rectangular offset fitting with different size ducts

[HVAC-Novice]

I have an AHU intake and a louver. They are offset in height and width and also have different connector sizes. An Ogee or eccentric transition or similar should be able to connect them. Problem is, auto-route can't do it. and if I just add a transition on one side, it doesn't let me adjust the transition (manually change the second connector size). All the transition properties let me do is adjust the offset, which doesn't help dealing with the different size. I also tried the oob offset fittings, but they also don't let me adjust the second connector size.

My workaround was to draw the duct (with ogee transition) from the AHU and enter the louver size. That gives me the transition to go from AHU size to louver size. Then I can manually adjust offset for height and width to move that connector to the louver location. I ended up aligning the louver to the transition to make the final connection to the louver. 

Is there a way to make that auto-route or more automatically? I think the problem here is that the distance is too small and the angles too sharp to auto-route. But obviously the fitting can work if I do it manually. 

I think I tried all oob transitions and even used offsets (made them into transitions). Nothing worked. 

This should be easy.. I made it work, but wonder if there is a better way to do that more automatically?

[iainsavage]

My take on automatic routing is that it tries (maybe not perfectly) to mimic real world installations and generally fails if you just don't have enough space to incorporate the fittings that you want to incorporate.

You can often fool it by moving parts further away, then automatic routing will work to fill int the larger gap and then you can move them back closer together, however if you ever have to upgrade your model or rebuild your system then the connections will break again.

For one-off unusual fittings like you've shown I would usually just model them in place.

[HVAC-Novice]

Thanks. I have seen such fitting in the field. Contractors that have their own sheetmetal shop actually like them since they can move most of the labor and complexity into the shop and use less labor time in the field. That fitting is complex to make, but it is less complex to make in a shop than to make a plenum and multiple taps and other connections in the field. Actually for a good metal shop it isn't that complex. They enter the sizes into their software and then they put the sheet-metal on their laser or plasma cutting table and it spits out the 4 sides to be assembled. It is complex if you make all the calcs and cutting manually, but no one does that anymore. So it isn't like they can't be made and Revit shouldn't exclude routes just because Autodesk thinks they can't be done or are not economical. And given the " ductsize"  is very large for the airflow, I'd be open for the contractor to make it in a less smooth way. I actually first had a plenum attached to the louver in the wall, and then the AHU tapping into that. But that seems to use much more metal and labor. 

I didn't try the idea to move the AHU far away and then let it auto-route. That sure sounds like a good idea next time. Fortunately a duct system doesn't have many of these odd transitions. 

[RobDraw]

Definitely disagree. These are custom fittings. Revit's content doesn't cover every possibility. Automatic routing is not a reliable solution. 

[iainsavage]

Yes such fittings do exist but they are not exactly standard off-the-shelf items and would probably be site measured and manufactured after the louvre and AHU have been positioned.

I think you’re expecting too much of Revit to automatically create such a fitting using the elbows and transitions included in the routing options.

I think there’s a way to make such fittings using Dynamo to extrude a variable cross-section along a path - I think I saw it in a Diroots video but I’ve never tried it.

[HVAC-Novice]

Most if not all the large fittings are made-to-order. Even if it wasn't such an extreme off-set, none of the ones are "off the shelf"  already in a warehouse. There is just too much variety to make and store them them and they are too large to transport economically over long distances. So they get made locally, often by the HVAC contractor if they are large enough to have their own shop. 

I'm not the one programming Revit, so I can say it is easy. But the Ogee fitting (or others) can be made to fit. So they should be able to use them in " auto-route". But it really doesn't matter what I want or what I think about this. It seems what I want is not possible (yet) and I have a workaround. 

Thanks for all the responses. At least i know the limits and don't miss some neat feature. 

[RobDraw]

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@HVAC-Novice wrote:

So it isn't like they can't be made and Revit shouldn't exclude routes just because Autodesk thinks they can't be done or are not economical.

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What makes you think this? They can be done and the tools to make them are there.

Not economical? Please explain that.

[iainsavage]

I know how ductwork is made.

My point is that even large standard components are made to a known pattern (elbow with certain centre-line radius, transition with certain taper angle etc) and are generally quite simplistic mathematically speaking. Yours is more complex and in terms of Revit's automatic routing solutions what component would it be? Elbow? Transition? 

[RobDraw]

I would call it a double curved transition with an offset and I doubt it would ever get built like that. The curves look like they might actually have a smaller cross-section than the smaller end. I might be wrong but I think a rectangular transition is more appropriate.

There are standard sizes for many fittings and around here SMACNA is the standard that is used as a reference by many.