I previously wrote an article on the Proof-Build for the Rusty Stumps/Rail Scale Models HVAC Ducting Ver 2

This will contain some of the same info as in the link but will focus on the actual build .. how and why the parts go together like I arranged them.

Straight up people. I am NOT a HVAC engineer, installer or even someone with a vague understanding of the mechanisms. I am a modeler and put together some parts to make something that looks ‘nifty’ on a roof of a HO structure.

I put up a concept rendering for almost 10 days and invited people who DID know what all the bits and pieces were supposed to be used and got amost ZERO responses. I got more responses of course after I had glued and painted the bis and pieces which is par for the course .. for some reason people like to wait until you are finished with something to be critical. It is MUCH more useful if that criticism and/or suggestions were given prior to the job starting. I suspect this is closely related to a “Monday Night Quarterback”.

The Kit

Walt came up with a HVAC Ducting Ver 2 and sent me the bits and pieces which I used 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 (in the same sense of close as “Horseshoes and Hand grenades”).

We have an assortment of parts to play with. They are:

  • Extraction Unit (2 ea) : There are two of these. They appear to me to be Extraction Units, so I called them that and use them as such. These pull processed air from the building and either exhaust it directly into the open air of if it is cold weather can cycle it back to the system so the heat can be extracted. We can image internal baffles to direct the flow either to the outside or back into the system.
  • Chiller/Heat Pump (1 ea) : Air from the ACU can be cycled through this unit which can either chill or heat the air. It can also get waste heat from a Extraction Unit so that waste can be used to increase efficiency of the system Heat Pump).
  • ACU (1 ea) : Air Cooling/Cleaning/Conditioning Unit (Shrug) – contains pumps, fans, heating and cooling coils and filters .. lots of filters .. probably …
  • EA73 (4 ea) : This is a 90° elbow that fits Evergreen #254 1/4″ square tubing
  • Evergreen #254 : 1/4″ square tubing. There was about 1″ of tubing used for my layout.
  • EB11 : (1 ea) – This a 90° elbow that fits Evergreen #229 tubing
  • Evergreen #229 : .281″/7,1 mm tubing – There is 5/16″ inches used.
  • P21 (7 ea) – This is a 90° elbow that fits Evergreen #224 tubing.
  • Evergreen #224 tubing (.125″/3,2 mm OD : I used about 10-1/2″ inches for my setup.

RTU – Roof Top Units

Like I said .. I haven’t a clue .. much .. as how this stuff actually work so I sat through a YouTube video titled – Rooftop Units explained – RTU working principle HVAC – which was enough information to get myself into trouble (this is where I got the names Extraction Unit, Chiller, Heat Pump, ACU and so on) and a vague idea of how they fit together. If that were not enough I also watched How Air Handling Units work AHU working principle HVAC ventilation .. in any case I more or less pulled information from both videos, put that into a can which I shook vigorously and pulled out bits here and there which I then used .. most likely incorrectly but it looks good so that is what counts for a modeler. I did re-name the main unit from AHU (Air Handling Unit) to ACU (Air Cooling Unit) to try and ameliorate the “Apples vrs Oranges” thing I had going on ..


This seems to be a reasonable flow which dictates how the ducting is connected depending on the number of units.

  • The ACU (Air Cooling Unit) pulls in outside air and filters it. It can then be sent directly to the building.
  • The filtered air can also be sent to the Chiller/Heat Pump. Depending on what is required the air can be heated or cooled and returned to the ACU. From there it goes to the building.
  • The Extraction Units pull process/used air from the building and can either (1) exhaust directly or (2) warm/hot air can be sent back to the heat Pump to recycle that heat. In this scenario Extraction Unit #2 simply feeds Extraction Unit #1. Baffles determines airflow from Exaction Unit #1 to the Heat Pump or Exhaust directly into the surrounding air.

Following this plan, you can plumb your RTU to ensure your “Little People” are comfortable at work.

Next page – let’s look at the units ->

4 thoughts on “HVAC #2 Tutorial”

  1. Looks good to me. Plus gives me a partial list. I’ll need to know other fittings you use. One thing we need to remember is your setup will be one way as the modeler could do it different. I’d tell them that and see what they come up with on their roof top.

  2. If we assume that my theory on “Airflow” is correct then a modeler can run his/her duct work however he or she wishes and still make sense (assuming my logic is worth following) and you could plumb any number of units.

    1. Well, we tend to disagree on a few things. My take, based on the original photo of this equipment is.

      1. Air is drawn from the building buy the ACU unit and cooled and sent to the chiller. Thee are no return ducts to the building from the ACU unit.

      2. The chiller further cools the air from the ACU unit and sends it to the two extractor units.

      2. The extractor units deliver the air to the building work areas thus the reason for the two ducts going into the building from each one.

      1. So draw up a diagram showing the location and piping and I will draw it up and include it. My version is based on watching videos on HVAC systems and attempting to create some sense from the parts I had. I stated specifically that this was MY version and that any and everyone else could use the parts as THEY wanted and THEY thought right. I did not change any part but used what was there. This is why the kit is in pieces .. so the individual can use it and connect the parts as he/she/it feels works best.

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