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Revit 2022 duct fitting pressuredrop - how to manipulate or how tomake then calcualte correctly

HVAC-Novice
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Revit 2022 duct fitting pressuredrop - how to manipulate or how tomake then calcualte correctly

HVAC-Novice
Advisor
Advisor

I have a generic round elbow and discovered the way it calculates pressuredrop makes no sense under some circumstances. it is the oob elbow:

At a given flow outside the below distinct angles: no pressuredrop and no loss method used

At 30°: pressurdrop is 0.0503 and it uses CD3-18

At 45°: pressuredrop is 0.0669 and it uses CD3-17

At 60°: pressuredrop is 0.0874 and it uses CD3-16

At 90°: pressuredrop is 0.0208 and it uses CD3-11 by default, but also offers CD3-12 and so on (s. picture)

 

This is a problem with all kind of fittings. In this example of a round elbow, it uses mitered elbows, which has higher pressuredrop. As an optimist i say this isn't too bad since it is more conservative. But the fact the 90° defaults to have a lower pressuredrop than the 30° fitting doesn't make sense. Even if i don't know a value is correct, seeing something illogical like that makes me nervous. Also the fact that it is zero at unusual angles isn't good, but knowing it I can adjust to that. 

 

I opened the family but did not see a way to set the type of fittings it uses and the connectors are type "fitting". Since I use Revit to get at least  a reasonable number, I like to resolve this or at least improve. Here my questions and ideas and I wonder if someone has ideas:

- are there families that work better or more reasonably?

- When it gives me multiple ASHRAE fittings to choose (90° in this case), is there a way to set that default in the family? I don't want to manually adjust that for every fitting. 

- Is there a family editing option to fix that or to assign calculated pressuredrops to the connectors? I envision i could create my own calculation with k-values or something to make it work. 

- In the project it gives me the option to use a "specific coefficient". But how can that be fixed tot he family? 

 

I'm willing to put in time to build or edit families so they work a bit better. but fittings seem to be a black box. There is an old thread from 2013, but links in there are outdated.

There is talk about using a fitting schedule to see or assign the CD values. But I wasn't able to create a duct fitting schedule that shows any such values (even with all parameters included)

 

enkus_0-1643904047840.png

enkus_1-1643904139274.png

enkus_2-1643904295992.png

enkus_3-1643904471077.png

 

 

 

 

 

Revit version: R2025.4
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iainsavage
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https://forums.augi.com/showthread.php?123571-ASHRAE-Duct-Fitting-Database

From 2009! and still the same.

As far as I can tell Revit accesses another file which runs algorithms to identify the correct tables, including looking at bends in close proximity to one another etc. 

I’ve had similar problems to you and resorted to using the user-specified k-coefficient option rather than letting Revit identify the applicable ASHRAE table because l was getting loss values which were 3 times what I expected and realised that the tables being used were completely inappropriate.

This setting is however an Instance setting and I can’t see a way of setting the default method/value so what I’ve been doing is selecting all in model and changing the k-coefficient value.

PITA.

There’s probably a better way to do that though, maybe using Dynamo?

HVAC-Novice
Advisor
Advisor

Thanks for the response. 

 

I started manipulating the elbow duct fitting and replaced the connectors with regular global/bidirectional connectors and assigned a pressuredrop. So far I have to manually enter the pressuredrop, but it seems to work in a system, and also works with auto-route.

 

Next step I will have it calculate the pressuredrop based on the SMACNA/ASHRAE loss coefficients. I need to find an approximation that changes the coefficient based on the angle parameter. But it looks like it could be done to calculate pressuredrop based on actual flow (and a velocity that derives the velocity pressure).

 

I also saw ASHRAE has different coefficients based on duct size, while SMACNA has one coefficient. I guess that can be implemented somehow. At least to a point that it makes more sense than what Revit does oob. 

Revit version: R2025.4
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Kevin.Lawson.PE
Advocate
Advocate

This is the way. Build the pressure drop calculations into the fittings, and assign the pressure drop parameter to the appropriate connector. 

 

KevinLawsonPE_1-1644008478890.png

 

Then in the project, set all the Loss Method for all of the fittings to "Specific Loss" (you can do this with a Dyanamo script or using a duct fitting schedule).  After all this, the built in pressure drop calculator will work fine. 

