I bought a new portable air conditioner for my condo. I bought this one: a/c. I want to use it to cool either my living room or my bedroom. My apartment is 1100 sq. ft. so I believe this should be more than enough to cool down either portion of the condo. However my condo is not A/C friendly. It has only narrow casement windows and one roof to ceiling sliding window in the living room. I am also not allowed to install a window mounted A/C. However, it does have these 3 inch holes in the wall in many of the rooms (presumably for ventilation or A/C). I heard some of my neighbors use those holes to vent their A/Cs. I wonder whether this is safe or not (fire, mold). If it is, can I just hook the hose into a 6 to 3 reducer? What material is good for this purpose: PVC, metal, something else?

I know there are ways to install portable A/Cs in casement and tall sliding windows but both seem too complicated. I was hoping I can just use a reducer and be done, but I want to make sure it is safe. BTW the condo exterior walls are concrete.The holes clearly lead to the outside (I can see the street through them)

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    You should ask the condo management or owners of other units what these 3" diameter holes are intended for. How are they currently sealed on the inside and the outside? Is this a multi-story building? Jul 7, 2017 at 17:40
  • I have only received vague answers, but I will ask again. This is a multy-story building. I am on the 6th out of 10 stories
    – cloudraven
    Jul 10, 2017 at 3:29

4 Answers 4


The reduction of the diameter from the 6" of the hose to 3" through the wall may not cause a significant problem. The resistance to fluid flow of a pipe is [inversely] proportional to the square of the diameter and directly proportional to the length of the pipe. So if the length of the 3" diameter hole is short compared to the length of the 6" hose, then the constriction to 3" may work OK. You could ask the mfgr, but you may have to try it. I bet it will work fine.

The key to making it work would be to make a gradual transition from 6" diameter to 3" diameter over say 8 inches to 1 ft of length. This would prevent the creation of turbulence which could restrict flow. Presumably the 3" hole in the wall is lined with a pipe, right?

I wonder if those 3" diameter holes are positioned vertically so that a ductless mini-split a/c could be installed. How high above the floor are these holes? How far are they from the ceiling? Would the building management allow that?

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    Another intervention would be an inline booster fan to increase the flow rate of the hot exhaust. This would have to be automatically switched on when the condenser would come on. Does the portable a/c unit have a switched receptacle on it? Jul 7, 2017 at 18:03
  • Thanks Jim for all your helpful comments. The length of the 3" diameter hole is short (9 inches). I will try to contact the manufacturer. The 3" hole is lined with a pipe. that is correct. It is horizontally. There is one of those holes in each room. In the lower half of the wall, about 1 foot from the floor. About a year ago the concierge sent me a picture of that outlet suggesting I should connect the portable A/C to it, but I have no clue how now that I realize it is narrower than the hose. Since they suggested it, I would assume it is allowed.
    – cloudraven
    Jul 10, 2017 at 3:33
  • The position low on the wall would mean it is not really suitable for a mini-split ductless a/c. The holes for that should be higher on the wall--mid wall to 18" from the ceiling Jul 10, 2017 at 3:47
  • I can't see the other end of this fitting but it is described as a 6" to 3" transition: homedepot.com/p/… Jul 10, 2017 at 16:54
  • I agree with Jim give it a try. The compressor has tempature safety switches that will prevent a fire, there are also high pressure safetys that will shut down the unit if it gets two hot. It could shorten the life of the unit but that would be the worst I would expect. If it has problems with the 3" size an inline blower or a duct booster could be added many of these are tempature controlled and that would increase the flow through the hole but I would give it a try.
    – Ed Beal
    Nov 17, 2017 at 21:04

If your air conditioner came with 6 inch hoses it was designed to work with the flow rate available in that size. It is not advisable to be reducing the operational size of the hoses because it will compromise the operational functionality of the AC unit.

Since flow rate (at a given pressure) is related to the cross sectional area of the pipe be aware that a 3 inch pipe has only one quarter of the area that a 6 inch pipe does. If you were going to try using those existing 3" holes you would have use four holes in conjunction with an elaborate manifold to split from the 6 inch. (And do you even have 4 holes to use per 6 inch hose?)

Even though the venting through the window seems complex it is going to be way simpler than the above described manifold system!

  • Compromise the operational functionality means that it will break sooner? or that it will cause a fire or something awful. If I am just reducing the expected life of the a/c I guess that is acceptable. It only runs for about 3 or 4 weeks during the summer and that's it Only a few hours a day during those weeks.
    – cloudraven
    Jul 10, 2017 at 3:35
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    Buy or make up an adapter to go from 6" down to 3". You may have to stack ABS plastic drain adapters to get there, 6" to 5" or 4" , then that to 3". I think that you should try to see if this will work through one hole before you resort to rigging up something in the window. Jul 10, 2017 at 3:42
  • @cloudraven - I cannot say what will happen to your A/C unit if you constrict its ducting down to 3". Clearly the A/C unit was designed to work with 6" hoses and thus the reason I do not recommend that you do what you are suggesting. The A/C unit could overheat inside. Too much overheating could cause any one of a number of failures or critical situations to arise.
    – Michael Karas
    Jul 10, 2017 at 8:19

I actually tried reducing the exaust pipe on a portable AC unit. It comes out at 6" and we reduced it to 4". I knew it wasn't right at the time but did it anyways for some reason. Anyways the AC worked fine for a few weeks, but then it would click off and just pump out warm air. However, waiting 10 minutes and restarting it seems to reset the unit as it still works fine. Tomorrow I am installing an inline fan ~5ft after the reducer and hope that works. Otherwise I will be cutting a larger hole and replacing with 6" this weekend. At the end of the day, I'm pretty sure that having its exaust capaciry so restricted overheated the unit and it triggers the AC off. Now whether it does this in advance of and in order to avoid causing damage or as a result of some damage I'm causing, I'm not sure, but the air seems just as cold as before.


Was researching a similar issue. If you use an in duct exhaust fan that has a higher cfm than the portable ac, it will negate the constriction caused by downsizing the exhaust from 6" to 3". And therefore will cause no adverse effects , shorter life, overheating, etc. to your portable ac.

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