It appears Revit randomly generates airflow direction.

[nathanMB9D4]

I'm a bit puzzled.  I have a project I'm working on, but I cannot seem to get the airflow direction to work correctly.  I have checked the fittings and equipment as best as I could, based on ME204-3, published by Autodesk.  What is frustrating is that I've got two systems in this project that are drawn the same way, but react differently.  (And these are out of the box' ducts and fittings).

-One system shows no airflow through the diffusers (even though they have a volume specified).  The ductwork shows random airflow directions.

-Another system shows the correct airflow on the diffusers, but a few segments of duct show zero airflow.

-What am I missing

Thanks!

[RLY_15]

Hello,

A quick way to start debugging - remove this tee fitting temporarily and Split Systems. Check if the duct branch heading left is properly or improperly flowing. If it's not flowing correctly after being split from the rest of the system, the issue is either in the duct tap family or in the air terminal family.

If it is flowing correctly after splitting the system, start working your way back to the central equipment with the disconnections/reconnections until the issue manifests. It might give you an idea of what family is responsible.

If you can screenshot the connector settings for the air terminal families, the duct taps, and whatever is upstream on the duct system, that would also help.

Also, the air terminals in 02 are displaying as 0 airflow. It looks like that's an Air Terminal tag, not a Duct Tag, so the air terminal itself is zero'd out.

[iainsavage]

Air terminals must have single direction connectors (not bidirectional) and must all have the same classification and direction e.g. Supply and IN.

There must not be more than one open end.

Then it depends on what else you have in the system such as main and in-line equipment.

There are many posts on this topic, several of which I have solved for others - take a look at some of them.

If you're still stuck please upload your model for interrogation.

[dbutts7]

I've got some detailed lab handouts from AU I've written on this along with some training manuals from over a decade ago that hasn't really changed - check out these labs:

Perfecting Piping and Duct Systems in Revit | Autodesk University

Charging Ahead with Revit 2020 MEP Engineering | Autodesk University

Let me know if these help - thanks! David B.

[nathanMB9D4]

Gentlemen,

Thank you for your ideas.  I've gone through a few of these things- There a quite a few things I've learned over the day and a half.  I cannot get system inspector to 'inspect' this.  I also seem to have trouble with T fittings and unions (reducing duct sizes), or perhaps the diffuses themselves.  This is a similar project I'm working on, with similar problems.  I find it odd, though, that the duct to the right gets larger after the T.

[iainsavage]

When you enable "Check Duct Systems" you get the following warning:

By selecting the system you can use Divide System:

The bulk of the system becomes SUPPLY AIR SOUTH (2) and then works fine:

After a bit of interrogation, checking schedules etc I couldn't find anything which was on the other part of the divided system - SUPPLY AIR SOUTH (1) - so I was able to just delete it without any apparent adverse effect.

Duct sizing on the SUPPLY AIR SOUTH (2) seemed to work fine.

Hope this helps.

[nathanMB9D4]

Here is the original project I am struggling with.  (revit troubleshoothing 2).  I am suffering many similar problems on a newer project (troubleshooting).

I appreciate the help that has been offered, but I'm still getting frustrated with this.

-N

[nathanMB9D4]

It is pushing me, hopefully, in the right direction (still working on it, but at least I have a direction to go).  A morning of frustration, checking parts, and a half hour at Autodesk trying to find "check duct systems."

I appreciate your time.  Thank you for your help.

-N

[iainsavage]

Then click on the warning triangles to see the accompanying message.

I'll look at your other project tomorrow.

[iainsavage]

"I am suffering many similar problems on a newer project (troubleshooting)."

See my response at part 6 and follow my steps to solve this one in TROUBLESHOOTING.rvt

Re Revit troubleshooting2.rvt:

You have created two separate networks with separate furnaces but put them both on Mechanical Supply Air 1.

These need to be two different systems.

Use the Divide System tool to split them into two systems.

On the supply system you have disconnects shown by the warning triangles below. (activate this view by going to Analyze > Show Disconnects > Ducts):

These two taps are not in line with the main duct, they are too low down and not connecting, you need to realign them to coincide with the main duct.

[nathanMB9D4]

I'm still having A LOT of trouble getting this to even pretend to work. 

-So, after trying what I understood so far from people who commented, I smoothed over a few errors.  I probably created some more.  (And I aplogize if I missed something, or made the same mistake again).  

-Apparently, Revit likes you to delete and re-do exactly the same thing 2 or three times.  That's the only way I got one of these systems to work.  (Another one was working, until I saved, went home, and opened the file the next morning, where it instantly had to be re-done).

-The system shown in the pictures (Return Air South)- I cannot get this to work.  I can't find an error even if I'm looking for one.  But once I cap the open end (highlighted in the second photograph- all of the flow in my ductwork goes to zero.

-I'm beginning to think the developers intentionally set out to drive people crazy.  I cannot even seem to get the program to fail in the same way consistently.  It likes to give me different failures for the same thing. 

Please help!

[iainsavage]

-Apparently, Revit likes you to delete and re-do exactly the same thing 2 or three times. 

---

Nope, not in my experience.

I cannot even seem to get the program to fail in the same way consistently.  It likes to give me different failures for the same thing. 

---

If the system is badly connected Revit has no idea what your intention is for air flow directions so it basically (my assumption) guesses what direction air flow goes. This leads to variable wrong answers in the air flow values in the system (if you look closely you'll find ducts with negative flow values). It is because your systems have not been well connected by you, not because the developers are trying to make your life difficult.

I'll take another look at your latest model and get back to you.

[iainsavage]

The top system, Supply Air South still has the same warning as before:

If you use the Divide System tool as I suggested then Supply Air South (1) calculates fine and the System Inspector will work.

Supply Air South (2) then seems to have nothing in it and can be deleted.

Then you can rename Supply Air South (1) to Supply Air South.

All as I said before.

Supply Air Middle South seems to be okay

Supply Air Middle North seems to be okay

Supply Air South - there is something not right about the duct connecting to the rooftop unit. I deleted and remade it and the system works okay.

Return Air South is the same problem.

Supply Air West has a warning:

There are disconnects in this section of duct:

Fix the disconnects and the system works okay.

Mechanical Return Air 1 is associated with one of the rooftop units but has nothing connected to it

Mechanical Return Air 2 seems to be okay

Mechanical Return Air 3 seems to be okay

Mechanical Exhaust Air 1 seems to work but your "fan" is classified as an air terminal rather than mechanical equipment.

Mechanical Exhaust Air 2 seems to be an isolated transition piece with nothing connected:

Mechanical Exhaust Air 4:

The air terminal isn't connected

and your fan is classified as an air terminal when it should be Mechanical Equipment:

Mechanical Exhaust Air 5 seems to work but your "fan" is classified as an air terminal rather than mechanical equipment.

Mechanical Exhaust Air 6 seems to work but your "fan" is classified as an air terminal rather than mechanical equipment.

Mechanical Exhaust Air 8 seems to be okay.

There are isolated unconnected grilles: