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Old 01-26-2012, 11:48 AM   #1
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Enlarging two 6" runs


I have two 6" flex runs to our bedroom that I need to enlarge.

There is simply not enough supply air to our bedroom.

I've attached a crude sketch of how it is right now.

The trunk is 16' total. It starts as 25"x9" and reduces to 16"x9" after 8'.

The shorter flex run is about 25'. The longer one is another 8' further at 33' -- so they're pretty long runs.

There are 9 runs that come off of that trunk. All are 6" runs -- however two of them are to the basement and are somewhat dampered off.


Some questions:

1. Should I try and figure out a Manual J/D for this or is that overkill? The room is 22'x24' with a tall 18' cathedral ceiling. I'm just thinking I could add more supply and then damper at the trunk and the wye if necessary. Also, it seems that trunk is adequately sized for this but I honestly have no way of knowing exactly.

2. Instead of running two separate runs from the trunk seems like it would make sense to combine them into perhaps a 10" run. Then I can pull a 6" off for the first drop, and then run 8" to the second.

3. It's all flex right now. With all those turns and such the flex duct is definitely not stretched out nor are the corners smooth. Would it make more sense to run rigid pipe for most -- if not all of this?

Help?
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Old 01-26-2012, 01:25 PM   #2
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Enlarging two 6" runs


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Originally Posted by Rewound98 View Post
I have two 6" flex runs to our bedroom that I need to enlarge.

There is simply not enough supply air to our bedroom.

I've attached a crude sketch of how it is right now.

The trunk is 16' total. It starts as 25"x9" and reduces to 16"x9" after 8'.

The shorter flex run is about 25'. The longer one is another 8' further at 33' -- so they're pretty long runs.

There are 9 runs that come off of that trunk. All are 6" runs -- however two of them are to the basement and are somewhat dampered off.


Some questions:

1. Should I try and figure out a Manual J/D for this or is that overkill? The room is 22'x24' with a tall 18' cathedral ceiling. I'm just thinking I could add more supply and then damper at the trunk and the wye if necessary. Also, it seems that trunk is adequately sized for this but I honestly have no way of knowing exactly.

2. Instead of running two separate runs from the trunk seems like it would make sense to combine them into perhaps a 10" run. Then I can pull a 6" off for the first drop, and then run 8" to the second.

3. It's all flex right now. With all those turns and such the flex duct is definitely not stretched out nor are the corners smooth. Would it make more sense to run rigid pipe for most -- if not all of this?
Help?
for best results, run a 3 rd supply.....enlarging the other two will do nothing......also if you want to gain a bunch of air flow switch to hard pipe....30 gauge

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Old 01-26-2012, 02:59 PM   #3
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Enlarging two 6" runs


In my scenario adding another supply is very difficult but I'll strongly consider it.

Sure, I can convert it all to hard pipe.

So just two 6" rigid pipes replacing the 6" flex?

I'd think combining them would have some advantages -- if only for lower cost and less to install.

Are there advantages to keeping them separate?
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Old 01-26-2012, 03:08 PM   #4
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Enlarging two 6" runs


I wouldn't run two 6" hard pipes over there. If you want hard pipe, run one 8" hard pipe to the center of the registers and tie in with the 6". The time you spend running, sealing, and insulating two long runs will far surpass the time you spend on one long run and some minor trunk line modifications. To each his own, but that's what I would do.
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Old 01-26-2012, 03:22 PM   #5
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Enlarging two 6" runs


For a vaulted ceiling run the 8" to the first register and tee off to the second one. No need to overshoot the register, my first post assumed a flat ceiling.

I just read your post again without the wife and dogs yapping in the background. You seem to have it mostly figured out. If those two existing registers are 4x10 fed with a 6" supply, a 8" hard pipe will do fine.

Last edited by brandonmcginnis; 01-26-2012 at 03:26 PM.
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Old 01-26-2012, 04:09 PM   #6
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Enlarging two 6" runs


again..........increasing the size will LOWER the volosity.....you will have even less flow then what you have now......for more "air" into a space, another run has to be added. you are losing some air flow by using flex, the more flex you use the more air flow you lose........

another thing......how are you going to put a 10 inch take off on an 8 inch duct? do not TEE in thats the worst thing to do.....you are robbing peter to pay Paul when you TEE in.
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Old 01-26-2012, 05:18 PM   #7
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Enlarging two 6" runs


Get rid of that 9X16 duct. Run 9X25 in its place. The trunk is short enough that no reduction is needed. this will help all the supplies that are currently connected to that 9X16 trunk.

