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Old 09-08-2010, 05:35 PM   #1
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HVAC Trunk Line Design


Hello and thanks for looking.
S.E. PA heavely wooded lot
1500 sqft ranch 90btu fha oil fired furnace in the basement
Recently raised the roof to add 800 sqft of livable space (total house = 2300sqft). R-19 walls r-30 ceiling. Looking to add a second hvac unit in the attic space for the second floor to accomodate the 800 sqft added.

Currently in the framing phase but would like to solidify the trunk dimensions. Because of space limitations 6 "flex branch lines were run under the floor in preperation for this additoin. Flat roof does not permit duct work.

Questions:
From the attached sketches
Will 1.5ton unit suffice? (propane fha or heat pump are my options)
Will 1 return of 16"x8" be sufficient?
Will my trunk line be adequate...specificaly where it is reduced?
Will additional supply registers be required in the 2 bedrooms because of the neccesity to use 6" flex?

Thank you!
Shawn
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HVAC Trunk Line Design-hvac-7-2-2010-schematic.jpg   HVAC Trunk Line Design-hvac-7-29-2010-floor-plan.jpg  

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Old 09-08-2010, 06:28 PM   #2
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HVAC Trunk Line Design


Both bedrooms will probably need a second supply. Because of them being 6", and because of the length.

16X8 is fine if your putting a 16X8 in both bedrooms, and a 16X8 in the hallway.
Unless your doing under cuts on the bedroom doors, and don't care about privacy, you want a return in both bedrooms.

May or may not get away with a 1.5 ton A/C, depends on how much heat is going to rise from the first floor.

A load calc will tell you. http://hvaccomputer.com/talkref.asp Its worth its 49 bucks.


Ducts in a flat roof joist space, gain a lot of heat in the summer, and lose a lot of heat in the winter. So you may need 2 tons for that small addition.

I would start the side that shows 2 supplies with 10X8, but only reduce it to 8X8, and add the third supply. The side that shows 3 supplies. I'd start off with 12X8, catch 2 supplies, and then reduce to 10X8, and catch the last 2 supplies(yep, the second supply you should add for the bedroom).

This lowers air flow resistance.

A return in both bedrooms, and one in the hallway.

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Old 09-08-2010, 07:15 PM   #3
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HVAC Trunk Line Design


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Originally Posted by beenthere View Post
Both bedrooms will probably need a second supply. Because of them being 6", and because of the length.

16X8 is fine if your putting a 16X8 in both bedrooms, and a 16X8 in the hallway.
Unless your doing under cuts on the bedroom doors, and don't care about privacy, you want a return in both bedrooms.

May or may not get away with a 1.5 ton A/C, depends on how much heat is going to rise from the first floor.

A load calc will tell you. http://hvaccomputer.com/talkref.asp Its worth its 49 bucks.


Ducts in a flat roof joist space, gain a lot of heat in the summer, and lose a lot of heat in the winter. So you may need 2 tons for that small addition.

I would start the side that shows 2 supplies with 10X8, but only reduce it to 8X8, and add the third supply. The side that shows 3 supplies. I'd start off with 12X8, catch 2 supplies, and then reduce to 10X8, and catch the last 2 supplies(yep, the second supply you should add for the bedroom).

This lowers air flow resistance.

A return in both bedrooms, and one in the hallway.

I would do as Been has said....


But I would also add balance dampers to the main trunk on both sides as well as turning vanes.

It looks like you need more air flow to the east. So by putting dampers in the suply duct will help you balance it.
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Old 09-09-2010, 01:19 PM   #4
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HVAC Trunk Line Design


Beenthere & Newtech...THANKS!

I am not a fan of the door undercuts but I was planning on putting a return (8x10) from each bedroom in to the hallway via the cooresponding closets.
Not a "true" return more an air passage to eliminate any door undercutting.

Would it be a problem if I mounted the trunk perpendicular to conventional?
I would prefer to have the 8" side horizontal and the 12" side verticle.

Newtech...turning vanes..I am confused. I do not propose any 90degree bends in my trunk. All branches from the trunk will be via take off boots to 6"round flex.
Did I miss something?

-Shawn
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Old 09-09-2010, 04:46 PM   #5
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HVAC Trunk Line Design


Quote:
Originally Posted by M-skills View Post
Beenthere & Newtech...THANKS!

Newtech...turning vanes..I am confused. I do not propose any 90degree bends in my trunk. All branches from the trunk will be via take off boots to 6"round flex.
Did I miss something?

-Shawn
On the outlet of the main trunk, where it branchs to both sides, are yor going to use a tee ____l l____ If so I would add the turning vanes............................................. ............................................... ............. ____\/____

Last edited by newtech; 09-09-2010 at 05:53 PM.
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Old 09-09-2010, 06:52 PM   #6
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HVAC Trunk Line Design


Make your central return bigger then 16X8.

Also, use large transfer grilles then 8X10. Atleast 8X14(should be bigger). Since you'll have 2 for each bedroom. 1 on the bedroom side, and one on the hall side.

Turning vanes for the tee, or the warm air collars at the supply plenum. Will reduce friction by anywhere from 65 to 150 foot of equivalent length.

Doesn't make a difference if the 8" is the horizontal or vertical dimension.

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