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Trunk and Flex branches

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Robdev

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Discussion starter · #1 ·
I have a 1500 sq house, 2.5 ton ac. 7 runs. Bedroom is 250 sq addition with original 1970’s round metal ducts. I want upgrade to flex, trying to figure out the right sizing. I believe I need an 8 inch flex duct for bedroom, but do I branch this off a trunk or run directly off plunum? I think I go off a trunk but if so what size trunk do I use?
Thank you
 
#3 ·
Duct design and layout is a large job. Best to find a contractor willing to design it for you.
Replacing metal with flex of the same diameter will cut your airflow by about 20%, so take that into consideration.
 
Discussion starter · #5 · (Edited by Moderator)
Vstech thank you for the replay. Though I do agree hiring a professional is the best move, itÂ’s just not an option at this point. My recent quotes come in around quote $**** and this is just to far out of my price range. Jbhenergy you may be correct the flex is not an upgrade, but at this point pretty much anything is an upgrade from my original 1970Â’s bent, leaking and dented duct work. Again IÂ’m just trying to get some direction on fitting a new duct system. In my research I believe I need to run branch duct work off a trunk duct work from the plunum. My calculation came to 8 inch flex for the 250 sq room. So IÂ’m curious what the trunk size would be. Also if flex is not an upgrade than even going professional is an issue since thatÂ’s what both places quoted me for.
Thank you both for your help.

Please do not add pricing to your posts. It is not allowed by forum rules.
 
#6 ·
Has the house been comfortable before the bedroom addition? Where is the extra air/cooling BTU's(CFM) coming from w/o taking away from the rest of the house? At what outdoor air temp does the existing AC run continuously? 85*F,90*F,95*F? Has a Load Estimate been done to see if what total BTU's are needed to do the new sized house? If you have the knowledge to guesstimate that an 8" flex (6" hard pipe w/ just 100CFM ++/--, depending) is needed then what is the total CFM needed for the rest of the house(total)? What is the length of the new addition flex? How many new exterior walls on the new addition? I'll give you this, you make duct work sound so simple but in fact it is not that simple.
 
#7 ·
Robdev said:
I have a 1,500 Sq Ft house, 2.5 ton ac. 7 runs.

Bedroom is 250 sq addition with original 1970’s round metal ducts.

I want upgrade to flex, trying to figure out the right sizing.

I believe I need an 8 inch flex duct for bedroom,
but do I branch this off a trunk or run directly off plenum?

I think I go off a trunk, if so,l what size trunk do I use?
Thank you
Click to expand...
__ 1,750 Sq Feet __ 1970's Built __ Might require 3.0 Tons in LA

Duct sizing: Main: 600 FPM. Branches: 400 FPM

3 Ton = ~ 1,100 CFM
2.5 Ton = ~ 900 + CFM
 
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#8 ·
Robdev said:
Vstech thank you for the replay. Though I do agree hiring a professional is the best move, itÂ’s just not an option at this point. My recent quotes come in around quote $**** and this is just to far out of my price range. Jbhenergy you may be correct the flex is not an upgrade, but at this point pretty much anything is an upgrade from my original 1970Â’s bent, leaking and dented duct work. Again IÂ’m just trying to get some direction on fitting a new duct system. In my research I believe I need to run branch duct work off a trunk duct work from the plunum. My calculation came to 8 inch flex for the 250 sq room. So IÂ’m curious what the trunk size would be. Also if flex is not an upgrade than even going professional is an issue since thatÂ’s what both places quoted me for.
Thank you both for your help.

Please do not add pricing to your posts. It is not allowed by forum rules.
Click to expand...
Good luck, you're going to need it!

47 Years doing refrigeration and A/C yet I still hire a Duct man to do that end of it!
 
#9 ·
So here’s the 30 second run down of how duct is supposed to sized….

First you need a manual j load calc for the room to determine how many btu you need.
Then you need to calculate the cfm based on the delta of your air
Then you need to find the remaining available static pressure of the air handling equipment
Then you need to calculate the total equivalent length of the duct the blower is working against.
Then using the available static pressure and the tel you can calculate your friction rate.
With your friction rate you can now use a ductulator to find what size pipe you need to deliver the btu and cfm from steps one and two.

Or you can take a wag and hope it works.

There are also charts plastered all over Google on how many cfm a certain size duct can hold. Just make sure you look at the Astrid at the bottom that says the friction rate for those numbers is .1. And know that in the years of me doing this, I have never calculated a FR that came to .1.

The above paragraph is what the difference is between real HVAC companies and equipment swappers.


Sent from my iPhone using Tapatalk
 
#10 ·
DIY is fine for most projects if you have the knowledge and skill level to do it.

Ductwork and air flow is a different story. There are people that have installed duct their whole life and still can't get it right ( maybe the weren't trying).

The thing is that you mechanics of doing it is not that difficult but still takes some knowledge to get close, the design part is a whole different story.

The problem you are going to have is that there is a better chance than not that you will make things worse instead of better. Then when you hire a professional to fix it he won't be able to (see the second line).

So now you have spent good money, twice, lost levels of comfort, and increased operating costs.

Now you need to find the guy to start over and fix it right. This will cost more than if you find the guy now and let him have at it.

Maybe the smart money would be to add a mini split to the addition and call it good . . . or not!?
 
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#11 ·
Instead of replacing with flex, try getting prices on actually upgrading your ductwork… air sealing, insulating etc… it might be better overall than flex, and less expensive.
Also, some companies teach you diy…
There is a reason why the job is being quoted high… ductwork is the #1 most important aspect of the Hvac system, and done poorly, affects the entire future of the home… get it done right, or leave it alone… please don’t attempt to diy ductwork.
 
#12 ·
vstech said:
Instead of replacing with flex, try getting prices on actually upgrading your ductwork… air sealing, insulating etc… it might be better overall than flex, and less expensive.
Also, some companies teach you diy…
There is a reason why the job is being quoted high… ductwork is the #1 most important aspect of the Hvac system, and done poorly, affects the entire future of the home… get it done right, or leave it alone… please don’t attempt to diy ductwork.
Click to expand...
I try to compare ductwork to the veins in your body. Without them heart is nothing


Sent from my iPhone using Tapatalk
 
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