Then run a 10" over to the MB and use a 10X7X7 wye and run that to each of those registers.

Hard pipe will work better.
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Old 01-26-2012, 05:54 PM   #8
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Enlarging two 6" runs


First you are losing about 30% of the designed air flow with flex pipe (maybe more with all the bends and saging you have).
You will pick up air flow with hard pipe 30 ga. With the hard pipe go all the way to the end before using a 90 degree ell.You will eliminate a 90 degree bend doing it that way and eliminate some friction.
You could put 4 runs into that bedroom and if you have no return air in it you still won't be comfortable.
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Old 01-27-2012, 10:37 AM   #9
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Enlarging two 6" runs


Thanks. Some good info here.

I'll post up after I finish up the changes ...

Oh, one last question.

Of course I'll put a damper in the new 10" takeoff.

Should I put dampers in the wye also or is that not necessary?
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Old 01-27-2012, 04:26 PM   #10
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Enlarging two 6" runs


Dampers at the Wye will help for balancing.
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Old 01-27-2012, 10:55 PM   #11
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Enlarging two 6" runs


I would try shorting those two runs up. The way there run there is additional length and bends that do not need to be there. Also, could there be any other issues causing low static? A couple of things you may want to look at are your filter. Is it clean? If your using a pleated try a poly. Is the fan performing as it should? clean? Are all the joints in your trunk line taped up good?
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Old 10-23-2012, 12:09 PM   #12
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Enlarging two 6" runs


So I'm finally getting around to this and am asking for some advice based on new information.

Here's few things I didn't know or didn't post at first that make this difficult so I'm trying to get the biggest bang for my $$ here.


All of the duct work is enclosed in a soffit running in the basement. Underneath the bedroom is a garage and underneath the living room is the game room.

I've cut some openings into the soffit to see what I'm up against.

The flex is junk. Seriously. It's just merely laying on top of the soffit. The three bends aren't sharp but they're very inefficient and the flex duct is hardly stretched like it should be nor supported.

Half of each run is 30 year old flex and the other half is much newer flex -- so somebody has looked at this before but made a substandard repair. I can tell it's newer because of the R4.2 VS R8 insulation around the flex.

Near the joist cavity I can feel some hard pipe inside so it's either a booster fan or some attempt at joining the flex. I see no wires though but they could have cut them.

The worst part of this design is that the joist cavity pinches both flex runs.

The opening is approx 12"x9.5" -- and that's not enough room for two 6" insulated flex ducts -- so that's bad.

That is surely causing some of the reduced air flow, no?



So any advice on how I could proceed?

Part of me says to only:

1. replace the section that goes through the joist cavity with two hard 6" pipes

2. add a few rigid 90s where possible

3. insulate it externally and call it a day

That would help enormously I'm sure as this is definitely constricting air flow.


The other part of me says to:

1. leave the newer flex and replace the sharp 90s with rigid 90s and pull the flex tight.

2. run 9" hard pipe from the plenum through the joist

3. using a 9x6x6 wye to connect

4. run 9" hard pipe back to the plenum


This will avoid much demo in the garage. I can more easily demo the soffit nearer the plenum and the joist cavity.

Also, the joist cavity is only 9.5" high. I don't think I can get 10" in there without some deflection. It's close...so maybe the 10" would work? I could run 10" until it got to the joist then reduce it to 9" before the wye?

Help?!
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Old 10-23-2012, 12:16 PM   #13
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Enlarging two 6" runs


It has been asked in prior posts, do you have return air in the MB?

If not then you are fighting a losing battle, kinda like trying to fill a jar thats already full.

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Old 10-23-2012, 02:09 PM   #14
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Enlarging two 6" runs


Ah. Yes, there is a return in the room.

Our energy audit also checked the air pressure difference with a manometer to ensure that wasn't the issue.

I can't overstate though how pitiful the air flow is.
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Old 10-23-2012, 04:29 PM   #15
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Enlarging two 6" runs


If you can replace those 6" with 7" you'll get much better air flow. A 9" can feed 2-7". A 6" flex is about equal to a 5" hard pipe. A 7" flex is equal to about a 6" hard pipe.

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