Understanding design duct sizing

[Marc_L_]

When using design duct, I'm not understanding what Revit is doing to the duct sizes. I want to draw a 12x1.75 duct, yet i always end up with a 11 13/16"x2" duct. I have gone into Mechanical settings>Duct Settings>Rectangular and added both 12" and 1.75" for sizes. However this STILL doesn't make sense to me, as Autodesk help states 

"When Rectangular is selected, the right panel lists rectangular duct sizes available for a project and shows which sizes can be specified from the Options Bar."

However the sizes listed in settings ARE NOT the same as what's listed in the options bar, and nowhere is 11 13/16" listed, so why does my duct keep changing to this size?

Please help me understand wtf is going on here

[RobDraw]

The answer to this question might make the problem go away.

Why are you using non-standard sizes in an inadvisable configuration? Custom sizes can be a lot more expensive and that width x height ratio is out of normal range.

Rule of thumb is to use standard sizes and may actually be a project requirement. 

[Marc_L_]

I'm trying to draw the below product for coordination purposes. It's a 12x1.75 rectangular spiral that's embedded in the slab. This is a common product in my area.

http://www.eccoduct.com/specifications.html

The revit default sizes are all pretty non standard, and i don't understand why they're like this.

[Marc_L_]

To expand on my other reply. I originally was going to draw this as a MEP fabrication duct, however as part of the process for layout and install of this product, i need to create a line drawing, which isn't possible with MEP fabrication parts, so I decided to switch to design duct for these.

[ToanDN]

What version?  I have no issue draw a 1.75" x 12" duct in Revit 2024.

[Marc_L_]

2023. I suspect it's an issue in my project settings somewhere, as if I start a new drawing with the default imperial mechanical template it stays at 12" when drawn. 

Irritatingly, despite the fact that i spent a good 20 minutes messing with the sizes in the settings, and drawing pieces of duct with the same result, all of a sudden now it's working. I started testing some other sizes to see, and now it's resolved itself😡

I'm still left wondering why it occurred in the first place though, this is not the first time I've run into a similar issue with design duct. Since i mostly work with MEP fab duct, i haven't really worried about it before. I'm also curious as to why the sizes in my duct settings are all fractions by default, although i do see why they don't match the options bar drop down now.

[RobDraw]

Obviously, the project has different settings than the default imperial mechanical template. I too, started a new project with the default template which I know hasn't been changed on this machine as it is used for testing purposes only.

These are the default duct sizes:

I also had to change the HVAC units settings in order to get fractional duct sizes to be available for selection.

[HVAC-Novice]

Glad it works now. My default settings look like RobDraw's screenshot. In your project or template you can adjust that to your needs (inc. your undergrounds ducts). 

I suspect your template was edited from oob but wasn't set to what you needed. 

Note that rectangular duct flips dimensions of width/height depending on floor or section view. this can be a bit irrigating at first.