Defining Loss Coefficient in Revit Family

[rcarterWL4FE]

Hello,

The Loss Coefficient for my Revit family for mitered elbows defaults to the ASHRAE Table value for xR3-1, where x is the system type (S, E, R) as opposed to the CR3-15. I've tried changing the connector element system classification to Global and that doesn't change anything. I've also tried to set the Loss Method as Coefficient and it does not use that value in the model either. How do I either A, get the correct ASHRAE Table fitting to apply or B, how do I define my own Loss Method that gets pulled into the model automatically and is used for the Pressure Loss calculation?

[HVAC-Novice · Accepted solution]

That doesn't work well in duct fittings. it works better in pipe fittings. And the ASHRAE database is crap. Either the fittings ASHRAE uses aren't the ones that actually are used in real world. Or the values make no sense. For example, a 90° elbow has less pressuredrop than a 45°. 

Look at my older thread (there may be more) what I ended up doing. Actually that my not be my most recent thread, but can't find a better one. I basically assign the pressure to the duct connector in the fitting and use the C-value from SMACNA book. this is what it looks lice in the family:

Only downside, you have to select all fittings (use 3D view wireframe) and set calculation method to "none" (would be nice if one could force to do that by default).

Use conservatives c-values if you have to interpolate between SMACNA fittings that only approximate the ones you need. 

[iainsavage]

In this linked post @Martin__Schmid explains how Revit selects the default fitting type from the database:-

https://forums.autodesk.com/t5/revit-mep-forum/duct-fitting-amp-ashrae-table/m-p/10943304#M84281

This link has some advice on how to build values into families:-

https://forums.autodesk.com/t5/revit-mep-forum/revit-2022-duct-fitting-pressuredrop-how-to-manipulat...

If you search the forum for threads containing the word coefficient you will find other posts too.

[rcarterWL4FE]

Thank you, that seems to work for me. I wish there was a way to get the Loss Method to default to "Not Defined", but the select all method doesn't take more than a second anyways, so I'm not too worried about that. Are there any drawbacks from changing the connector System Classification from "Fitting" to "Global"? Or is it a wash as long as the ductwork I'm connecting it to has a defined system and flow associated with it?

[HVAC-Novice]

Pre-set or automatic would be nice. I often swap out fittings or add to the duct system and then you do that over and over. Still better what ASHRAE fabricated. 

If you don't already , buy the SMACNA duct design guide. It has many of the actually used fittings. For some it only has flow in one direction, or if in both, different values. I select the conservatives larger value. that way my error (if any) will be on higher calculated pressure drop.