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Old 11-07-2008, 05:23 PM   #1
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Furnace install in attic - Duct configuration


Hello all, I am installing a furnace in the attic. I have all the gas line and electrical done. I have a lot of experience with gas line and electrical but little with sizing duct system. I have a 1200 sq ft house with a open floor plan. Only 3 doors on the floor to the bedrooms that would keep airflow from getting around. I know there is a ton of calculation into duct systems. I was planning to oversize the duct system to ensure a low velocity and low noise air flow. My furnace is somewhat oversize for my application but it is a new trane that I got a very good price on. Anyway I am in Southern California so I really won't be running much other than to run the cold out in the morning. I had the heat on less than 10 days last year so everything doesn't have to be perfect. I attached a very simple drawing of a duct system. I plan on having one duct per bed room (3) Two ducts in the main room which is 1/3 of the total square footage, 1 duct in the kitchen and 1 smaller in the bathroom. The kitchen and main room is all wide open and could almost be considered the same room. I had to compress the image before it would let me upload. You might not be able to read the text. The trunkline is the same size as the furnace outlet. There is also one smaller line off the trunk which would be six inch flex for the bathroom. let me know what all my problems would be for such a simple design and how I could improve. I plan on making the main trunk line metal.
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Old 11-08-2008, 06:32 AM   #2
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Furnace install in attic - Duct configuration


How long is the trunk line.
If its too long, without a reduction in size, you have no velocity regain, and won't get good air flow to all the supplies.

What is your definition of low velocity?

How much insulation will the trunk line have.

What is teh furnace opening size?

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Old 11-08-2008, 10:11 AM   #3
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Furnace install in attic - Duct configuration


I drew up a different plan using A plenum right off the furnace. Off the sides of the plenum I would use 10" round metal pipe extending 10' each side. I would say the plenum would be roughly 10" wide X 20" Tall X 24" Long. Is the metal pipe coming right off the sides of the plenum a problem without air vanes in the plenum to direct the air? Any thoughts to this layout vs. the other layout. Each Flex run should be less than 15' long. All being 8' except for the 6 for the bathroom. I am not sure about velocity. Just so I have enough flow to not overheat the furnace.
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Old 11-08-2008, 11:56 AM   #4
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Furnace install in attic - Duct configuration


A 60,000 BTU 90% furnace.
Temp rise of 40F needs 1250CFM
For 50F its 1000CFM
For 60F its 833CFM
For 70F its 714CFM

If an 80%, then air flow is
40F 1111CFM
50F 889CFM
60F 740CFM
70F 635CFM

Your 10" round, tapping into the plenum, is about 135' of total equivalent lenth.
Each 8" tap will also have an El of 15' mimimum.

Depending how close together they are, some will have to be added together. So some could be 30' or more EL.

So your supply as you have it shown, will have a longest TEL of about 210'.

Since you don't have any return info. We'll say its only 100' TEL.

Allowing for normal filter, supply registers, return grille, air filter, and A/C coil pressure drops.
Furnace rated ESP .5"-.32"=ASP of .18"
.18*100=18/310=FR .056"

(Trane recomends .2" ESP for heat only applications)

10" round at .06"FR is 320CFM at 590FPM

That would give you a total of 640CFM, or a 70F temp rise on an 80%, or 78F on a 90%.

Yes, vanes, or a diverter should be used. And blower speed should be choosen wisely.

Vanes will decrease the EL of the round tap into the plenum by more then .

So what is your definition of low velocity.
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Old 11-08-2008, 02:35 PM   #5
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Furnace install in attic - Duct configuration


I can't put a definetion on low velocity. I guess I would say what ever it would be to prevent whistle. I followed some of your post but don't understand everthing. I will put vanes in the plenum like you suggested. Would using 12" instead of 10" be better for the round? It looks like the round metal will be closer to 7' in length each side instead of 10'. and the flex runs will be closer to 10'. At this time I was going to use a 14" flex return or a 12" metal return which would be about 15' in length. Basically would this setup work or should I change any sizes or configuration? Like I said I don't have the experience to actually calculate what I need. Thanks for spending the time to help me. I am learning a lot.
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Old 11-08-2008, 02:35 PM   #6
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Furnace install in attic - Duct configuration


Also I forgot, It is a 90% furnace and no a/c coil

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Old 11-08-2008, 06:46 PM   #7
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Furnace install in attic - Duct configuration


The best way, is to know how many CFM each room Needs, to know that a load calc is needed.
Then, you don't have to guess at how much air to move to each room/area.

A good chance, that if you did a load calc, you would find that 1 side oif the house needs more air flow then the other. Meaning one side may need a 10" trunk, and the other a 12" trunk(kitchens don't need as much heat as a bedroom).

You can use the 10" round for both trunk lines, that put you at about 920FPM, but you would want to use flex for the supply branches then for noise prevention.

14" flex is a bit small for 1000CFM of return.
Have you considered installing returns in all the bedrooms. That way, those rooms have good return during the night when the doors are closed.

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