By Dennis Collin
When working with Revit’s MEP tools the transition piece from one duct size to another is created automatically for both round, rectangular and oval duct.
Whilst fine in certain situations, when trying to avoid obstructions or gaps, an eccentric transition is required. For round duct and pipes the default transition is a concentric taper
But for the default rectangular duct systems, the concentric taper either errors with a routing problem or creates twisted geometry. This rule is defined within the routing preferences which can be accessed by selecting a duct. This can be done by editing the Properties and then choosing Edit Type. From the resulting Type Properties dialogue box, select routing preferences.
Editing the routing preferences displays all the connection settings and fittings.
The default fitting for transition is Concentric Taper, this can be replaced by a more flexible eccentric taper. This process can be achieved by using the load family button to import the Eccentric Taper.
Once loaded it can be assigned to replace the Concentric Taper as the default transition. The eccentric transition will work with concentric arrangements, but also it can be eccentrically aligned and provide a similar result to the image below. This arrangement is possible in both plan and sectional views.
A method to achieve this is to split the duct around the approximate transition points.
Then change the centre duct to a different size, this will create a duct with a concentric transition.
Delete the transitions and use the Align command to define the required alignment and then drag the smaller duct to the ends of the larger duct where the correct transition will be applied.
An alternate approach is to use Revit’s Duct justification alignment tool. On one of the open ends, select the duct and right click drawing a duct of a reduced size.
To rectify this, select each duct and choose Justify on the ribbon.
The ribbon changes to display the justify option; choose the required alignment reference e.g. Bottom centre
The ducts should now be justified to the required edge and an eccentric transition applied.