 

KevinLawsonPE_0-1644008306265.png

 

This is also nice because you can then tag all of the duct and fittings with the pressure drop and they update in real time, which makes QA easier. 

 

The tricky part is that some fittings have two different loss coefficient tables.  A booted tee, for instance, has a different pressure drop if it's diverging (like on a supply system) or converging (like on and exhaust system).  You can either build two different families and remember to use the right family, or you can add a Converging/Diverging check box which changes which loss coefficient table is used.   My goal is for the Ripple plugin is to keep track of connector flow directions in the background and automatically set your converging/diverging setting.  

 

-Kevin Lawson, PE
www.rippleengineeringsoftware.com
Revit heating and cooling load calculations in one click!

HVAC-Novice
Advisor
Advisor

Actually it seems I don't need to set the method in the project. I set the connector to be a regular duct connector and assigned the pressuredrop parameter to one of them. 

 

In the project it still is set to ASHRAE. But if I try out and set the pressuredrop to 1", the system pressuredrop raises by 1" and that branch becomes the critical path. Obviously this isn't a validation of my family yet....but tying the pressuredrop value to the duct connector seems to override whatever the project does with fittings. (Don't quote me on that.....:-)

 

But I created a duct fitting schedule and included ALL available parameters, but I can't see one that looks like it is setting the calculation method. which parameter do yo mean to set the "loss method"? I came across similar advice in a different thread, but seem to not be able to see such parameter in a schedule.

 

As for other fittings, I will look into each one I use if they behave OK oob, or if they need adjustment. Elbows seem to be the most urgent since a duct system has many elbows compared to other fittings.  I saw for tabs Revit uses a different fitting that is less efficient than mine. but I'm less concerned about that since that likely just gives me a tiny bit too much pressure drop and would be conservative. Maybe i will, but i wasn't planning to re-create all the fittings. 

 

Kind of odd that Revit has a pretty good duct tool, but doesn't' use the correct fittings, or makes it quite hard for us to manipulate the fittings. I'm sure I'm not the fist one to point this out. 

 

enkus_0-1644016096984.png

 

Revit version: R2025.4
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iainsavage
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Mentor

Is  it not easier to just use this option?:

iainsavage_0-1644058328242.png

 

Kevin.Lawson.PE
Advocate
Advocate

The Loss Method isn't schedulable, but you can create a schedule of all of the duct fittings, select them all in the schedule, then click your Default 3D View and you will have all of the duct fittings in the project selected, you can then change the loss method of all of them to Specific Loss in one click as opposed to having it done fitting by fitting.  

 

KevinLawsonPE_0-1644094555306.png

 

I'm not used to dealing with the OOTB fittings, so this may have some limitations. 

-Kevin Lawson, PE
www.rippleengineeringsoftware.com
Revit heating and cooling load calculations in one click!

Kevin.Lawson.PE
Advocate
Advocate

Either would work, but once you've gone through all of the work to calculate the K factor, the pressure drop is just one more calculation and it allows you to tag the fittings with the pressure drop, which is nice. 

-Kevin Lawson, PE
www.rippleengineeringsoftware.com
Revit heating and cooling load calculations in one click!

HVAC-Novice
Advisor
Advisor

Thanks. I was able to select them and in properties I can change the "Loss Method" data. i didn't need to highlight them in a 3D view, though. 

 

but there are 2 potential pitfalls with this method:

- I have to manually do this after I added the last elbow (or do it again if I add more). I also would have to look up the loss-factors again. 

- The schedule doesn't seem to differentiate by angle (30°, 90°-fitting). But I need to apply different loss factors based on angle. Otherwise a 30° would show me the same pressuredrop as a 90°. 

 

If possible, I like to code that all in the family. That way I just use it and don't have to worry and it automatically adjusts. Revise instantly 🙂

 

enkus_0-1644110134296.png

 

Revit version: R2025.4
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iainsavage
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@Kevin.Lawson.PE wrote:

 then click your Default 3D View and you will have all of the duct fittings in the project selected, you can then change the loss method of all of them to Specific Loss in one click as opposed to having it done fitting by fitting


Right click on one fitting in any view and “Select all instances in model”

iainsavage
Mentor
Mentor

Unless you’ve got dozens of 45 degree bends on your index run I wouldn’t overly worry about the minimal impact of a slightly reduced coefficient. I would just use the coefficient for 90s.

