Duct Size vs Length parameter

[brane1012]

What is difference between parameter HVAC - Duct Size and Common - Length?

Is it important to pay attention which parameter to use when defining duct connectors?

[brane1012]

Anybody? :S

[Mehdi.Kardehi]

Hi mate...not sure what you are after...see below the difference between duct size and length

[brane1012]

So the only difference between those two parameter categories is how are they shown in schedule? Thera are no consequences on pressure loss calculations and stuff like that?!

[CoreyDaun · Accepted solution]

These two Parameter Types can be used interchangeably in a Family, and appear to function identically as Length Type Parameters. I suspect that the only reason that Duct Size is a separate Parameter Type is so it can have its own configuration under Project Units. By the imperial default, Length is formatted to feet and inches (i.e. 1' - 3") while Duct Size is formatted to just inches (i.e. 15"). I do not believe there are any special calculations or considerations regarding the Duct Size Parameter Type.

Edit:

The "Size" that is being displayed in image of a schedule in a previous post is not the same Parameter as you are describing, and the two are unrelated. What is shown there is the System Parameter "Size" that combines the Height and Width into a single value, the format of which is controlled under Mechanical Settings > Duct Settings.

[bvm76]

I've done some test to duct fittings and found out that using the length parameter will not adopt the size of the duct.

[brane1012]

This seems like logical explanation... Tnx again! You are my savior! 🙂

Best regards!

[brane1012]

I didn't notice that...Can you clarify that little bit more?

[bvm76]

As an example, when you are creat a duct layout using an elbow with length parameter, the size of the elbow will not adopt the current size of the duct. Say if you are drawing a 150Ø duct and the elbow size it was created is 200Ø, it will then add transition to connect to 150Ø (see image 1). Whatever size you assign to the elbow on the family creation will be fixed. Another thing is you cannot use the duct size on the properties palette to change the size of the elbow (see iamge 2).  

[matevz.petric]

9 years later, here i am...

My mind is blown right now. I had no idea that this is the way it works. And it still does, in 2023, i just tested it out myeslf. Attached is a short video. 

I modified the OOTB elbow family and created my own length parameters using "Common" category instead of HVAC Duct size. Afterwards, the size did not adjust, just as you stated.

The weirdest thing is that, if you click on "assign parameter" button before you defined them, Revit will force you to use the wrong parameter type, meaning it'll pre-select "Length" from "Common" category (instead of "Duct Size" from "HVAC"), while also not allowing you to change it?!

Pretty crazy TBH.

Well, i learned something today.
Cheers!