Ducted, portable air conditioners suffer from terrible efficiency inherent to their design. Here is how they work and how to improve portable air conditioner efficiency.

Portable (Air-to-Air, Monoblock or Ducted) Units

If you, like me, cannot install a split or central system because you don’t actually own the house, and because you have insulated windows, cannot use window units, you are left with either suffering in the heat or using a portable air-to-air unit.

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Mobile AC with vent hose

They truly are the worst of the bunch. They work like any heat pump or air conditioner – ambient heat is removed from the room air and dumped into air that is then vented outside. However, this method comes with a vast array of drawbacks.

  • Since the unit’s heat exchangers are not outside, they need a hose to vent the excess heat. That hose needs to be installed in a manner that reduces the amount of hot outside air to be leaked inside. This in of itself can be a nightmareish undertaking.
  • Since the compressor and the fans are inside, they can be quite noisy. The air inside the vent hose can also create a lot of noise due to turbulences.
  • The worst drawback however is the design of 99% of the available, commercial units…

Why They Suck

The required vent hose truly is the largest drawback of a portable unit. Which is why most product pictures tend to downplay that little detail, as a majority of photos only shows the product itself. You may notice however, that they seemingly all only have one hose. But that is a big problem.

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Can you see the problem?

The air that is vented outside of the room needs to be replaced. It creates a negative pressure differential inside the room the unit is in, thus the vacuum draws in air from the path of least resistance. This will be in all likelyhood the place where the exhaust hose is installed – causing hot outside air to come back inside the room!

This vastly reduces the efficiency of the portable AC. But we’re not done yet – to make matters worse, the AC will inevitably use some of the room air it just expensively cooled down – and use it to dump heat into and vent outside, further reducing the efficiency.

The Energy Efficiency Rating (EER)

Many electric appliances in the EU need to be sold with an energy label, which is supposed to tell you how (in-)efficient an appliance is. With single- and double duct air conditioners, this metric is calculated using the Energy Efficiency Rating (EER). This rating however, is only theoretical. It is based on the coefficient of performance, or COP. It is a ratio of useful heating or cooling divided by work (energy) required. Higher COPs equate to higher efficiency, lower energy (power) consumption and thus lower operating costs. The equation is

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where Q is the useful heat supplied or removed by the considered system and W is the net work put into the considered system in one cycle.

This value completely disregards environmental variables, such as whether the system draws in hot air while it is running. That is why ducted air conditioners can receive an A energy efficiency rating – despite, realistically, being many times less efficient under real world conditions than other air conditioner types. The only reason why single and dual duct systems are even still sold in the EU is because the EU made an exception for them, as they are so inefficient that they’d have been banned otherwise.

How to make portable ACs suck less

The biggest problem is the negative pressure created by venting room air outside. The easiest and most obvious solution is the addition of a second hose – creating a separate loop for the hot side of the heat exchanging cycle. Outside air is sucked in, heat is dumped into it, and the air is then vented outside. Thus creating no pressure differential inside, and therefore not drawing additional hot air in.

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Single and dual hose portable ACs compared.

For reasons beyond my understanding however, there seem to be no dual hose ACs on sale anywhere in Germany. I can only conclude that this is done purely because the manufacturers don’t need to do it. The energy label rating, as stated previously, is entirely fictitious regardless of single or dual hose use. And consumers are likely already annoyed by having to install a single hose. The installation of a second hose therefore may be the straw that breaks the camel’s back.

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The different vents on a portable AC unit

That leaves us to take matters into our own hands. It is possible, and actually fairly simple, to convert a single hose portable AC to a dual hose portable AC. First off you need to identify the vent where your AC draws in ambient room air to cool down the hot side of your AC.

I have a fairly cheap whitelabel AC I bought out of dire need at a local grocery store. It does have a lot of bang for the buck, but also has the flawed single hose design.

  • RED: This is the hot side air intake. This is where we will install the air intake hose.
  • GREEN: This is the hot side air exhaust. This is where the regular exhaust hose is installed.
  • BLUE: This is the cold side air intake, which takes the room air, where it is then cooled down and vented out the other side.

The Quick and Dirty Way

What you need:

ComponentDescriptionWhere to buy
AC air hoseThe air that draws in the outside air. It should be identical to the exhaust vent hose to reduce the strain on the motor.Amazon.de
AC air hose flangeThis will allow us to attach the intake hose to the intake cover.Amazon.de
Duct TapeRegular duct tape. I said this is the quick and dirty way after all.Amazon.de
Window AC adapterAn adapter to fit the intake side into a smaller slot on the window. Not strictly necessary, but recommended.Amazon.de
AC Window Seal KitLikely only necessary for other Europeans out there, but a must-have for us for sure!Amazon.de
Carton or other building materialTo cover the intake vent. I’ve used carton boxes, but nearly anything will do.Be creative!
  • Cut the carton in a shape large enough to cover the intake vent. Before installing it, cut a hole about center of the cover and attach the hose adapter.
  • Proceed to attach the cover to the AC unit and seal it with duct tape.
  • Now attach the hose, once again seal it with duct tape.
  • Proceed to hang the intake vent outside the window. Make sure to keep a distance to the intake hose, to prevent an air short-circuit.
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The quick…

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… and dirty way.

It ain’t pretty, but it works! I don’t have the tools necessary to take accurate measurements, but not only has the AC now managed to cool the room down far more, the compressor now occasionally turns off as it has reached the target temperature, something it had not done before.

The Proper Way

Now, while the above may work, surely there is a more aesthetically pleasing way? Well, you’re still going to be stuck with two ugly hoses hanging out of a window, so it’s going to be a losing battle, but I’ve designed a replacement attachment for the intake vent:

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The 3D Printed adapter alternative

Because I currently lack access to my 3D printer, I was unable to print the adapter myself. The installation process is much the same.


Once again, where manufacturers fail, makers like us can make up the difference. With just a little bit of duct tape and carton, you can vastly improve the experience of your AC (and lower the electricity bill to boot!). I hope this helped – and if you know someone who also uses one of these abysmal AC units, tell them about this to make their misery slightly less agonising!

I have recently discovered the joy of using ceiling fans. I was skeptical at first, but there’s a reason why they enjoy an immense popularity around the world. Because of the draft they create, I’ve been comfortable in room temperatures of 30 °C, while consuming only 30 Watts of power, and being much quieter overall. This has worked so well this summer (2022), that I’ve decided to sell my portable AC unit.

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  1. Thank you so much for this older article I just found. I have a truck camper with a portable AC and the efficiency was horrible. Plan this week to make the modifications in prep of a very warm camping trip to Virginia.


  2. Sorry but the idea is sound in theory but it’s got to be wrong.

    If the air outside (which is why you’re having an AC on in the first place), is say 35oc and you want 20oc inside, you’re effectively trying to cook 35oc air.

    If you use the indoor standard version you’re cooling the 30oc air, 25oc air.. Etc etc, as the air in the room will be significantly cooler than the outside air.

    This is like trying to do this for electric heaters asking then to pull in air from outside to warm up instead of heating the air inside.

    1. No, you are incorrect.
      How do you think split AC systems work?
      The coolant is enough to combat the hotter air.
      You don’t “cook air”, you use the thermal energy to compress a gas to liquid form.
      Your comparison to heating air with electricity is incorrect.

      But the idea portrayed here is only half efficient, in the real world a lot of room air will be pulled from all sides and openings, including the recently cooled air, sending it out the exhaust. Dual hose ACs have insulated compartments, making sure only air from the intake hose is being pulled through the condenser.

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