Ductwork liner

[jon_wilcoxJQWEZ]

Is there a solution to have ductwork show up as actual size when adding duct liner? So for a 24x24 duct with 2" of liner, the exterior of the duct should show as 28x28 with a free area of 24x24. I know this has been asked before, but after all these years, has Autodesk come up with a viable solution other than hand inputting in the difference every time? #fixthesystemfamily

[iainsavage]

Personally I would say that a 24x24 duct with internal 2" liner has only a 20x20 flow path.

If you want a 24x24 flow path then I would spec that as a 28x28 duct with 2" liner so in my personal opinion Revit is correct. Just my opinion though.

[robert2JCCH]

It's a difference of perspective.

For a designer, duct size can (sometimes) be noted to refer to free size. Depends on the firm, from what I can tell, and usually described in the 'how to read the drawings properly' section of whatever general notes coverpage or spec exists.

For an installer, duct size is fabrication size.

So, it's a (sometimes) competing opinion on what the drawing sizes should be noting, for a product marketed to multiple fields.

It would probably be really cool or convenient for Autodesk to allow adjustment from either perspective (similar to how they eventually added top/bottom elevation interactivity). You should pop the idea back into Revit Ideas and hope it gets picked up in the next major MEP tuneup. In the meantime, you can try using the Duct/Pipe Sizing tool to re-apply whatever Velocity/Static Pressure constraints you design by, and/or spot-verify your work by checking this part of the properties palette:

[RobDraw]

---

@jon_wilcoxJQWEZ wrote:

I know this has been asked before, but after all these years, has Autodesk come up with a viable solution other than hand inputting in the difference every time?

---

That would require it to designated as something that needs addressing. Just because people ask for it doesn't mean it needs fixing. Be careful what you ask for. The change could effect insulation also.

[hmunsell]

In Revit terms..... the Liner is INSIDE the duct. the Insulation is OUTSIDE the duct. so adding a Lining would not effect the duct size. Adding insulation on the other hand could.

You would need to make 2 calculated parameters. one for Total Height and one for Total Width. make sure the Height, Width and Duct Insulation is in the schedule. hide the Width and Height parameters in the appearance tab. use the Calculated Parameter tool to make a new parameter with a formula adding the Height or Width with the Insulation (see attached Video).

[iainsavage]

I just checked if Revit adjusted air velocity and pressure drop when a liner is added and it does - it bases the velocity and pressure drop on the reduced cross-sectional area caused by the liner.

[hmunsell]

correct, it takes the liner, inside the duct, into account 👍. the liner materials have a Roughness parameter on them that can be adjusted as needed. 

[fabiosato]

Hello,

I suggest you include this request in Revit Ideas.

[shawnchambers]

Yes.  Draw it actual size (28"x28").  Tag it with the Free Size label rather than the Size label.  So it'll read 24"x24", but be drawn as 28"x28"

[HVAC-Novice]

I don't think Revit needs improving here!

Since Revit accounts for the smaller cross section due to the liner (and increases velocity accordingly), all seems correct. 

If you have 28"x28"duct (that sheet metal part being the duct) and add 2"liner (we add twice on both sides) your air-cross section is 24"x24". And this is exactly what one would expect. The sheet metal guy still needs to make 28"x28", and that will be shown and the air knows it has 24"x24" to move. This is how a duct will be constructed in real life. 

I think what OP wants (showing smaller than real life ductsize) would be incorrect and lead to wrong installation since it goes against how a Sheetmetal contractor would read plans and fabricate. 

[tawilson]

I could be wrong but I thought op was asking if Revit could automatically resize the duct when liner was added.

[shawnchambers]

I think you are right.  But I also think he didnt understand that he could make a tag that reads the free area and not the actual duct size.  So his plans could read 24x24, but have the duct drawn at 28x28.

[iainsavage]

It would need to be very clear that the tags are stating internal free sizes and not the sheet metal sizes otherwise you're going to end up with a whole pile of undersized ductwork because I'm pretty sure that if a fabricator saw a tag just stating 24"x24" they would make the sheet metal that size. 

[HVAC-Novice]

tawilson: To me it sounds OP are asking for Revit to show the wrong size. The duct is the metal part and that is shown as it will be fabricated. If you show a different (wrong) size for that, you most likely end up with the wrong size sheetmetal. 

A sheetmetal guy will make 28" metalduct. Then a liner guy will install 2" liner. The latter never measures the cross-section of the the resulting air duct and doesn't care that it is supposed to be 24" across (28"-2x2"). They just put in 2" liner per plans. 

Revit shows the dimensions each trade needs (28"for sheetmetal, and 2"for liner) . There isn't a need to show the resulting 24". Showing the 24" would just create confusion. A lot of things in Revit are based on how things get constructed in real life. No one creates a liner with 24" inner width, and then sheet-metal around it. Sheetmetal first, then liner. 

