Cargo Trailer Conversion – AC Install

AC installation is generally the most complex part of the conversion process, only next to the electrical system. Hopefully, this article will provide some guidance on the subject and in particular the use of a windows AC unit installed INSIDE the conversion.


Here is a quick writeup as to how AC units works:

“Air conditioners use refrigeration to chill indoor air, taking advantage of a remarkable physical law: When a liquid converts to a gas (in a process called phase conversion), it absorbs heat. Air conditioners exploit this feature of phase conversion by forcing special chemical compounds to evaporate and condense over and over again in a closed system of coils.

The compounds involved are refrigerants that have properties enabling them to change at relatively low temperatures. Air conditioners also contain fans that move warm interior air over these cold, refrigerant-filled coils. In fact, central air conditioners have a whole system of ducts designed to funnel air to and from these serpentine, air-chilling coils.

When hot air flows over the cold, low-pressure evaporator coils, the refrigerant inside absorbs heat as it changes from a liquid to a gaseous state. To keep cooling efficiently, the air conditioner has to convert the refrigerant gas back to a liquid again. To do that, a compressor puts the gas under high pressure, a process that creates unwanted heat. All the extra heat created by compressing the gas is then evacuated to the outdoors with the help of a second set of coils called condenser coils, and a second fan. As the gas cools, it changes back to a liquid, and the process starts all over again. Think of it as an endless, elegant cycle: liquid refrigerant, phase conversion to a gas/ heat absorption, compression and phase transition back to a liquid again.

It’s easy to see that there are two distinct things going on in an air conditioner. Refrigerant is chilling the indoor air, and the resulting gas is being continually compressed and cooled for conversion back to a liquid again. On the next page, we’ll look at how the different parts of an air conditioner work to make all that possible.”

A window AC unit contains both the condenser coils with air drawn from the outside via the sides and exhausted out the back and evaporator coils which draw air from the inside and return it from the front. See the below illustration.

In order to use a window AC unit within a CTC (Cargo Trailer Conversion), you have two options for installation: 1.), install in an opening where the condenser coils are pulling air directly from the outside, or 2.) inside with ducts (or plenum) to provide outside air to the condenser coils.

I chose the second, allowing me to maintain a stealthy appearance to my CTC. Here is how I did it!


  • I purchased a GE 5,000 BTU window unit from Walmart online for the excellent price of $116! (
  • On the driver’s side of the CTC, I installed a 12″ vent for the condenser coil: (
  • I built a cabinet which will also be used to house the AC unit.
  • I set the unit in place and built a simple barrier (vent, plenum) between the back of the AC unit and the outside vent. You will see in the below image the opening, then it is sealed against the vent using polystyrene insulation and ducting tape. (Not the same as duct tape!)
  • I also added a funnel-like cup holder (a marine cup holder with a moisture drain connected to a 1/2″ rubber hose going down through the floor.) as a condensation catch below the condensate drain hole in the AC unit. (See the above images)
  • I drilled some vent holes in the shelf on the left side of the AC unit, which will be used for the fresh air intake from the floor.
  • Once again, using polystyrene insulation, I enclosed the AC unit so the coils pull in only air from the floor duct (in the next line.)
  • Next, I drilled vent holes in the floor just below the vent holes in the shelf, and build a plenum using polystyrene insulation (handy stuff!) from the underside of the shelf to the floor. I used a wire mesh to keep insects out of the plenum.
  • I then finished taping all of the ducts and plenums, sealing them all up so that there is no air flow from the outside in or vice versa.

The result, and highly efficient AC unit that was cost-effective initially, and in the long term! (Draws only 4 AMPS on high!) The AC install is complete, and I will now finish the cabinet face! (I will update with final photos once that is complete!)

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Born in California, raised in New Mexico. Current home base, Florida!

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3 years ago

Question, instead of building a full vent plenum, could you use pvc pipe to create plenum effect? Would this work?

Devin Lawrence
Devin Lawrence
3 years ago

Hey Brian, do you have any photos of the right side of your ac unit? Where did you drill the holes for the fresh air? Thanks!

Arturo Est
Arturo Est
2 years ago

Does it work efficiently? It looks like there may be issues getting air through that long narrow channel leading to the air intake.

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