We see ducting all the time without being aware of it as it just becomes part of the background much like telephone poles, fireplugs and storm drains. Our eyes will usually just pass over such common objects without triggering anything. Homes often will have an external AC unit with ducting running to the house.

Industrial Examples

We have glimpses of HVAC ducting on top of one story buildings and if we are higher such as driving on an overpass we can see HVAC ducting all over roofs.

Notice that some of the sheet metal have X-shape ribs to strengthen the part while others lack it.

In industrial areas we can often see convoluted weaving of ducting on the sides of industrial structures. The photo to the left is of a structure at the Invista plant in Lugoff, SC.

A quick guess is that the larger pipe is ~30″ diameter while the smaller perhaps 12″


It came to me one day that I could use Evergreen tubing to replicate the duct and then create 3D printed fittings for all of the elbows, tees, connectors, adapters etc. etc.I sent the files off to Walt Gillespie of Rusty Stumps Scale Models and Walt sent back the first of a test run. I used this relatively small assortment to create this test run.

It was successful enough that a ‘kit’ was sold. The kit is now being sold by Rail Scale Models – as “Roof Top HVAC System – O Scale“. There was also a HO version. The plan is to supply RSM with parts for a future release.

This is a look at “Wise Engineering Works” – an O scale scratchbuilt structure I originally built as “show and tell” for a NMRA class.

The pipe is Evergreen tubing and the elbows are 3D prints. The HVAC unit on the roof was printed on a filament 3D printer.

I love the clutter.

Exactly what is needed ranges greatly but currently here are the general categories (this may change depending on beer). Click the link to Converters, Joiners, Center Joiners, Elbows, Tees or Exhaust Ports to go to a Product Page for each of these items. Note that for the time being, this is “Old Stock” from Rusty Stumps.

  • Converters – These adapt different sizes and type of Evergreen tubing. There are square, rectangular and round tubing in sizes that gives us a total of 20 different tubes.
  • Joiners -These ‘join’ a tube to a flat surface such as a HVAC unit.
  • Center Joiners – These ‘join’ two lengths of the same size tubing
  • Elbows – Round and Square elbows can be used either horizontally or vertically. The rectangular elbows will require a vertical and horizontal version for each size.
  • Tees – Like the elbows the Round and Square tees work in any direction while Rectangular tees may require additional versions for when the duct turns 90°.
  • Exhaust Ports – Exhausts are used everywhere it seems at some point. Heck. You use one connected to a bathroom exhaust fan after all.

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