HVAC Ducting

Way back in 2018 I worked with Walt Gillespie of Rusty Stumps to create HVAC fittings to fit Evergreen tubing to mate up with his HVAC units. The result he released as a kit, now available from Rail Scale Models The Roof-Top HVAC System

Walt has come up with a HVAC Ducting Ver 2 and has sent me the bits and pieces which I will use for a proof-build. With the box of goodies he sent me a photo that he used as a guide .. not exactly  what he made but close. That’s the rendering to the left. He also did a pen sketch on the back to show the layout he visualized using the parts sent to me.

Now comes the fun part .. opening up the box to see what detail goodies Walt sent.

Bits and Pieces

First look at what I have to ‘play’ with.

  • There are the HVAC units of various and assorted shapes and sizes in the top left. Walt had started to play around a bit looks like as I see some HVAC duct elbows and short lengths of square tubing
  • next to the right are EA73 90° elbows – to fit Evergreen No.254 tubing – 1/4″ sq. tubing with 0.194 ID
  • Top is a bag of assorted Evergreen tubing and elbows .. to be examined more closely later
  • At the top is a 6 inch long piece of Evergreen No.254 tubing
  • EB11 .. thee 90° elbows are to connect to Evergreen No.229 round tubing – 9/32″ OD / 0.281″ ID.
  • P21 – 90° pipe fittings for 1/8″ rod/Evergreen No.224 tubing.
  • A piece of 0.080″ thick 5″ x 8″ cardstock for the roof


Well. Here we co. There are two HVAC units .. two are duplicated. I have also included all of the HVAC parts that I identified earlier. There are EB11 and P21 elbows and the EA73 which .. well .. is also an elbow. We have Evergreen #254, #229 and #224 tubin.

The next step will be to close one eye and squint the other at the drawing Walt sent of how he suggested they be set up. I do have to wonder if Walt always draws with the pen held by his toes .. but that is another story.

Layout 1

I laid out the HVAC units per a sketch Walt sent me. It was at this point that I thought that … perhaps … I should see if they would fit on the roof sheet he had included. Seems not.

Layout 2

Second try. I rearranged everthing … basically sliding the two units with the downturned square ducting to the left.

A look from overhead.

I think that … even if this is just a section of the roof for display, the bottom unit should be moved a bit away from the edge of the roof section if for no other reason than ,pre balanced visually.

Layout 3

I sat through a video on Rooftop HVAC (RTU – Roof Top Units) and AHU (Air Cooled Units) and got something like a near vague understanding .. possibly .. almost .. on how to arrange ‘stuff’.
The two smaller units are Extraction Units (EU). These suck air out of the building through the square elbows. The air is vented out except during cold weather when hot air is sent to the Chiller/Heat Pump to use that wast heat. Air is pumped from the ACU (Air Cooled Unit) to the Chiller/Heat Pump and depending on the ambient temperature hot, cold or mixed air is returned from said Chiller/Heat Pump.

The reason for the U-Bend in the Processed air pipe is because the outlet/inlets on the CHiller/Heat Pump and ACU are not inline.

A view from the top. I moved everything in from the outer edge of the “roof” just for aesthetics .. and that is pretty much .. that.

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