Real duct size???

[dracri]

I don't know how is it for you, but when we are talking about a duct and say 300x300, we are talking about air stream size, I am not interested in what external duct size is, this will come later. If you have a duct running outside, exposed to weather you may need lining (let's say 25mm) and for inside you go from lining to insulation (also 25mm). See picture attached. Revit doesn’t know that the air stream size is the important one.
Why is that? Can anybody tell me? Am I right or wrong?

[srosen]

Revit does know the air stream size is the important value - that is the duct size reported by the duct tag and the size reported to the calculation engine (the net free area). You can select the duct, right click & select Element Properties, and add a lining thickness (represented by dashed lines inside the duct) or insulation thickness (thermal insulation represented by lines outside of the duct).

[dracri]

Hi srosen, thank you for your post, maybe I’m missing something, please somebody enlighten me, in this screenshot (see attached jpg) are a few examples of arrangements I came across in Revit.
You say that Revit is reporting the “net free area” that I like to call air stream size of a duct when tagged “Revit does know the air stream size is the important value - that is the duct size reported by the duct tag and the...”
(1) is showing a 300x300 duct with 25mm thermal insulation, when tagged is 300x300 which is correct (air stream size is 300x300)
(2) is showing a 300x300 duct and we apply 25mm insulation, when tagged is still 300x300 which is not correct should be 250x250 (air stream size) according to what you said above. (see Element Properties as well, Lining and Size)
(3) easiest example of a supply / return duct. You have to have the duct thermal insulated due to the amount of heat you have to transport. Inside the building you can have thermal insulation (is cheaper), outside the building you cannot have thermal insulation due to whether condition (can get wet from rain, snow…) so you need to line the duct, does the air stream size changed? No, according to Revit it does so you need to increase the physical size to keep the air stream size, resulting the #(3)
(4) example you need to have just 1 bend lined for acoustic reasons, what is the air stream size looking to the picture? To keep the size you have to enlarge the bend, resulting (5) or (6) if you need a longer portion of lined duct for acoustic purpose.

Maybe you are referring that Revit is indeed reporting the airstream size but doesn’t draw correctly, when applied lining could this be the reason? Which again is that I might be too picky and go again to the other extreme, require that Revit should be a drafting tool not only a design tool.

But now in the end I understand Kyle B (see http://discussion.autodesk.com/thread.jspa?threadID=655087) <> I don’t know of any other convention, because I didn’t need to work with another convention other then this one, when I worked in Europe I used same convention I’m using now, sory to be an ignorant from this point of view.

[srosen]

Maybe you are referring that Revit is indeed reporting the airstream size but doesn’t draw correctly, when applied lining could this be the reason?

Exactly. You are correct that the drawing should increase the sheet metal physical size for lining to maintain the net free area - Revit does not do this. It is important to note to the contractor your model (and documentation in terms of plotting drawing sheets) specifies net free area - when viewing the dashed lines in the duct (or more importantly your specifications) the contractor must make allowances for increasing the sheet metal size to accommodate the specified lining thickness. Theoretically this could mean your duct in a model (which is indicated 1 or 2 inches (25 or 50mm) on the small side) may not show actual interference - however it would not be prudent to have such a low tolerance for any such installations.

[dracri]

I hope this will be addressed in future release.