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Old 08-05-2012, 06:13 PM   #1
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Duct question


If limited space on plenum and wanting to give each area a supply vent, whats your thought on combining supply ducts to say 2 adjacent areas like: Kitchen and Breakfast? (This open area, about 204 square feet, currently has only 1 supply vent.)

Provide a single supply duct of 8 inches, providing about 230 CFM for the total area. Knowing that this could be noisy, split into 2 supply vents, 1 over each window in each area, fed by individual 6" ducts from the 8". In theory giving each area 50% of the volume delivered by the 8".

or if supply plenum space allows

Would you rather run individual supply lines to each: Kitchen (104 sq feet) 7" providing 160 CFM (allowing for appliance heat gain) and Breakfast area 100 sq feet getting 6" 110 CFM.





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Old 08-05-2012, 09:44 PM   #2
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better IMO to run separate ducts per register. An 8" does push two 6" and will work

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Old 08-05-2012, 09:48 PM   #3
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Thanks for taking the time to respond.
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Old 08-05-2012, 09:56 PM   #4
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It would depend on how much flex or rather how far away the registers are from the unit's supply plenum. For the most part I always remove Y connected duct runs and make dedicated runs to correct air flow issues ( as well as install manual dampers), but if the total run is let's say far less than a full 25' bag of flex than you can certainly get away with a Y.

A Turner has said, dedicated is best.
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Old 08-05-2012, 10:02 PM   #5
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Duct question


Yes it is less than 25, more than likely in the 12-15 foot range so the "y" runs would probably be less than 6 foot.

Would a "y" be better than a "t"?

Thanks for your response.




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Originally Posted by Doc Holliday View Post
It would depend on how much flex or rather how far away the registers are from the unit's supply plenum. For the most part I always remove Y connected duct runs and make dedicated runs to correct air flow issues ( as well as install manual dampers), but if the total run is let's say far less than a full 25' bag of flex than you can certainly get away with a Y.

A Turner has said, dedicated is best.
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Old 08-05-2012, 10:14 PM   #6
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Yes, a Y would be best. I know you said limited plenum space so will these be complete new addition and registers or a change up from what's existing and if so, what is existing? And where on the plenum do you plan on taking this run off of? Can you post some pics?
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Last edited by Doc Holliday; 08-05-2012 at 10:28 PM.
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Old 08-05-2012, 10:28 PM   #7
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Yes, a Y would be best. I know you said limited plenum space so will these be complete new addition and registers or a change up from what's existing and if so, what is exiting? And where on the plenum do you plan on taking this run off of? Can you post some pics?

Change up. Want to install a new supply plenum, increasing its size in horizontal length, to 36 inches from 30.

This run already exists, but share to dining room. We want to change that so that the kitchen and breakfast area share or are independant.

Dining room could now be shared on a line with the formal living room since they are next to each other and this would be less than an8 foot run to a "y".

The runs off of the new plenum would be similar to how they are now: both sides and the top.

I have an excell spreadsheet that I can share with details. See attachments.
Attached Files
File Type: zip jm duct.zip (19.8 KB, 18 views)

Last edited by digitalplumber; 08-05-2012 at 10:30 PM. Reason: typos
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Old 08-05-2012, 10:52 PM   #8
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I don't foresee a problem with a Y, you'll be fine. Might want to think about manual dampers installed at each duct take off. That way you can balance the air however you like it by closing off/adjusting a few here and there to support greater velocity to and through other ducts if you want to.
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Old 08-05-2012, 10:55 PM   #9
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Originally Posted by Doc Holliday View Post
I don't foresee a problem with a Y, you'll be fine. Might want to think about manual dampers installed at each duct take off. That way you can balance the air however you like it by closing off/adjusting a few here and there to support greater velocity to and through other ducts if you want to.

Yep, they are there now and intend on them at the plenum on single runs and at the "y" on any doubles.

Did you by chance look at my spreadsheet and see anything that may be glaringly wrong or a bad idea?
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Old 08-06-2012, 04:14 AM   #10
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Well, if your using flex duct. You aren't going to get any of those "actual" CFMs you listed.
What size A/C do you have? Is your area a high humidity area? Are you at a high altitude? Few homes need 1 CFM of air flow per sq ft of home area.

Looks like you took CFM values from a duculator for metal duct set to a .1" friction rate. .1" is almost never the correct friction rate.
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Old 08-06-2012, 07:59 AM   #11
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Well, if your using flex duct. You aren't going to get any of those "actual" CFMs you listed.
What size A/C do you have? Is your area a high humidity area? Are you at a high altitude? Few homes need 1 CFM of air flow per sq ft of home area.

Looks like you took CFM values from a duculator for metal duct set to a .1" friction rate. .1" is almost never the correct friction rate.
BT, Houston Texas and using the following based on the furnace to be replaced to a 1600 CFM, currently a York Diamond 1200 CFM:

First, Planning and Installation Instructions - Mater Flow Ductwork by Master Flow. Picked it up at HD.

Second using a Shoemaker duculator.

these 2 default to use the .1

third, I have a chart called Field Duct Size Estimate that I found on the net. It apparently uses .05.

Thanks for your review, corrections and ieas.
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Old 08-06-2012, 09:24 AM   #12
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Looks like I have some recalculating to do. See some suggestions of using .05 or .08 for friction. Makes a big difference. May just leave the old metal in!

By the way, I know that additional ceiling returns are taboo in: bathrooms, utility rooms, closets and kitchens but how about just outside of a kitchen in the den ceiling which is at the same level as the kitchen ceiling and just outside of the entry into the kitchen??

I would love to be able to pull some of that heat out? Also this den ceiling slopes upward at the opposite parallel side of the entry to the kitchen about 12 inches. This was the alternate place I was considering and is not in direct line of the kitchen. This is an all electric kitchen.

Thoughts on this return?
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Old 08-06-2012, 10:13 AM   #13
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Duct question


A simple rule of thumb when going from metal to flex is to simply upsize the ducs by one inch to accommodate for friction rate. I wouldn't recommend not actually using a ductulator to actually determine duct size for an entire home but not that big of a deal for one Y run, if that's all your changing.
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Old 08-06-2012, 04:53 PM   #14
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Quote:
Originally Posted by Doc Holliday View Post
A simple rule of thumb when going from metal to flex is to simply upsize the ducs by one inch to accommodate for friction rate. I wouldn't recommend not actually using a ductulator to actually determine duct size for an entire home but not that big of a deal for one Y run, if that's all your changing.

I think he's changing a bit more then one run.
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Old 08-06-2012, 04:54 PM   #15
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Quote:
Originally Posted by digitalplumber View Post
Looks like I have some recalculating to do. See some suggestions of using .05 or .08 for friction. Makes a big difference. May just leave the old metal in!

By the way, I know that additional ceiling returns are taboo in: bathrooms, utility rooms, closets and kitchens but how about just outside of a kitchen in the den ceiling which is at the same level as the kitchen ceiling and just outside of the entry into the kitchen??

I would love to be able to pull some of that heat out? Also this den ceiling slopes upward at the opposite parallel side of the entry to the kitchen about 12 inches. This was the alternate place I was considering and is not in direct line of the kitchen. This is an all electric kitchen.

Thoughts on this return?
the return must be a min of 10 foot from the ranges exhaust hood. So if any of those areas your thinking about re 10' or more from the hood,your ok.

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