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Old 07-26-2012, 12:25 AM   #1
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What Diameter Ducting for This Project?


Hello. See pic below.

I am using a portable A/C unit that exhausts through the lower window. I will be running aluminum ducting from the upper window to the lower room to get fresh air to the A/C unit.

The ducting will be about 28 feet long. I will use a 3 speed 20" Lasko box fan to help draw the air through the duct. I will be using some sort of 20" furnace filter in front of the box fan to filter the air, nothing too restrictive.

The A/C unit has a powerful fan that draws air into it from the lower room, exhausts cold air into that room and hot air through the lower window. This powerful fan will also help draw the air though the ducting. The upper and lower rooms will be relatively air tight.

I estimate the force of the air taken into the A/C unit to be at least 2000 CFM. I would like the CFM rate from the 20" box fan (and filter) to be 1000 to 2000 CFM, creating a slight negative air pressure in the lower room. I will control that rate with the speed selector on the box fan and the type of air filter used with it.

I'm hoping 6" or 8" diameter aluminum ducting will suffice. I would appreciate your advice on the diameter of ducting that should be used for this project. Thank you.


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Old 07-26-2012, 05:25 AM   #2
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What Diameter Ducting for This Project?


Ain't gonna get 1000 CFM through a 8" round duct. Would need closer to 20".

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Old 07-26-2012, 06:06 AM   #3
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What Diameter Ducting for This Project?


Quote:
Originally Posted by BR549 View Post
Hello. See pic below.

I am using a portable A/C unit that exhausts through the lower window. I will be running aluminum ducting from the upper window to the lower room to get fresh air to the A/C unit.

The ducting will be about 28 feet long. I will use a 3 speed 20" Lasko box fan to help draw the air through the duct. I will be using some sort of 20" furnace filter in front of the box fan to filter the air, nothing too restrictive.

The A/C unit has a powerful fan that draws air into it from the lower room, exhausts cold air into that room and hot air through the lower window. This powerful fan will also help draw the air though the ducting. The upper and lower rooms will be relatively air tight.

I estimate the force of the air taken into the A/C unit to be at least 2000 CFM. I would like the CFM rate from the 20" box fan (and filter) to be 1000 to 2000 CFM, creating a slight negative air pressure in the lower room. I will control that rate with the speed selector on the box fan and the type of air filter used with it.

I'm hoping 6" or 8" diameter aluminum ducting will suffice. I would appreciate your advice on the diameter of ducting that should be used for this project. Thank you.

ain't going to happen ..whats plan b..
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Old 07-26-2012, 07:55 AM   #4
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What Diameter Ducting for This Project?


I don't think this is going to work but here is what you asked for

6" = 100 cfm
8"=200 cfm
14"=1000 cfm
20"=2000 cfm

these are based in our world of staic pressures. I can't say about window unit and box fan staic pressures.
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Old 07-26-2012, 10:49 AM   #5
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What Diameter Ducting for This Project?


Thanks for jumping in everyone, just what I needed. Really helps.

After giving this more thought last night and now learning from your help, I think I will be better off with less air anyway, maybe 500 to 1000 CFM.

Plan B
Run double lengths of 7" aluminum ducting. This size because I can get it shipped from Home Depot within a reasonable budget for me at about $120.

Also, I will be fabricating these to use with the ducting, made from cheap foam core panels and aluminum tape.
- connection with the intake window
- junction box between that connection and the ducting
- junction box between the other end of the ducting and the box fan

Please post any input on Plan B before I order the ducting later today. Thanks again.
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Old 07-26-2012, 10:52 AM   #6
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What Diameter Ducting for This Project?


7"= right around 150 cfm
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Old 07-26-2012, 11:33 AM   #7
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What Diameter Ducting for This Project?


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7"= right around 150 cfm
That would be a problem. So would spending too much more or sourcing bigger ducting. So, no need trying to fit a square peg into a round hole.

Plan C
10" x 10" ducting fabricated from cheap foamcore panels and aluminum tape.

This foamcore is made of thin polystyrene with paper on each side. I can get 20" x 29" panels from a couple different dollar stores. Probably do the whole project for under $50, and I don't have to wait a week for shipped ducting.

This would be a more temporary solution, but then I'd only need it to work for about 8 months.
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Old 07-26-2012, 01:07 PM   #8
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What Diameter Ducting for This Project?


Another point of concern would be the introduction of untempered outdoor air into a space. What's the reasoning for this? Some buildings do need fresh air, but for the most part a residential structure will get plenty of fresh air through door/window openings and other areas. If you're in a humid area (or even a semi-humid area), you could be spending money needlessly.
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Old 07-26-2012, 02:11 PM   #9
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What Diameter Ducting for This Project?


Good question, should have stated this up front. The reason for this ducting is to resolve a severe air quality problem that I have with the current intake for this portable A/C unit. I have all other potential issues covered to my satisfaction: humidity, indoor and outdoor air, legal, etc

Any comment / advice on my Plan C ? Thanks!

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Originally Posted by scottmcd9999 View Post
Another point of concern would be the introduction of untempered outdoor air into a space. What's the reasoning for this? Some buildings do need fresh air, but for the most part a residential structure will get plenty of fresh air through door/window openings and other areas. If you're in a humid area (or even a semi-humid area), you could be spending money needlessly.
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Old 07-26-2012, 02:28 PM   #10
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What Diameter Ducting for This Project?


10x10 INSIDE = right around 500 cfm

Please remeber this is based on staic pressure of a modern heating an cooling blower. your box fan or window unit will not produce the same staic pressure. I have no way to know what a window unit or box fan can produce in pressure. I would guess not as much as a blower in a unit.

please post pic when done. I HAVE TO SEE THIS
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Old 07-26-2012, 03:33 PM   #11
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What Diameter Ducting for This Project?


Quote:
Originally Posted by turnermech View Post
10x10 INSIDE = right around 500 cfm

Please remeber this is based on staic pressure of a modern heating an cooling blower. your box fan or window unit will not produce the same staic pressure. I have no way to know what a window unit or box fan can produce in pressure. I would guess not as much as a blower in a unit.

please post pic when done. I HAVE TO SEE THIS
And I was hoping to keep this ducting relatively small. I'm guessing a modern blower could yield at least twice the static pressure of the two fans I'm using.

So, on to plan D... 20" x 20" ducting with foam core panels...

Regarding the pics... as planned, I will post close up shots that show most of what I've done, but not everything... so as to expose the train wreck that is my apartment.

I've got a lot of work to do with this, so I will post again when done... unless there are any helpful comments / advice posted for my Plan D.

Thanks!

Last edited by BR549; 07-26-2012 at 03:39 PM.
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Old 07-26-2012, 03:39 PM   #12
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What Diameter Ducting for This Project?


12x16 or 10x18 or 14x14 are all more than twice the 10x10. Are all more than 1000 cfm @ .1 static pressure
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Old 07-26-2012, 03:45 PM   #13
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12x16 or 10x18 or 14x14 are all more than twice the 10x10. Are all more than 1000 cfm @ .1 static pressure
With 14" x 14" being more than twice as efficient as 10" x 10", then by my estimation that should suffice. Definitely would be far more convenient for me than having a 20" x 20" box running the length of my apartment. I'm going to have to submit your name to the record books for being so helpful.
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Old 09-11-2012, 12:10 AM   #14
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What Diameter Ducting for This Project?


Shortly after my last post, I determined the cost of this pipe dream to be too high and the chance of success too low. Since then I've been on a quest for better air filtration and purification. It's taken me until now to get a system that works half way decent. See the pic.
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What Diameter Ducting for This Project?-air-system.jpg  

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