Correct Duct Sizing

[tonynace]

I am finishing an attic, turning it into two rooms, a master bedroom and master bath and need to run duct from my furnace in the basement up to the attic. The room dimensions will be 10 x 11 for the bathroom and 10 x 21 for the bedroom, both with 8' ceilings. I will also have a 7 x 8 walk-in closet off the MB. I've gotten a couple quotes to do this job, but they are slightly different on the duct size. Both want to supply the upstairs with an 8 x 12 rectangular duct, but on the return, one wants to use 8" round, and the other 10" round or another 8 x 12 rectangular. They will feed plenums in the attic which will then have flexible ducts to the registers. Since I am bringing the feed ducts up along a wall, I will have to build out the wall some to accommodate them, and I don't want to bring out the wall any further than absolutely necessary, as space is an issue in that location. First off, is round duct better for airflow than square or rectangular? If it is, can I go with 8" instead of the 10"? If round is really not any better, I will just go with the 8 x 12 as this will give a bigger duct area assuming the contractor does not want a whole lot more to do it that way. Also, does the walk-in closet need it's own register and feed? I plan on having the whole underside of the roof and walls insulated with spray on foam, so the whole attic will be a conditioned space, if that makes any difference in the equation. Do the plenums in the attic then have to be insulated?

[wahoo]

All ductwork in unconditioned spaces needs to be insulated. I assume you are NOT zoning this system? Reducing the size of the ductwork for looks or ease of installation is usually a horrible idea with really bad results. Remember that cold air falls, so if this is an upper floor you'll need plenty of air flow to influence this upper floor. Keeping it cool could be a REAL challenge if not zoned, or seperate unit.:.02:

[skippedover]

Sorry to disappoint you but the equivalent duct size for an 8x12 rectangular is a 12-inch round. And yes, round has better airflow than rectangular. You've thrown various duct sizes out but no specifics as to total airflow. Has a load analysis been done? From where are they taping off for the new duct run? Does the existing blower have sufficient capacity to accommodate all? Manual 'J', 'D'????

[superd77]

Wondering if your existing unit would be big enough to handle that new load?!?

[tonynace]

I've been told that my furnace is sufficient to handle the upstairs as well. They will be tapping off the main plenum. It is only a small house, about 1000 sq ft on the first level. I would prefer round as far as it being better for airflow. And I know I don't need a 12" round, even though it may be equal to an 8 x 12 rectangular. I was quoted with an 8" round and a 10" round. And the contractors said my furnace was sufficient to handle the extra space. Since the round pipe is better, I want to know if I can get by with the 8" on the return so I can fit it in to the space I have. Otherwise, I will use the 8 x 12 rectangular. How much CFM do I need for those two rooms? I understand that that I don't need as Much CFM capacity on the return as I do the supply, or at least that's what I was told. Is that correct?

[wahoo]

It seems you want to "engineer" your pipe size to fit whatever you wish so why ask us "will it work?". This is subject to "what do you expect from this duct system?". "Work" needs definition. Heck, you can reduce that 8X12 down to a simple 4" round pipe if you don't expect this pipe to do "anything" and you only have space within your design for a 4" round pipe. Me? I'd hire an experienced and competent contractor and let him do it as he wants. Then have someone cover whatever is installed. But that's only my opinion.:.02:

[tonynace]

It seems you want to "engineer" your pipe size to fit whatever you wish so why ask us "will it work?". This is subject to "what do you expect from this duct system?". "Work" needs definition. Heck, you can reduce that 8X12 down to a simple 4" round pipe if you don't expect this pipe to do "anything" and you only have space within your design for a 4" round pipe. Me? I'd hire an experienced and competent contractor and let him do it as he wants. Then have someone cover whatever is installed. But that's only my opinion.:.02:

If you had read my post, you would know that I am hiring a contractor. It's just that I've gotten somewhat conflicting views on how to accomplish what I want done. One says I need a 10" round for the return, and the other says I can get by with an 8". I've given the room dimensions and have been told that the furnace can handle it, so you "professionals" should be able to tell with the room dimensions how much CFM is needed and the correct size of the supply and return ducts.

[kdean1]

I caution you against trying to use the smaller of the duct sizes. You should NOT use an 8" for the only return as it is less volume than the supplies. If your contractor told you "don't need as Much CFM capacity on the return as I do the supply" fire them NOW! How can you circulate the proper quantity of air without the same quantity in the return?
Is your contractor a general contractor or an HVAC contractor?
Also have whomever you hire give you a written assurance that the ducts will supply enough air flow to keep the area at the temperature you specify.

[skippedover]

Within certain limits, the size of duct can be reduced or expanded, all at the expense of velocity and static pressure. As the velocity goes up, so goes the static. A blower is designed to operate within a certain static range and the actual airflow through the unit is a product of motor speed tapping and static pressure. So the more you reduce the duct size, the less airflow through the air handler or furnace.

The first step to know what size pipe is needed is to do a load analysis so that what you do is established on known facts rather than just guessing. From what the thread is about, I'd say there's a lot of guessing in the equation. So you can guess at the size, have it installed, box it all in and then suffer the consequences of an under sized return with its associated low airflow and discomforts. Or you can guess differently and put in a larger pipe than needed, box it in and live with the over sized bump in your wall. Or you can have it done without guessing, know that the airflow is correct, the duct is sized properly and the bump in the wall is a small as it can be without compromising the heating and/or cooling system.

Of course, you are in control of the choices as it's your home and you're paying the bill to have the work done. So if you like gambling, go for it. If you're not a gambler, then I've given you the information you need to seek a scientific answer. Wish you well either way. SO

[wahoo]

We've always designed the return ductwork at a LOWER static than the discharge which means the return is always LARGER than the discharge. An 8" round metal duct will return a little less than 200 cfm which means it'll return slightly less than two 6" discharge registers should put out. How much air do you wish to discharge into this bedroom? And how much air can your system deliver to this new addition when combined with present system? Lots of variables. As Skip said, there's a lot of figures and formulas that need to be considered before the ductwork is finished and covered. At the least have a manual J done on addition and size the ductwork to satisfy the heat/cool loads.:.02:

[beenthere]

I've been told that my furnace is sufficient to handle the upstairs as well. They will be tapping off the main plenum. It is only a small house, about 1000 sq ft on the first level. I would prefer round as far as it being better for airflow. And I know I don't need a 12" round, even though it may be equal to an 8 x 12 rectangular. I was quoted with an 8" round and a 10" round. And the contractors said my furnace was sufficient to handle the extra space. Since the round pipe is better, I want to know if I can get by with the 8" on the return so I can fit it in to the space I have. Otherwise, I will use the 8 x 12 rectangular. How much CFM do I need for those two rooms? I understand that that I don't need as Much CFM capacity on the return as I do the supply, or at least that's what I was told. Is that correct?

You were told wrong.

The contractors that were there can tell you how many CFMs each room needs. We can't cause we can't see the rooms, so we don't know what direction they face, or how many windows, and what type of windows. let alone where you are located at.

[genduct]

is how much air you need to deliver. Then we need to know how close to the supply plenum the take off is going to be and how long is the run.
THEN you can get an answer

[tonynace]

Now I'm more confused than ever.

[kdean1]

You're starting the calculation at the wrong end.
As genduct is saying, the FIRST thing we need to know is how cfm's are needed (obtained from a load calculation) AND the static pressure of the existing duct system. THEN it is possible to determine what duct sizes will be needed to deliver that volume of air to the new rooms.