Forgive me for my poor paint skill:

air conditioner setup

I have this small room, one door, no windows, one skylight. I'm planning to buy a portable air conditioner and put the hot air pipe near the lower area of the skylight, pressed against the mosquito net. Note that air will still be able to pass through all the net, not only the part near the pipe.

Room is about 3x3m width, 2.5m height. Skylight is about 1x1m width, 2m height.

The plan is that hot air should be able to get outside because it leaves the pipe with a certain speed and also hot air should go up because that's what hot air does.

During the day, when the air conditioner will be on, all windows in the rest of the house will be closed, so air should have no reasons to enter from the skylight. But even if they were open, 90% of the time air flows from windows to the skylight.

My doubts are:

  1. Will hot air really go out?
  2. In the "all windows are closed" case, if air goes out, it must also get in somewhere... the rest of the house is like six times the size of the room and in the kitchen (the furthest room) there are two small holes in the wall (10cm radius) for security reason (in case of gas leaks).

EDIT: The air conditioner that I'm looking at doesn't have a second input pipe, so I guess it will pull air directly from the room, and the room will pull it from the skylight. Now... It might create an air flow like the one in the updated picture where most of the hot air still get's out, or just suck back the hot air inside... What's the most probable outcome? Even if I find one with the input pipe, it will be useless if I can't put them both outside AND far from each other right? If so, I guess I'm out of luck...

air conditioner setup v2

  • 4
    Poor paint skill?? Is there some other picture somewhere you're referencing because those are great
    – Brad
    Commented May 15, 2017 at 20:08

8 Answers 8


Except you are pumping hot air out of the room. You are trying to create a vacuum in the room, and nature abhors a vacuum. It will push an equivalent amount of air back into the room via every possible path, in proportion to their air resistance.

Most likely it is simply going to push air down your skylight, handing your hot air right back to you.

Most of those portable air conditioners that put a hose on the condenser output, also have options to also put a hose on the condenser input. You'll want to figure that out.

keep in mind you only drew the two air conditioner outputs. There are also two air inputs - one to the condenser, and one to the evaporator (which outputs cold air). On most units they are separate, or at least separable.

Since only the condenser output leaves the room, only the condenser input needs to be drawn from outside the room. The evaporator outputs into the room, so the evaporator input can also be inside the room.

  • I've found different products have different designs. My unit has two hoses, hot air out, outside air in and can be directed in different locations. For flagg19's application, the second hose would be better applied to a low window or opening in the wall, aiming for the coolest outside air, keeping the system nearly balanced.
    – fred_dot_u
    Commented May 14, 2017 at 14:58
  • Thanks for answering, I've updated the question, even though you and @fred_dot_u already said enough of what I should aim for
    – flagg19
    Commented May 14, 2017 at 15:22
  • "Since only the condenser output leaves the room, only the condenser input needs to be drawn from outside the room. The evaporator outputs into the room, so the evaporator input can also be inside the room." This was the key concept that I was missing, thanks.
    – flagg19
    Commented May 14, 2017 at 15:31

There are portable units sold that are already split. You put one piece outside and one inside, with hoses connecting. Start with that type.

split system


If possible, you should install a second vent pipe in the skylight above the insect screen, leading far enough outside the skylight to avoid mixing the exhaust: second vent pipe Do not puncture or pierce the screen. Just allow the lower end of the new pipe to rest on top of the screen. Press the top of the lower pipe against the screen from the bottom.

The hot air will have to pass through the screen from one pipe to the other. If you remove the AC and lower pipe, insects will still not be able to enter the room.

  • 1
    That's brilliant but sadly I can't have the pipe outside of the skylight (it has to be able to close), still the idea of installing the second part of the pipe without having to cut the screen is good. Anyway, I see you have great paint skills too ;)
    – flagg19
    Commented May 14, 2017 at 18:11
  • 4
    Thank you, that's very kind. I learned everything I know from the great @flagg19, the most famous artist in this question thread. Commented May 15, 2017 at 0:00

Based on my experience, I can only advise against the type of AC you have in mind.

