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Old 10-25-2012, 06:50 PM   #1
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Duct Sizing Confusion


Iíve been searching the internet and looking at threads on various forums for information to design a decent vent system for my home. Iíve found a lot of information that has helped, but a lot has confused me as well, so Iím hoping someone can point me in the right direction.
Hereís the scenario:

My home is a 70íx32í ranch on a walk-out, unfinished basement, but is well insulated. Iím installing a wood furnace at one end of the basement which will serve as my sole source of heat (hopefully), so the duct will be running quite a long way. This is a temporary installation, so my plan is to use flexible duct until I have time to replace it.

The furnace has three blower settings, 1,000, 1250, and 1,300 CFM. My plan is to use the 1,100CFM setting if possible.
I found an online Manual J calculator, so I have a pretty good idea what I need to supply to each room, but I will be adding some air returns to match the furnace output.

My questions:

1. Iíve found several tables listing the CFM for various duct sizes as well as the maximum CFM ratings from the duct manufacturer, but none of them agree with each other. In some cases they are not even close, differing by as much as 35%. Why do they differ so much? I have a feeling Iím looking at peopleís opinions on this, so is there a source that gives a definitive answer?

2. I am looking at register box sizes vs. duct sizes and I see that the box size is not the same as the duct size. For example, I can purchase a 14x6 box with both 6Ē and 8Ē connections for the duct. I can also purchase other box sizes with the same 6Ē and 8Ē connections. Why is this? I was under the impression that airflow is determined by the duct size, not the box area.

3. How often should the main trunk be reduced in size? After each take-off, or after the volume of air is reduced enough to accommodate the next smaller size duct?

Any answers and help anyone can give would be greatly appreciated!

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Old 10-25-2012, 08:03 PM   #2
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Duct Sizing Confusion


If you have done the "hard part" of doing a manual J,why not continue on with a maual D.
I don't know what cahrts you have been looking at,but if you have a hard metal duct chart you can deduct abot 30 % for the same size in flex, because of all the ridges and sags in flex duct.
Return air FROM EACH ROOM is also as big a factor as the supply side when you want comfort.If comfort is not the ideal then just get the same amount of air back to the furnace as its putting out so that it dosesn't bake each time it turns on.

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Old 10-25-2012, 08:06 PM   #3
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Duct Sizing Confusion


Register velocity is what is important. A 14x6x6 register box/boot is a low velocity low throw size. A 14x6x8 is a higher volume high throw size. And yes, you still need to select the correct register for the throw you need.

Flex duct from one manufacturer to the next is not the same. So one brand will have better air flow then the next. Its best to ue a duculator for flex duct, then to use a sheet metal one.
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Old 11-02-2012, 07:15 PM   #4
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Duct Sizing Confusion


In doing more research, I've come up with a few more questions.

In another thread I found that blowers are rated at "X" CFM at "Y" inches wc. This is not indicated on my blower, so I called the manufacturer, and they've been less than helpful.

Is there a standard that blowers are rated at, or am I over thinking this and simply need to know the CFM of my unit?

From what I'm seeing, static pressure is a floating value. It changes based on the volume of air through a particular size duct. If the volume stays constant the static pressure increases as the duct gets smaller, and decreases as the duct gets larger. Am I correct on this?

I would like to find more information on register throw, but I am not finding much that is useful online. The local libraries don't have anything on it either. Is there a good resource out there that I could find this information in, short of purchasing the books?

Again, thank you for any help you can give.
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Old 11-02-2012, 09:25 PM   #5
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Duct Sizing Confusion


Most furnaces are rated at .5"WC of static pressure. Usually you'll end up with .6 to .8" depending on the A/Cs evap coil, air filter, and of course duct design/sizing.

The larger the duct the less restrictive it is, so static is less.

Hart&Cooley have all their engineering ifo on their site and it can be downloaded for free.
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