furnace ducts

[Evil Baby]

Just so nobody gets confused, I've just re-used a old thread about flex duct.


now to the question, I've realized I can use normal ducts to get air to attict. Question is I have limited space so I was think 6inch for heat/air and 4 inch for cold air return.


Is that wise, or should they be same size idealy?

[Drthorne]

Install 8" duct if it fits, there's a lot more air flow resistance with this type of duct compared to the standard sheet metal stuff. flow through 8" flex duct is roughly equal to 6" sheet metal. I'd recommend installing a return line if there's not one on that floor already.

[Evil Baby]

I may still be able to install sheet metal but thanks for the advice.

I'm still trying to figure out where the best place is to attempt to get this to my attict.


any idea how much a professional would generally charge?

[Evil Baby]

Just so nobody gets confused, I've just re-used a old thread about flex duct.


now to the question, I've realized I can use normal ducts to get air to attict. Question is I have limited space so I was think 6inch for heat/air and 4 inch for cold air return.


Is that wise, or should they be same size idealy?

bump for new quetsion

[Pete_Coach]

Install 8" duct if it fits, there's a lot more air flow resistance with this type of duct compared to the standard sheet metal stuff. flow through 8" flex duct is roughly equal to 6" sheet metal. I'd recommend installing a return line if there's not one on that floor already.

What are you saying? A flexible duct is not good. The smoother the interior of the duct is the better the flow of air. The larger, the more air will flow (less velocity, but more air, it is basic fluid dynamics).
OP, keep in mind, every 90 degree turn or elbow equals an additional 10 feet to the run. Try to make it as short, as straight and as smooth as possible.
A return duct back to the furnace is a must, you have to have the circulatory air flow for it to work.

[Evil Baby]

What are you saying? A flexible duct is not good. The smoother the interior of the duct is the better the flow of air. The larger, the more air will flow (less velocity, but more air, it is basic fluid dynamics).
OP, keep in mind, every 90 degree turn or elbow equals an additional 10 feet to the run. Try to make it as short, as straight and as smooth as possible.
A return duct back to the furnace is a must, you have to have the circulatory air flow for it to work.

I was saying that flex is not as good as sheet-metal. From what I've been told on this thread 8 inch flex is cmpairable to 6 inch of sheet.

I was unaware that 90 degree elbow's were that bad for air flow. From the work we did today we should only need one 90 degree and one 45 degree elbow for the ac/hot air duct.

[Pete_Coach]

Calculating and installing ductwork is as much a science as figuring out what size your furnace or airconditioner should be. The air flow produced by your furnace fan is hugely important and this affects what size ducts will be needed fr each room.
For calculating duct equivalent lengths use this http://www.siriushoods.com/pdf/ductworksheet.pdf.
It is also very important that you also run a return duct (which needs to be bigger than the supply duct) back to the furnace. It was once explained to me that a room is like a bottle. If you fill it up with water then it is full and no more can be put into it. If you want to add more, you must let some out the other end. Same as a room full of air (warm or cold), if you want more, there has to be a means of getting or letting some of the staler air out, hence the return duct.
There are several HVAC specialists that are on this board and perhaps they can give some definitive answers and advice?

[Evil Baby]

Calculating and installing ductwork is as much a science as figuring out what size your furnace or airconditioner should be. The air flow produced by your furnace fan is hugely important and this affects what size ducts will be needed fr each room.
For calculating duct equivalent lengths use this http://www.siriushoods.com/pdf/ductworksheet.pdf.
It is also very important that you also run a return duct (which needs to be bigger than the supply duct) back to the furnace. It was once explained to me that a room is like a bottle. If you fill it up with water then it is full and no more can be put into it. If you want to add more, you must let some out the other end. Same as a room full of air (warm or cold), if you want more, there has to be a means of getting or letting some of the staler air out, hence the return duct.
There are several HVAC specialists that are on this board and perhaps they can give some definitive answers and advice?

Thanks for the very helpfull reply. We are installing a air return duct, so hopefully that will help. Also hopefully my furnace will be able to force the air up into the attict. It's a fair ways up but we're almost done instlling it so I guess I'll find out the hard way.

I did check to make sure my furance could handle the square footage of my house and it had enough power for it.