An AC unit consists of two pieces: a compressor and a condenser. The compressor compresses the coolant (a gas), which causes it to heat up. That heat is then dissipated by means of a heat exchanger, cooling the hot gas down to ambient temperature. That compressed gas then goes into the condenser, where it is allowed to expand, which will cause it to cool down. The condenser has another heat exchanger in which warm air from the room moves past the coolant pipes. The air cools down and the coolant warms up. The coolant then goes back to the compressor, where the cycle repeats.

With a single-unit AC, you have both pieces inside the room. Of 2 liters of air the unit draws in, about 1 liter goes back into the room, cooled. The rest is used to cool the coolant in the compressor and eventually gets blown out of the room via the exhaust pipe. Since this would create a pressure difference, outside air flows back into the room—through the gap under the door or through the skylight in your case.

This means you’re constantly drawing (presumably warm) air from outside into the room. Also, the compressor and the exhaust pipe are extra sources of heat, which you wouldn’t want inside the room. All in all, this type of AC unit is not very efficient. This, together with the extra noise due to the compressor being in the room, has caused me to ditch my portable AC in favor of a split device, a step I’ve never regretted since.

I strongly recommend you get a split device (as JDługosz writes, they are also available as portable units). Place the compressor outside the skylight (assuming you can place it on the roof). You will need a hole to feed the tube through (probably best in the side wall of the skylight, a few cm above the roof to avoid moisture issues). If that is an issue (e.g. if you do not own the building and your landlord will not agree to such a setup), feeding the tubes through the open skylight would also work but the open skylight would give you a constant flow of warm air from outside.


I have used this type of air conditioner before. What you thinking of doing "should" work but you have to be realistic about your goals.

I have used a portable like this, when the house wide A/C was broken and it was 100 degrees outside. It got the room I was in down to around 85. That's not awesome, but it's better then the 110 or 120 the rest of the house was.

They can be effective, as long as your realistic about what you want.

As for your vent, that's your real problem. You need to essentially "close" the skylight, except for the vent. Air has to come in from somewhere, and if the skylight is open, it will just bring all the exhaust air back in. The best way to do this is to rig a cooler. The kind you get for under $2 at the gas station.


You can cut that cooler all kinds of up, then use it to block off the airflow. If you can, pair it with something like this dryer vent.

Esentially you would create a

|       |
   | |
   | |

And fix that to your skylight/screen so little or no air could come back in.

Now I want to be clear, these things are not awesome. They will work, but there are better solutions out there from a economical, and energy cost sense. Not to mention solutions that will just plain work better. But in a pinch, this will work.


I have 3 of these units, 2 for a beach house and 1 in a city apartment with lousy AC.

To all the people saying these units don't work-- not really true. They work, just not like central AC or a wall/window unit.

They will chill the area around the unit, and maybe up to a 10'x10'room.

So, if you aim it towards your bed, you can get a great nights sleep; or aim it towards your couch and you can game away or watch TV in AC comfort. But it won't cool a large room.

In terms of venting, for the city unit we could not use the window, so we cut a 4" hole in the base of the drywall. The vent fits the hole perfectly, so no air back currents. With the unit venting into the inter-wall space, it has worked fine for 2 years. Just be sure to periodically check the water reservoir and empty as needed.

These are the units we have: Honeywell 14,000-BTU 550-sq ft 115-Volt Portable Air Conditioner with Heater. Good luck!!


Most portable ac units are not efficient because most of the heat is not removed There is a new product out there called a portable split system. it is designed for small rooms and is completely portable


Your system will not work.

The condenser has to be outside or otherwise fully ventilated.

At a mininum you would need some kind of blower system to ventilate the condenser. Since that would involve creating or modifying a cowling for the AC unit, it is not something you could do easily.

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