# Thread: Spacing of duct openings on box, and size of box

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## Spacing of duct openings on box, and size of box

I have a really limited attic space - 22 from the top of the trusses to the the bottom of the 2 x 4s under the apex of the roof, then a low slope from there) with a bunch of angled supporting studs taking up space every two feet.

I need to fit a supply box that will have 10 ducts coming out of it, 5 off one side, 3 off the other and 2 off the back. I need to know how much space there needs to be between duct openings in the box. A salesman told me there needs to be 1 between them, and I want to see if hes right, and also does the 1 refer to the space between the actual duct, or between where the insulation on each ends?

For example, if I have two 8 ducts side by side, and they are 10 with insulation, do I need to 17 total space (8 plus 8 plus 1), or do I need 21 total space (10 plus 10 plus 1)? The question still applies if 2 inches is needed  is it 2 between the actual ducts or 2 between where the insulation ends  do I need 18 total space or 22 total space?

Also does there need to be 1 inch between the edge of the box and a duct hole, or can an hole go right up to the edge?

Does it matter how big a box is relative to the size of the unit. I ask because it looks like my box would be about 20 x 22 x 64 for a 3.5 ton package unit due to the studs. It would likely be put about 22 feet from the wall where the supply trunk duct line enters the attic. The supply duct runs coming off it will be about 8 to 22 feet long. Does making the supply box smaller than the above have a benefit, and how large would the benefit be? I could potentially get it to be 20 x 19 x 60 depending on the answers to how far the holes need to be apart, and if they can come up to the edge, and where the studs are located. I'm not even sure I can fit a 20 x 22 x 64 box, which is why I need to know these things.

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1" is good

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Get it wrong now and you will have to live with it forever. Your HVAC contractor will tell you what works.

This is why you need a pro to do the job...and when i say pro, I don't mean the cheapest guy willing to tell you what you want to hear.

Good luck and BTW- no dyi questions are allowed on the forum.

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If it is so tight, why not look into mini-splits like fujitsu or lg. If you are in Florida you should not need a tremendous amount of heat, and the air conditioners can be extremly efficent.

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You have 22" in height, what about sideways? Can you run a piece of duct off of the plenum?
Pics would allow a more intelligent response

6. Try using some real duct. Should work much better than a spider-web.

7. Sorry, this is not a DIY site.

Also, going to one 4 ton unit will cost you more in the long run.

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Originally Posted by beenthere
Sorry, this is not a DIY site.

Also, going to one 4 ton unit will cost you more in the long run.
Hi Beenthere,

I'm not doing the work myself. I work 65 hours a week for a small nonprofit so I don't have time to install an HVAC - nor the knowledge or skills at that.

I need to figure out though if a supply box, a return box, the trunk lines to those 2 boxes, 10 supply ducts and 2-3 return ducts can fit. And I don't want to rely on a salesman saying whether it can be done or not. To convert from two separate units to 1 unit, I'm going to open up the apex of my roof about 25 feet long so it can be done.

My roof is 1 x 12s not plywood so it's not real hard to open it up and a friend of mine who builds houses will do it for \$15 an hour, but I don't want to sign a contract with an AC company and have him do that only to find out everything cannot fit. It needs to be determined ahead and I don't want to rely on a salesperson saying "sure it can be done." I need to graph it out on paper to be sure before committing a lot of money to an HVAC company. And also to show to the HVAC guys who come to give quotes.

My friend cut an 18 x 18 hole to give me very partial access to the attic so I could see the layout of the studs and the spacing, but there are already some ducts kind of blocking some site lines so it's impossible to stand there and know if everything can fit by sight. It's got to be laid out in graphs with exact measurements, and sales guys are going to do it. So I need to know some basics, and I assume this forum will help me learn them.

Thanks

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Originally Posted by genduct
You have 22" in height, what about sideways? Can you run a piece of duct off of the plenum?
Pics would allow a more intelligent response
There is enough room that it might be possible to fit a box that's 20" high by 22" wide before it starts to hit the main roof studs. Enough room needs to be left on 1 side to fit 5 supply ducts all going 1 direction and 1 return duct going the other, and there are interweaving support studs further down. If the box can be smaller, it would definitely make doing this possible. I've got graphs set up in a graphics app but need to input the correct info. Then I can print it and show it to the HVAC folks who are coming Monday afternoon and Tuesday to give quotes. If I know it can be done, and can show them clearly that it's doable, then they will quote that (and they are going to sit here while I remove things around the graph - I need to do it before they get here), thus I need help. My county has rebate program that will give \$1,000 rebates for new HVACS, but they are running out of funds so I need to move pretty fast. If Plan A will not work, I need to move to Plan B, but I favor Plan A far, far more.

For photos, it's tiny dark attic so photos would be hard to see and would just show some studs. But just picture any supply box like this -

Links to direct purchase sites are not allowed

If there has to be 1” of space between the outer insulation of each duct, then the box might not fit in my attic with the beam structure up there.