Its engineering, the idea is to get a big enough fan, not work out values to the third decimal place.

iainsavage
Mentor
Mentor

….ultimately though Autodesk should fix the algorithms which select the ASHRAE tables so that we don’t need these workarounds.

Kevin.Lawson.PE
Advocate
Advocate

Yep, curve fit those and put the formula in a parameter to calculate your actual loss coefficient.

 

[Angle Correction Factor] = 0.000058 * ([Angle] / 1°) ^ 2 + 0.016296 * ([Angle] / 1°) + 0.00312

[Loss Coefficient] = [Elbow Loss Coefficient] * [Angle Correction Factor]

 

KevinLawsonPE_0-1644164630084.png

 

-Kevin Lawson, PE
www.rippleengineeringsoftware.com
Revit heating and cooling load calculations in one click!

HVAC-Novice
Advisor
Advisor

My fit is a bit different and i ignored the values below 30°. I usually correct the fit equation so that all relevant values are like the measured value, or a bit higher. but I think this is reasonably close either way. 

 

I agree, I may be over-thinking this. but before applying safety factors, i like to get at least close or have a solution that makes sense. 

 

But I discovered one problem. Revit adds its own pressure drop to what I calculate. That shows in the properties and also in the pressure drop report and the system inspector. In the below screenshot I set it to specific loss = 0 and the oob parameter "Pressured Drop" shows my calculated value (0.0106 in htis case). if i set the specific loss to 1 in-wg, the Pressure Drop goes to 1.0106 and so on. Same if I set it to ASHRAE fitting, then the "Pressure Drop" will be what the ASHRAE fitting calculates plus my calculated value. The only solution would be to select all the fittings and set the Loss method to specific at 0. That obviously is inconvenient, I and has the danger the user doesn't do that or doesn't remember to do that again after adding fittings. 

 

So I'm kind of back to where i was unless i can find a way to permanently set the Revit pressuredrop to 0. 

 

Another practical idea would be to just leave these fittings that have some sort of pressure drop alone and only manipulate fittings that have no Revit pressuredrop set already. Those obviously are the most dangerous ones since they make Revit underestimate the pressure drop. I agree with iansavage that we want conservative design values, and not necessarily the absolute correct values. 

 

I wish Revit would either fix the fittings and make them all be correct, or at least would allow us to override them. But they chose to set it up with partially wrong calculations AND prevent us from overriding them (at least not easily). 

 

enkus_0-1644171881661.png

 

enkus_1-1644172100541.png

 

Revit version: R2025.4
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HVAC-Novice
Advisor
Advisor

I added a yes/no paramter that lets me enable to use "my" calculated pressure (and then i set the Loss Method to "not defined). and if i decide to use the ASHRAE Revit values, I uncheck this. 

 

I noticed another oddity. I used the SMACNA fittings since i onw the SMACNA HVAC System Design guide. thsoe C-values seem to be higher than the ASHRAE values. I don't have the ASHRAE fitting database or design guide, so i rely on what Google shows me and what is in the pressure report. but for gored and die-stamped fittings, the SMACNA values seem to be higher by a bit. Did anyone else notice that or have an explanation? The engineer in me would use the higher (SMACNA) values. but this drives me even more to use my own calcs. 

 

And the Revit ASHRAE fittings seem to be the ones for 1.5D. So for a 1D elbow, they will be even more over-optimistic, which is exactly the opposite of what a designer wants. 

 

I'm entertaining the idea to buy the ASHRAE database, but it seems to be a subscription. and since I would prefer to use the higher values, it wouldn't be much use to me anyway compared to the SMACNA values.

 

The only explanation i have is that ASHRAE changes the coefficient based on duct diameter. The SMACNA values seem to be based on smaller diameter ducts. 

 

 

Revit version: R2025.4
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iainsavage
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See part 16 of this post for how to access the ASHRAE factors which Revit uses

https://forums.autodesk.com/t5/revit-mep-forum/duct-fittings-huge-pressure-loss/m-p/10928584#M84078

HVAC-Novice
Advisor
Advisor

Thanks. It looks like ASHRAE only has data for a few fittings. For example, many are only available for the 1.5D. 

For the fittings where ASHRAE adjusts the C-factor based on diameter, it looks like SMACNA data are equal to what ASHRAE has for 8"-10" duct size. For larger ducts, the ASHRAE factor gets smaller. So SMACNA will be more conservative for larger ducts. 