[jon_wilcoxJQWEZ]

The reason for the request was for engineering designer reasons. If we use calculations for airflows, we want the free area of the airflow path and not the duct work size. The construction design industry also shows the free area and not the duct size in drawings. I have already made tags that will show what we want, but if we add or remove liner insulation from the ductwork, we have to go resize every piece of ductwork again.  Currently we don't even show liner because it is so difficult to work with in the model, if the ductwork automatically sized based on the free area and not the ductwork, drafting/designing with liner would be a viable option. 

[HVAC-Novice]

My understanding of what others said is that Revit takes into account the reduced area due to  lining when performing flow and pressure drop calculations. Wouldn't that alleviate your concern about the effective free area? It doesn't need to "show" the inside dimensions as long as it calculates velocity and pressure drop correctly. 

I don't know what region you are in, so things may be different. Here (US) all plans I've ever seen show the nominal sheetmetal duct size  and then show the liner (or insulation) thickness. I don't know why a viewer would want to know the effective dimensions, but they would then mentally add or deduct those liner/insulation thickness. 

Sometimes plans don't actually show liner thickness and just specify that somewhere. So whoever then manually calculates pressuredrop, may not actually take that into account and then have too small cross-section. But those are not the designers who use BIM or would model with liner to begin with. Maybe they add some language to adjust the sheet metal duct size to make up for that. But I don't think a sheetmetal installer actually reads that deep into the specs and mentally adds duct width or height. and the estimator who bid this project likely also doesn't account for that. If a duct on the plans shows 26"x30", that is exactly what they will do. They don't provide a 30"x34" just because somewhere hidden in 1000 pages of specifications there is some language about duct liner. I think your concern is valid, but really only if you do flow calculations outside of Revit. 

[tawilson]

Actually, that is exactly what we do. I don't recall seeing the liner on the contract drawing. I have to read the specs or sometimes it's in the drawing notes when I create the coordination/fabrication drawing. And now that we've discussed duct sizes and free area and static pressures, I can see why the designers don't show liner. And reading specs is part of the process on the fabrication end. There's info there our shop or installers need to see.

[shawnchambers]

Yes.  It comes down to "who" is doing the drawing.  Is this the design engineer or the fabricating detailer.  I was assuming it was the design engineer (cad person) who wanted to have the duct drawn correctly (for initial design coordination)... but who wanted to have it tag correctly.  A detailer would see 24x24, read the specs to see the 2" of insulation, and create a 28x28 ductwork.  They would never tag 24x24.

[shawnchambers]

You are 100% correct.  The fabricating detailer would read the contract drawings, see 24"x24", read the specs to see 2" of liner, and then produce a shop drawing with 28x28 duct.   They'd never tag 24x24.

But remember that Revit was used by design engineers before fabricators came into the picture.  I was using Revit to draw duct when everyone was still use CadDuct in Autocad, and Autodesk had yet to purchase the fabrication suite from MAP and rename it CADMep.  And before they integrated it into Revit.  I also remember when most detailers said they'd never do fabrication drawings in Revit.  LOL.

There is definitely a need to show 24x24.  Contract drawings from the engineer need to show the free air space of the ductwork.  It is only later that they determine liner thickness and length due to sound and insulation specs.  So to an engineer, 24x24 is the most important.... to a fabricator, 28x28.

So when the OP wants it to read 24x24, I'm assuming they are on the design engineering side of things and want the tag to read 24x24, but would like to have the duct drawn at 28x28 so they could do some initial design coordination.  (nothing is worse than an engineer who thinks their ductwork design all fits because they didn't leave room for liner and wrap growth). 

So by showing the OP that you can draw it 28x28, and tag the free air space of 24x24, I'm helping them make sure their ductwork fits and that the fabricating detailer won't have to reroute everything to make it work.  🙂

[HVAC-Novice]

First of all, we don't draw anything, we model. 🙂

I understand that liner thickness is decided later. But then you just add liner to that section and increase the size by so many inches and a transition is added to the portion that doesn't get a liner.  

Why do you need to see the free area? Revit already assumes the smaller free area for lined duct. I create views with duct legends that show the friction rate. that way I would see if a liner inadvertently creates huge pressuredrop and can correct it. the only thing I ever need to see is velocity and friction rate. 

Yes the specifications should be read. But it is just nice if the plans already show what is supposed to be done and where exactly. Some specs just say to add so much liner for all some type of devices. But this sometimes isn't' even possible or needed for all devices (some specs are just recycled from what was written in 1960).

If you want something done and done right, you have to put it on the plans. Over the years I've seen many installers with plan sets. But the specs usually are not used by the field guys (maybe by a foreperson in the job trailer, if you are lucky). If I wanted a specific device to have 10 ft of liner on the downstream duct and wanted to make sure the duct size is increased, I definitely would put that on the plan sheet with the final sheetmetal duct size and the liner tagged.