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Old 06-19-2012, 01:18 AM   #1
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Advice on Running HVAC Ducts?


I'm building a 2-story addition on to the rear of my existing house. One of the things I have to do is get forced air from my HVAC unit to the addition. I have one spot where I can bring a duct in....

The 2x4's on the floor show the outline of the 'space' that the duct can come into. This will be a busy area because I also have 2 3" DWS vents coming up as well as 2 bathroom vents.



The duct (I'm thinking at least 14 insulated flex) will come in through that dark opening (existing house roof).

This view is what is below the above space.



My 'plan' is to fabricate an air box between the floor joists on the right side....about 12" width. From that box I will run flex duct to 3 different spots.

On the top side, I will need to run more flex duct to the master bedroom and bath.

My HVAC unit is a 60,000 BTU Carrier unit. I live in S California....so I don't need AC and in the winter, it just does not get that cold. I know the 60k BTU unit is borderline for a 2000 sq ft space.....but I'm also beefing up the insulation in the existing house and the new section is going to be well insulated.

I guess my main question is....will a 14" duct be enough to supply all the air for the addition....with half going into the 12" airbox between the rafters? And, is a custom air box in the ceiling the best way to interconnect all the ducts I need?

And, no, I will not be bringing in a profesional HVAC company.....that would not be any fun....kind of like building up a custom car and paying someone to do all the work.

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Old 06-19-2012, 02:50 AM   #2
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Advice on Running HVAC Ducts?


Whatabout return air???

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Old 06-19-2012, 10:26 AM   #3
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Advice on Running HVAC Ducts?


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Whatabout return air???
This is the 2nd story floor layout....

The area where the stairs are (bottom center of dwg) contains the return air duct for the HVAC....about as centered in the house as you can get. The downstairs is all open. Upstairs, I will install one vent (most likely in the stair well by the bedroom door) for that return air.



The false wall area I'm talking about is that little spot between the upstairs toilet and the stairwell.
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Old 06-19-2012, 09:59 PM   #4
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Advice on Running HVAC Ducts?


Are u trying to run it from ur existing plenum/ur existing duct work from ur air handler

Last edited by Jone32; 06-19-2012 at 10:02 PM.
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Old 06-20-2012, 01:12 AM   #5
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Advice on Running HVAC Ducts?


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Are u trying to run it from ur existing plenum/ur existing duct work from ur air handler
If I understand your question correctly....I plan to run a new duct from my existing AH. The existing AH already has serveral ducts attached to it going to existing outlets in the existing house.
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Old 06-21-2012, 06:57 PM   #6
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Advice on Running HVAC Ducts?


Ok this is what u do.. I don't think u can run that size flex for the amount of cfm's an btu's that u will need.. u said u where adding on a second addition to ur house which is already 2000sq feet an u have a 60,000 btu unit. which is a 5 ton unit. So u will need to increase ur blower motor speed to handle it. U won't get the correct amount of air flow to the room..an on the duct size if u go to ur neighborhood a/c supply house an buy a "DUCTUATOR" it will give u the correct duct size an the correct amount of cfm's an btu's that u will need... I hope that will help u progress in ur situation if not I will try an help as much as I can
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Old 06-22-2012, 04:38 AM   #7
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Advice on Running HVAC Ducts?


Quote:
Originally Posted by Jone32 View Post
Ok this is what u do.. I don't think u can run that size flex for the amount of cfm's an btu's that u will need.. u said u where adding on a second addition to ur house which is already 2000sq feet an u have a 60,000 btu unit. which is a 5 ton unit. So u will need to increase ur blower motor speed to handle it. U won't get the correct amount of air flow to the room..an on the duct size if u go to ur neighborhood a/c supply house an buy a "DUCTUATOR" it will give u the correct duct size an the correct amount of cfm's an btu's that u will need... I hope that will help u progress in ur situation if not I will try an help as much as I can
Clarification....

Existing house is 1026 sq ft....addition is another 1000 sq ft...so the new total is going to be around 2000 sq ft.

So...the new flex only needs to supply enough air for 1000 sq ft....

With that said...I am insulating the hell out of the house. One of the largest walls is the one on the side of the family room...18'L x 9'H...plans said 2x4 construction...I did 2x6....allows me to put in more insulation....

AC is not an issue...we will not have it...don't need it....

Heating? If this past winter is any clue....it's not going to be an issue....the existing house is getting insulated (original house only had insulation in the attic...back then, they didn't insulate the walls). Per the numbers....60,000 BTU is borderline for 2000 sq ft....given our location and mods to the house....I really don't think it will be an issue.

Jone...thanks for the hint on the "ductuator"....I'll check that out...

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