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How To: fix your DUCT system

8 REPLIES 8
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Message 1 of 9
wils02
7296 Views, 8 Replies

How To: fix your DUCT system

A while ago, I posted a message about fixing a piping system to make it "well connected" and thereby able to generate a piping pressure loss report. It seemed to be liked by quite a few people (98 views, 4 kudos), and hopefully it worked and was helpful for them. You can find it here:

 

http://forums.autodesk.com/t5/revit-mep/how-to-fix-your-piping-system/m-p/5297939

 

Well, as I progress through this large project at work, I am learning more and more so I thought I would write another How To for ductwork. So here it is!

 

HOW TO MAKE A "WELL CONNECTED" DUCT SYSTEM (on the Supply side):

 

  1. Make sure all connectors are logical in your duct system.
  • All air terminal connectors downstream of VAVs should be "Preset", "Supply", and "In"
  • VAV outlet should be "Calculated", "Supply", and "Out" (make sure the outlet connector is the primary connector!)
  • VAV inlet should be "Preset", "Supply", and "In"
  • AHU should be "Calculated", "Supply", and "Out"

Making sure all of these connectors are set appropriately will allow the airflow value assigned at the air terminal to propogate back to the AHU properly.

 

2.  Make sure all duct is complete and fully connected, this means there will be a non-zero flow value in all duct

 

If these two steps are completed, you will have a "well connected" system. Here's the kicker...Revit may not realize it and still give you problems. The "check duct system" tool may still give you errors. I think this is a fault in the Revit programming, flow information seems to get "stuck".

 

TO FIX THIS (ONCE THE ABOVE STEPS ARE COMPLETED ONLY!!): 

 

  1. Press F9 to pull up the system browser
  2. Select the system name only!!!
  3. Press delete (this deletes the system and not all equipment on the system)
  4. This next step is extremely important, and saves me loads of frustration and time. Here is why...At this point all equipment (VAV, air terminals, and AHU) should all be hooked up, but with no system defined for ANY of it, including ductwork!!! Let me reiterate, all duct and equipment for this particular system should read "undefined" for the system classification.

     Ok so you made it this far, now you have two choices. You can either choose an air terminal or VAV to create your new system. If you choose the air terminal, I have noticed Revit will sit there and "think" in the background for a suuuppppppeeerrr looooonnnnggg time (approximately 8 min) before it will add the single air terminal you selected.

 

OR

 

You can select the VAV, create a new system, and it will still take a looooonnnnggg time (approximately 8 min) to add the VAV to the system, but it will also add every other VAV at the same time. I prefer this method because it will literally save you hours. Once every VAV has been added to the system, now start adding the air terminals they are connected to and you will see that it goes quickly. Total time to add every VAV and air terminal and AHU to the system should be approximately 10 min (adding the very first VAV is what takes the most time). 

 

Now I know you are thinking this sounds crazy, and youre right...it is. Nonetheless, I have found it necessary to do it this way if I want to be able to generate a pressure loss report for my Multi VAV systems. For some reason these steps seem to flush the system and cause the proper propigation of flow data throughout the system, and this will fix the errors and create a "well connected" ductsystem.

 

There you have it! This is the process I use, and it helps me get all my systems set up for the pressure loss reports. Now go forth and complete your systems!

 

Post if you have any questions or suggestions! I will be checking to see if anyone has any questions.

Sincerely,

Wils02

8 REPLIES 8
Message 2 of 9
wils02
in reply to: wils02

I forgot to mention once you select the VAV to start the new system, be sure to select the outlet connector to "create the duct system" from.

Message 3 of 9
Thomas
in reply to: wils02

Very nice post.  I've worked with mostly ductwork myself for the past 2 years or so and you are right about the connectors.  The systems I model are mainly Air Handlers connected directly to terminals.  The Air Handlers are set to In/Out (SA/RA), and calculated, terminals to Preset (and reverse In/Out).  

