Duct flow Real life case study

[yobry]

Hi dear Revit colleagues,

I have a real life case study to propose you in Ventilation.

I join you a Revit file with a supply air (in blue) et Return air (in yellow) connected to an AHU with two external Cooling coil.

The problem is I have a lot of "Cutting" flow transmission and I can't find why.

I realised some previous project with 3 stairs offices building ans it was working well. 

In addition of the revit file (2020 version) I add some Screenshots to show the problem in preview.

In the Revit file you can open the 3D view "Case study".

1/Overview with theorical flow data I'm waiting

2/ Table of flow system:

in yellow: The sum of the air terminals, this is correct

in green: Correct in the Mechanical equipment

in Red: Wrong value

3/ Duct System Error

4/ Duct flows given by revit tag.

Some values are lost after elbows.

5/ The value is equal to 0 in the duct, but after in the Sound attenuator the value of my shared parameter display the expected value !!!

Thanks a lot if you take a small time to find something bad in my design.

Yoann

[iainsavage]

I've identified a few potential issues but may I ask you a question first - is it intentional to have 3(?) different supply systems or is it your intention to have all of the supply diffusers on one system?

[yobry]

About the sypply systems I think they are 3 because of the connectors settings of the cooling coils. So this is not intended. Do you think there is a link with the flow transmission problems ?

[iainsavage]

I'm struggling with this one I'm afraid.

I was able to connect your diffusers using my own duct network and using Autodesk families and everything worked fine so there is a component somewhere in your system which is causing the flow direction confusion but I'm having trouble tracking down the offender.

[yobry]

ok thanks for had a look. I will investigate more.

[yobry]

It would be nice if an Autodesk expert can solves this problem because I'm asking myself about the ability of Revit to use the flow and loss datas in all type of project. If not I need to search an other way...

[HVAC-Novice]

It isn't Autodesk's responsibility to resolve user errors. This is a user forum.

Something is set up wrong in your families or the duct systems. I'm not sure if that AHU is made of 2 AHU, or just has two inlets and outlets.

I recommend you start with a simple system and make sure it works. Do it by the book. Then you know the families all have the correct connectors. Then make the system more complex and verify the system still works after every change. That way you find out what is screwing up the system.

[yobry]

I posted my case here because Autodesk don't offers some helps to resolve this kind of problem. This is writing I have to subscribe an other offer to have this service.

The AHU is one with an air exchanger in the middle.

[iainsavage · Accepted solution]

It should work.

The way that your coils are setup they should, in Revit flow terms, act the same as the Autodesk single duct VAV families with a primary system on the intake side and separate systems downstream.

You could change them to be "in-line" components so that the upstream and downstream systems are the same system by changing the connector settings (see how the Autodesk default coils are setup).

When I split this part of the system, divide system and put in a temporary fan it works okay:

When I do the same with this part of the system, it doesn't solve the problem so there is something else causing the problem here.

But when I remove this fitting

and replace it with standard branches

the error disappears but the flows don't propagate through the sub-system.

Deleting these transitions and replacing with different fittings seems to cure the flow addition problem

When I remove the temporary fans and re-join the two sub-systems they form one system which works fine but as soon as I reconnect the system to the cooling coil the error reappaears.

Looking at the other cooling coil system it does not report an error but some of the ducts show a zero flow value, so something's not right there either.

Reconfiguring this junction seems to fix the zero flow problem

But when I then join everything back together the flow mismatch error reappears.

Disconnecting the lower coil system again and dividing the system , for some reason I'm getting a 4600 m3/h flow in the disconnected part but I can't figure out where that number is coming from.

That's where I've got to - almost solved but falling at that last hurdle.

I have suspicions that the pants and wye fittings might be causing problems but haven't been able to prove that yet.

Unfortunately I don't have R2020 so I can't send the model back to you unless you can open a R2021 version (or I could maybe export to IFC?).

[yobry]

Ok thanks a lot for your review ! You can upload the Revit I have until 2022, just this project was always in 2020.

I will check what I can do to resolve this.

[HVAC-Novice]

It is like Toyota sold me a Landcruiser. But now I have to learn how to crawl over rocks myself and no one from Toyota comes out to help me. When you buy Revit, you buy the software, but not a direct support. I just wanted you to be aware, 99.9999% of replies here are just users like you. Every blue moon someone from Autodesk is nice enough to chime in, but this isn't to be expected. So if no one provides a resolution, it is on you to figure it out (or pay someone).

Just managing expectations here. I've had flow problems, and after long research it always turned out to be something set wrong, not something wrong with Revit. There are limitations when you have parallel systems and there are some workarounds (like when space restrictions require me to use multiple smaller louvers in lieu of a large one). It all can be done. but best to methodically go from a functioning simple system to a complex one while watching what changes with every change.

[iainsavage · Accepted solution]

Think I've cracked it.

I changed your cooling coil families connectors to Global, bidirectional, calculated and linked (which makes the family "in-line" equipment) and .....Bingo!

[RobDraw]

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@yobry wrote:

It would be nice if an Autodesk expert can solves this problem

---

You've gotten some great advice up and after you stated this and it appears that the help provided by @iainsavage has resolved your user issue. Who are you going to ask next time?

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@yobry wrote:

I'm asking myself about the ability of Revit to use the flow and loss datas in all type of project. If not I need to search an other way...

---

You should be asking yourself about your ability to use the program correctly before doubting well established functionality. If you want an easy button, maybe you should look for another way.

The advice you have gotten here was spot on and quite valuable. I hope you can absorb good portion of it and use it going forward before doubting the program again.

[iainsavage]

Please don't be discouraged from posting more questions in the future. The forum users will always be willing to help but we are just users of the product, we don't work for Autodesk and we don't always get the answer right first time.

I've helped many forum users with flow failure problems similar to your own but yours just took a bit longer to figure out because there seems to have been a few different issues occurring together.

Hopefully you can complete your design now.

[yobry]

@iainsavage 

Thank you a lot for all tour staff. I accepted the two solutions about the Cooling coils and all the fittings problems.

I will continue to train with this model to be able to resolve the flow issues in the future.

@RobDraw 

Thank for yours advices and you faith in Revit. The readers of this post can make their own opinion about the Revit flow method.

[RobDraw]

Yes but those opinions should be based on proper use of the software without user errors.