If there has to be 1” of space between the actual metal part of the duct, then it will fit easily. In fact it could probably fit even if there 2” is needed between the actual ducts. But if it’s needed between the insulation, that adds too much space, the box gets too big to fit it plus 10 supply ducts, 3 return ducts and two large trunk lines, and a return box. The more opinions I can get on what amount of space is needed is better as I want to have confidence before I sign a contract, put a deposit down and pay my builder friend to open up the roof.

Also, if duct openings can go up to the edge of the box, that would also help the cause.

Thanks,

Peter
Last edited by beenthere; 05-09-2011 at 04:56 AM. Reason: removed link

10. Only contractors that see your attic lay out can give you good advise if it will work or not. No one here can see your attic to know what will or won't work. And what size it needs to be. It sounds like your designing a very restrictive duct system that could cause it not to work. And since its "your" design, the contractor you use wouldn't be responsible for it when it doesn't work.

Your also converting a 2 system house into a single system house, which will increase your cooling bill. And probably leave you with a much warmer second floor, then first floor.

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Originally Posted by beenthere
Only contractors that see your attic lay out can give you good advise if it will work or not. No one here can see your attic to know what will or won't work. And what size it needs to be. It sounds like your designing a very restrictive duct system that could cause it not to work. And since its "your" design, the contractor you use wouldn't be responsible for it when it doesn't work.

Your also converting a 2 system house into a single system house, which will increase your cooling bill. And probably leave you with a much warmer second floor, then first floor.
First I need to find out if the box and ducts would physically fit, then I can ask the contractors if it would work or not. I will also encourage them to recommend alternatives for where the ducts would run and the size of the box. Ive spent hours viewing the layout and brainstorming a way for them to fit up there and I feel lucky to have figured out a way that would work if the duct holes in the supply box only need 2 of space between the metal edges. If they 1-2 inches of space between the outer insulation of each duct, then it wont work.

Can someone tell me how many inches are needed between holes, and state whether the distance is between insulation or between metal?

Id be most grateful. Then I can show the sketch to the contractors who are coming later today and tomorrow and get their opinion on whether it would work. And also see if they have a better approach.

But I cant show it to them if I dont have the proper distance needed between holes. And if I dont know it would work and cant show it to them, then theres problem no reason for them to give quotes on the system I want.

It took me probably 10 hours trying to find an approach that would fit all the duct runs and boxes, and I have a good math and spacing/layout mind, and I just managed to fit all the things in after trying many approaches. I somewhat doubt any contractor is going to want to spend say 2 hours to map out the beam and studs pattern of my attic on a computer program and then spend a couple more hours trying to figure out an approach just to give me a quote. I could well take them longer than that. And Im not going to hire someone and hope they can figure it out later. Im beyond open to it if they want to do the above to give a quote, but I dont expect they will. So I have no choice. Once they see my graphs, they might well see some improvements to it or point out problems that would prevent it - I just dont think anyone going to do that from scratch.

On your note about a single unit, I have a single story ranch style house - it's not 2 floors. What percentage more would moving from two 2-ton units to one 3.5 ton unit cost in the long run? And why would it cost more?

Im pretty sure Ill only need a 3.5 ton and will know within a day as Im having load measures/calcs done as part of an energy audit today.

I dont run the AC much, nor the heat much so even if it costs 10% more a year to have a single unit package system and have 2 more closets to use, no noise in my living room, kitchen or back yard, and have access to repair or replace my water heater any time its needed (which I lose access to with an indoor unit), Id definitely do the outdoor unit. And might even go as high as 35% high for the factors above. Plus roughly every 15 years I assume I will save one or two thou as I assume one 3.5 ton would be a good bit less than two 2-tons. I work from home for a nonprofit and my home office is in the same zone as the BR unit, so I had to keep both units running the same and could not turn off the BR area unit during the day. Also, if I turn the other zone off at night, the heat rolls into the other zone and the AC on it cranks all night and I end up chilly.

The other factor is that I heavily want a whole home dehumidifier and on my tiny nonprofit salary I cant come close to affording 2 of them. And I need it throughout the house. Im in this place 20-22 hours a day and its humid year round yet I only need AC a lot from mid-May to mid-Sept (and occassionally for 2 other months) yet need dehumidification year round. So for all the factors above, I think a single system is best in my case. Just need to know some basic questions.

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## You would do well to heed this advice!!!!!!!!!!

Originally Posted by beenthere
Only contractors that see your attic lay out can give you good advise if it will work or not. No one here can see your attic to know what will or won't work. And what size it needs to be. It sounds like your designing a very restrictive duct system that could cause it not to work. And since its "your" design, the contractor you use wouldn't be responsible for it when it doesn't work.

Your also converting a 2 system house into a single system house, which will increase your cooling bill. And probably leave you with a much warmer second floor, then first floor.
How can you argue with these simple observations?

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## PS

The duct is best when installed INSIDE THE THERMAL/ BUILDING ENVELOPE!!!!!
Not in an attic space.

Find a good contractor and then use some hallways or perimeter soffits that your \$15.000/ hour guy can build to enclose your new, properly sized and insulated duct

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