to plant a tree and make it zero, direct sun.

Are you saying I could use flex for both trunk lines??

Nothing here is chiseled in stone and I am open to options.

Thanks for the responses!! Dr Octopus
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Old 06-06-2010, 09:26 AM   #19
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Yes flex can be used for both.

Depends how straight the 8" is ran.

Even if you use flex for supply and return. Use a plenum on the return and supply to connect the flex to.
A round to rectangler transition is better. But not required. And often, room prevents using it.
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Old 06-06-2010, 02:07 PM   #20
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Quote:
Originally Posted by jjrbus View Post
Morning


Thanks for the responses!! Dr Octopus
I am not an HVAC guy so you would do best to listen to their recommendations but I would like to toss in a couple of personal thoughts:

working around flex, ductboard, and tin for 20 years I have seen a variety of problems and I have seen where flex seems to cause problems.

The fact it is difficult to keep in a straight line, the whoop de doos seem, to me, that that would add length, restriction, and turbulence; none of which are desired.


the duct board is damaged by water so using it where it might be subjected to an overflowing drain pan for the evap coil would seem to be asking for problems. It also cannot be cleaned internally and if you listen to the naysayers, that fiberglass on the inside will cause glass fiber to be blown around in your house.


so, due to that, tin sounds like the best way to go in my mind.

and if the HVAC guys want to beat me up and say why I am wrong, I am more than willing to learn.

If are concerned about accessibility; you design the system so what needs to be disassembled is made to disassemble.
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Old 06-06-2010, 02:16 PM   #21
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Flex installed right doesn't have those whoopty doos. Using proper supports instead of 3/4" plumbers strap helps a lot.
Speed demonds won't spend the time to stretch it out right. A good company, or DIYer will.

Duct board. like sheet metal, doesn't need to be cleaned. Duct cleaning has never been proven to help efficiency. And, there is no proof that the dust or dirt that people are told is coming from their dirty duct. Is stopped by duct cleaning weather sheet metal or duct board.

Sheet metal will rust out when exposed to water.

Buy duct board with the guard, and no fibers can be blown around.

http://www.specjm.com/products/ductboard/enviroaire.asp
http://www.specjm.com/commercial/airhandling.asp
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Old 06-06-2010, 05:23 PM   #22
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Thanks for the input. I'm all ears. Here in SW Florida I do not think sheet metal would be an option for the homeowner. It is not as available as it is in the north. I am not in building, but I have never seen it in residential construction here in FL.

My 8" flex to the new room will not be straight it has to wander through some trusses and drop down through the ceiling!!

The duct board with guard seems like a good idea! It looks like the flex duct is already lined??

It would seem the best way to cut down on dust would be regular filter changing.
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Old 06-06-2010, 05:38 PM   #23
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Might need 2-8" then. or 1-8" and 1-7" to that room then.

Air filters are the best way. The duct work only gets dirty, if your filters are leaving dirt through.

If you really want to use sheet metal. You will find plenty of shops that do commercial can make it for you. Or, you can order it online.
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Old 06-07-2010, 08:53 AM   #24
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2 8"!! I'm starting to wonder if this is more trouble than it is worth?? A window unit or mini split may be the best option.
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Old 06-07-2010, 09:56 AM   #25
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Long flex runs are restrictive. So you don't get the air flow that you may first think your going to get.

At 100 foot of total equivalent length, and at a .1" friction rate. An 8" flex only delivers 185 CFM.

With the take off, register boot, register, a few bends and a few whoopie doos. You'll be well over 100' of total equivalent length.

If the room isn't going to be used as much as the rest of the house. And isn't open to the rest of the house for a return path. Often a mini split is the best way to go, in the long run.
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Old 06-08-2010, 05:57 PM   #26
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One more try at this, I'm hoping for special dispensation!! I keep the stat at 80 degrees in the house, I have the door to the room open, it is 95 outside temp and the temp in the room is 88. That is a 7 degree difference with no duct to the room, only the kitchen (room I took picture from) ceiling fan moving air.. Is it possible that this room would work with only one 8" flex? Or am I wasting our time??
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Old 06-08-2010, 06:05 PM   #27
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Hard to say.

When you run the duct to there. Your other rooms won't be getting as much air. So maybe it will stay cool with 1-8" run, maybe not. It won't be 7 warmer anymore though either way.

So does that room never get more then 7 warmer. or does it get warmer later in the summer.
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Old 06-08-2010, 11:00 PM   #28
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I'm wondering if it might be worth experimenting with a 8" flex duct off the current system and see what happens??

I have framed in for a window air, to use with a small generator during electrical outages, I could use that during hot spells as a supplement. I still have the option to add a cheap mini split.

I do not know about changes in the room temperature. I just finished putting in new windows and insulation so everything is new. I unintentionally left the door open and noticed it was not that bad in the room. So I put my thermometer there and watched it for 2 days. During the the day it averages 7 degrees cooler in the center of the room. 95 degrees is typical for summer here. Of course here in FL it is the humidity that can make life miserable.

Thank you for the time and effort you have put into responding to my posts.
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Old 06-09-2010, 05:27 AM   #29
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You can try a single 8", and see how it works.
If not good enough. Add another later.

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