 

I have also notice quite a few odd things with the systems.  I once had a system not working, but then I deleted an elbow down to a terminal and it worked!  Explain that one to me. lol  I've used similar methos to your deleting system to fix systems quite a few times.  To save the loading time you could try to disconnect 1 terminal from the system, then create the system with that terminal, then reconnect it.  It's a bit more work but I haven't had the issues with loading like you mentioned.

 

I would really like it if they added a button or feature that you could force Revit to re-examine the system.  Certain things seem to make it stop working, while others are ignored and don't update it.

Message 4 of 9
wils02
in reply to: Thomas

Tell me about it! I haven't looked into Revit 2015 yet, but hopefully they have added/will add a feature to do a better job debugging. It would help tremendously and alleviate a lot of stress. It sucks when you get everything connected and Revit will still not recognize it as a "well connected" system. Frustrating beyond belief.

 

For my particular project, the building is about 6 stories with 17 AHU and hundreds of VAVs and my method seems to be working. After thinking about it, it probably takes me soo long because of the size of my systems, but as a programmer myself I can't see why it would take so long to simply add another piece of equipment to a system. I have a feeling there are some behind-the-scene calculations going on that are slowing the process down. It would also be great if you could completely turn these off. I am aware of turning calculations off for the system, but it doesn't seem to help. Thats why I think something else is going on, but I don't know this for sure.

 

Anyway, glad you liked it! Good luck with your stuff.

 

wils02

Message 5 of 9
wils02
in reply to: wils02

To update my post about fixing the duct system, I have a tip that will eliminate the time it takes to add a piece of equipment to the system.

 

Just to re outline the steps below:

 

  1. Make sure everything is connected physically and there is a non-zero flow value all the way back to the AHU.
  2. Delete the system(s) that have formed from connecting everything. These are sometimes automatically generated system from Revit after you connect ductwork together.

This next step is where the time savings come in; yesterday I posted that it would take about 8 min to add a single VAV to the system. This is true, but it is mostly due to the size of my ductwork and building. However, if you find yourself in the same situation here is the workaround.

 

  1. Chose a single VAV to build the system from and disconnect the duct at the inlet (just pull it back an inch or so).
  2. Now, right click the VAV to select the inlet connector and select "Create duct system". This will create the system based on the inlet, seems odd, but this is the best way to do it.
  3. Name the system as you wish, and it should take about 2 seconds to generate the system with this single VAV on it. 
  4. Simply connect the duct back to the inlet and the system will automatically add the remaining VAVs and AHU (if it is all actually connected together by ductwork).
  5. Now your system should consist of all VAVs and the AHU.
  6. Go through and click a SINGLE terminal downstream of every VAV and it will add the air terminals as well.

Following these steps and assuming you have followed the other previous steps before, you will have a "well connected" system in the eyes of Revit. You are now able to generate your pressure loss report! 

 

Have fun

 

 

Message 6 of 9
smyyousef
in reply to: wils02

Outstanding, but the problem here not in the settings, but inherently in AHU never had a System classifications, all the times it gives: Undefined.Power which i don't know what does it mean, but VAV box always has a system classification. who did this?

Message 7 of 9
tanveer.aziz
in reply to: wils02

Thanks a lot mate, it was life saving. Hats off!
Message 8 of 9
eamaya3W2WQ
in reply to: wils02

Hello,

 

This is awesome! Thanks for taking the time to make this! 

 

After following your procedure. I continue to have this issue pop up:

eamaya3W2WQ_1-1676039522330.png

eamaya3W2WQ_0-1676039053686.png

 

 

 

Message 9 of 9
HVAC-Novice
in reply to: wils02

I have one correction or hint: use "global" as system type whenever possible. A VAV terminal device also can be used to control exhaust flow. Terminals like registers or diffusers should be the only items deciding what type of system it is. Everything else (like fans, or duct accessories) should just be agnostic to that. 

Revit version: R2024.2

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