Loss Method does not have Specific Loss & Specific Coefficient as options

Hi community,

I know this question is not quite related to Dynamo perhaps, but I suppose Dynamo forum is more vocal than MEP forum, so I decide to put it here since didn’t get much attention back there.

It is really bothering.

Here’s my situation. Our team mainly uses data from CIBSE to calculate pressure drop for duct fittings. What Revit has are: built-in ASHRAE table, Specific Loss, Specific Coefficient and Not Defined. It pretty much leads me to Specific Loss / Coefficient. So I thought about maybe I can map data from CIBSE to the fittings onto these two options via Dynamo or some API calls later. But I found that some of the fittings does not even have these options under loss method. At first, I thought I was just dealing with few bugsy duct fitting families. But it turns out Revit Sample project has it too, some of them have just only ASHRAE and Not Defined.

I found some discussion saying this issue exists in previous versions. Some found Revit wiped out these two options in Revit 2017. I cant say much about Revit 2017 but I’m experiencing it in Revit 2019.

Not the main purpose of his question but the guys did run into my question as well

Really need some help on it. Any suggestion would be appreciated.


Apologies but I’m not sure you’ll be able to fix this one, it looks like an issue with Revit under the hood?

Here’s one of the few posts I’m aware of which seems related, maybe it is helpful?



@Mark.Ackerley Thx! Very informative link, I could use it later.

After days of collecting info and testing, I found out that Revit had removed Specific Loss & Specific Coefficient for fittings with more than 2 connectors since 2017. But I dont think that is what they real mean by concluding to “more than 2 connectors.” Because in fact, my tap here only has two connectors. So I guess it becomes more like if a fitting is going to cause pressure drop at multiple spots (e.g. on main duct and branch duct), Revit won’t offer these two options because the specific loss/coefficient user gives won’t be applied to the spots in a way that user is expecting. So user has to rely on ASHRAE table then.

1 Like