 

The result for the ASHRAE/Revit fittings is, that they assign the 1.5D factor, even if it is a 1D. and we rely on the few fittings ASHRAE has in the database. 

 

I tried to "flush out" the default ASHRAE fitting setting in the fitting family. I took the family, converted it to a mechanical family, saved it. then converted it back to a duct fitting. but it still "remembered" the ASHRAE options it had assigned. When I create a duct-fitting from scratch (from generic template, then convert to duct fitting), the ASHRAE options are blank and I ASSUMME it would then not use an ASHRAE value, but only use my calculated value.  

 

I guess I could re-create the geometries from a fresh family and hope that works. But that sounds like an exercise in frustration with other potential pitfalls. 

 

Does anybody know how for a new fitting the ASHRAE options get assigned to begin with? Or how to "reset" the fitting family so it forgets that? I think if I could do that, the fitting could work the way I want without me having to manually disabling the ASHRAE option in the project. Maybe dynamo? (I'm a total noob in dynamo)

 

It also would help if one could just reduce the type of ASHRAE fitting available. For example, the round die-stamped (smooth) fitting would not have options for gored or rectangular fittings. 

Revit version: R2025.4
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HVAC-Novice
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Accepted solution

I think I got as far as Revit lets me. i got some help from a duct-fitting specific thread

 

I basically went through my duct system and manually verified if the fitting showed reasonable values compared to SMACNA. OOB the taps don't have any pressure drop, transitions default to a lower loss fitting than actual, elbows only use the 1.5D (even if it is 1D), some transitions don't have a pressure drop at all.... it is a mess. You can't use that for design. So for all the one I use I add "my" scheme and then select them to set loss method to "not defined". if you replace or add a fitting, you have to do that again.

 

I removed the fittings that I don't want to use from the XML-file and also removed the tbl files for those. after some re-starts they seem not be available in Revit anymore. So that hopefully will avoid me having to manually disable the ASHRAE calcs for each fitting. 

 

One caveat is, that if you replace a fitting (i.e. swap a 1.5D in instead of a 1D) it doesn't seem to read the flow for the pressure calcs (it passes the flow through, though). Sometimes i have to just wait, reconnect the duct, or change the flow value forth and back to trigger re-calculation. So for QC it is good to check each fitting if they actually calculate a pressure drop (tag helps). Carefully review the pressure loss report and each fitting. 

 

Thanks to everyone. I now have much more confidence in the results and know better how Revit works. 

 

Edit: You actually can skip the velocity pressure calculation and use the "coefficient Method" for the connector. Drawback is, you don't see the calculated pressure in the model, but it shows up in the pressure loss report. You still need to set the Loss Method to "not defined", though. 

 

enkus_0-1644424273940.png

 

Revit version: R2025.4

iainsavage
Mentor
Mentor

some transitions don't have a pressure drop at all”

According to the help, this is because they don’t conform to any of the ASHRAE table criteria and as explained by Martin Schmidt in that other post even some of the Autodesk supplied ootb content doesn’t precisely conform including a simple slip-jointed radius bend. It may be that the fitting families need to be modified e.g. the taper angles on transitions?

HVAC-Novice
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I don't own a ASHRAE fitting database, so I can't tell if they just don't have enough data or if they only have fittings I don't use. Like they often seem to only have a 1.5D version of an elbow, but not a 1D etc. 

 

Even some transitions that just have a 15°, 30°, 45° etc. type don't have ASHRAE data in Revit. Some of them have have multiple ASHRAE options, but the default is one with a low pressure drop. And they show the same pressuredrop for a 15° or a 60° transition.  I thought I can use at least some ASHRAE fittings, but ended up entering my data in all the one I use. My ducsystem went up from 0.28 in-wg to 0..37 in-wg by going from oob to my method 🙂 I don't have dampers in it yet, but for those I always used my method based on manufacturer data anyway. 

 

I think it would be better if Revit did away with the ASHRAE data and just did something similar to what I did in the family and the oob families then just have standard C-values and the user can adjust them. What they created just results in wrong (often too low) data and gives them a bad reputation. I read from many people they don't trust the Revit duct calcs. I think the whole duct calculation is really great and better than 3rd party software or manual calcs. But the fitting implementation really is bad. I bet they put a lot of engineering and programming in this - but they probably never talked to a designer. 

 

Revit version: R2025.4
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