Build duct plenum - tap into the endcap or how to enclose a duct so that taps work

[HVAC-Novice]

In my example I have a roof gravity ventilator that draws air from a plenum below. I used a rectangular duct (from the roof hood duct connector built down). I will tap a few lateral ducts and that should work fine since they tap into the rectangular duct. but the larges duct would tap from below. Ideally I can tap that in. 

But if I close the bottom of the duct with an endcap, the taps don't seem to work on the end-cap. I don't want to do a transition since that really isn't a transition unless I can find or make one that will have a 90° transition. In real life thsi will be a bell mouth for round duct going into the plenum, but the bellmouth could be shown in a 2-D detail and doesn't need to be into he model. 

This should work with different size and type of ducts and also ducts that are not in the center. I don't think including that into the hood family would work since I want to run multiple ducts in (and the flows should add up). And the number and size of ducts going into the plenum will differ. 

Is there a way to make an endcap behave like "duct" regarding taps? Or can the duct be closed with something that is just "duct"?  

Revit is calculating the flow, so i need a solution that works like the plenum and doesn't just look like one. 

[iainsavage · Accepted solution]

Customise a transition to be “abrupt” i.e. with a step in size rather than tapered sides.

[HVAC-Novice · Accepted solution]

thanks. I just use one of the transitions that have a 30,45, and 60° type and changed the angle to 90°. that didn't work. I then just manually adjusted the length of the transition to 1/4" (needs to be over 0!) to hack it. 

I used that transition in the duct system family type. However, it will use one of the regular transitions initially. I then manually swapped that out to the abrupt one. 

If the round duct is not in the center of the plenum, you need to place the ducts before at the location and the automatic transition will taper/align appropriately. Once everything is in the correct location, swap in the abrupt transition. Flow calculation works.

Only real downside is adding multiple ducts from that and won't work unless you really make a very custom family. but I doubt entering a plenum from below is common. 

Family attached (Revit 2022)

[HVAC-Novice]

OK, i wasn't happy it didn't show the bellmouth entrance to the plenum and some smart-ass contractor will use that as an excuse to not use a bell-mouth. 

Here the rfa with bellmouth and a yes/no parameter to show or not show the bellmouth. 

[RobDraw]

I used to use equipment families for plenums. It was easy enough to whip one out for occasional needs and could be made parametric. No workarounds and quite customizable. 

[HVAC-Novice]

did you use a mechanical family,  or what did you use? Is that something you could share?

I could see it being a very complex family if you have many ducts going into one plenum and the flow calcs also have to work. but this is the first time i cam across this issue, so i haven't thought too much about it yet. 

[RobDraw]

I used the mechanical equipment family template. 

I need to ask a question before I get into a lengthy explanation. Can you build an AHU family?

[HVAC-Novice]

Yes, I build my own AHU (with functioning connectors). I know how to position the connectors (with a small extrusion) on an AHU, if that is what you mean.

I can imagine attaching many duct connectors, and then move them around as needed and add up the flow rates. i kind of have a good idea how to do it. I just imagine it getting complex with maybe 10 ducts going into a plenum and they could go in on any of the sides. 

Well, for this project I got by with just ducts. Using ducts has it limits, but it is relatively flexible. If I come across more plenums, I may look more into the creation of a mechanical family. 

[RobDraw]

10 ducts into a plenum is complex for sure. The nice thing about mechanical equipment is you can mix systems. 

[iainsavage]

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@HVAC-Novice wrote

Well, for this project I got by with just ducts. Using ducts has it limits, but it is relatively flexible.

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I’m sure you’ve realised this yourself but just check how Revit assigns the loss factors to the connections - it probably won’t be able to analyse it and select a relevant ASHRAE table so you’ll probably need to assign factors manually (I’ve even had to do this with bends in sequence which Revit never seems to get right and generally assigns the wrong table or none at all).

[HVAC-Novice]

Loss factors for fittings are a bit iffy in REVIT anyway. For fittings there doesn't seem to be one in the family editor. and in the project often they have to be manually assigned, or are not even available. 

Sure a topic for a separate thread. But the plenum is relatively large compared to the flows, so I hope losses are relatively minor. And it being bell-mouth also should help. Where I have rectangular ducts into the plenum, the ducts are very large since they connect directly from AHU or louver and those devices are relatively large compared to the airflow since they have internal restrictions. So with low velocities, losses should be manageable. 

[drew.hasenkampDPSQE]

Do you have this made for rect-to-rect?

[HVAC-Novice]

I changed the overall duct layout in that project, so I didn't need it anymore. So I can't really tell 100% what I actually used. I think I used the attached transition and used the 180°(it actually uses 179° since 180° doesn't work, but looks the same). I don't recall if you first use a smaller angle (i.e. 60°), then align and then switch to the 180°  or if you can do it right away with the 180°.