Revit MEP

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Valued Contributor
Bill.Knittle
Posts: 63
Registered: ‎08-11-2006
Message 11 of 14 (565 Views)

Re: create duct system after ducts are drawn?

07-24-2013 07:25 AM in reply to: kschindel

One thing I would look at is the VAV's properties. There might be a ratio parameter in there doubling the aggregated flow from the downstream Air Terminals. The OTB parallel fan units have a ratio parameter which dictates the inlet flow based on the discharge flow.

Distinguished Contributor
kschindel
Posts: 148
Registered: ‎12-22-2008
Message 12 of 14 (555 Views)

Re: create duct system after ducts are drawn?

07-24-2013 02:53 PM in reply to: Bill.Knittle

I'm not sure I want to create a new duct system type. I'm fine just using the standard "supply" type. At least i don't think this is my problem here. What I want to do is create duct systems after i drew the ducts.

 

But I only can create a duct system (starting with VAV systems) when I select a diffuser that is not connected. And I only can add VAV boxes to the VAV duct system when the VAV boxes are disconnected on the discharge side. When they are still connected, I can't select them as long as they are physically connected. the only workaround is to delete the duct connecting the diffusers and the discharge of VAV box and re-connect after the VAV system is created.

all the other options with tabbing etc. don't seem to work since they don't bring up the "create duct system" command.

 

I now see why they want you to create systems before drawing ducts.

 

My only workaround now seems to be to re-do everything (disconnect diffusers, VAV boxes and create systems). Except Revit now decided to crash every time I select an AHU...

 

Valued Contributor
Bill.Knittle
Posts: 63
Registered: ‎08-11-2006
Message 13 of 14 (546 Views)

Re: create duct system after ducts are drawn?

07-25-2013 06:30 AM in reply to: kschindel

Did you check the VAV family in the Family Editor to see if there is a Formula doubling the Flow value? I would start there. When I look at your VAV networks in your System Browser snap-shot, everything looks normal except that the the flows double.

 

The Air Terminals and their VAV are your secondary systems. The VAVs and the AHU are your primary system. I would select a VAV and edit the Duct System. Check to see if you can make the AHU the equipment.

 

The only other thing you can do is Delete the logical systems in the System Browser and rebuild them from the ground up. I wouldn't blow away the ductwork to solve a logical system issue.

 

I've attached an image from my Revit MEP families course I teach. Take a look at it. It's a great graphic to understand the logic.

Valued Contributor
Bill.Knittle
Posts: 63
Registered: ‎08-11-2006
Message 14 of 14 (536 Views)

Re: create duct system after ducts are drawn?

07-25-2013 08:04 AM in reply to: kschindel

To help understand what is happening, I should reiterate a few things. Connectors are what make the world go around in Revit MEP. On the Duct side of things, We use (of course) Duct Connectors. A Supply Diffuser has a Duct Connector on it and it is set to the System Classification "Supply Air." The VAV typically has two Duct Connectors (1 inlet & 1 discharge) both set to the System Classification "Supply Air." The System Classification tells the device or equipment which logical systems it can take part in. Therefore, when you draw duct away from the VAV it automatically creates a logical Supply Air system. When you duct to the diffuser, the diffuser is added to that VAV's logical Supply Air system by means of the physical connection. Inside of each Duct Connector is also a Flow Configuration. When this is set to Calculated, it tells Revit that this Connector will be the upstream source and that all downstream Flows connected to it will be totaled here. When I put an AHU in and draw duct away from its supply discharge, another logical system is created. When I duct from the VAV inlet and tap into the duct coming from the AHU, the VAV is added to the AHU's logical system. This is Revit's way of automatically generating the logic. It does a good job but, it's 50% complete in terms of truly showing the distribution's hierachy.

 

The Preset Flow from the Air Terminal is propagated to the discharge on the VAV. The discharge on the VAV propagates the aggregated Flow to its inlet. Finally, the inlet Flow is propagated to the discharge on the AHU to be aggregated. The logical systems look like Image 1.jpg.

 

The only thing that I need to do is tell each logical system, who is the true equipment from the devices. I do that by editing the Duct System. The issue in Image 1 is that the VAV takes part in both logical systems so I need to be careful here when assigning it as the Equipment. When I select the VAV and edit the Duct System, I get an option to choose which of the two logical systems I want to edit. See Image 2.jpg .

 

  • First, I want to edit Mechanical Supply Air 1 (the VAV and Air Terminal) and make the VAV the Equipment.
  • Finally, I want to pick the VAV again, edit the Duct System, select Mechanical Supply Air 2 (the AHU and VAV), and make the AHU the Equipment .

The System Browser will now display the true hierarchy of information. See Image 3.jpg. Notice that the ratio in the VAV is reducing the overall flow upstream by 0.2. This is because the VAV also has a Return Air connector on it for a Plenum Return. So, It does not require the full flow from the AHU its diffuser is demanding.

 

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