I recently bought a portable air conditioner and was wondering if it is absolutely necessary to vent it to the outside. I plan on using it in a room that doesn't have a window as was wondering if I could vent it to another room in the house without doing any damage (eg mold growing in the house). I understand the air coming out of the AC will be warm and would warm up the room it was venting into.

  • You simply cut a hole in your wall to vent. That's the absolute best way.
    – user28567
    Commented Dec 5, 2014 at 18:28
  • 2
    Air conditioners work by making one side cooler and one side hotter. Since both sides are in the same room, it won't actually work. Commented Aug 4, 2015 at 18:22
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    According to thermodynamics, the room will heat up by the amount of electrical energy dissipated as heat by the air conditioning unit. So while you will have cold air blowing on you, it will progressively get warmer as the rest of the room heats up. "There's No Such Thing As A Free Lunch - TNSTAFL" Commented Aug 4, 2015 at 23:47
  • I'm wondering if one could just run the output hose of the portable to the return vent of a central AC (which would be running) and let that do the work? I'm sure the central AC would work harder and cost more energy but could that help? Kris
    – user55348
    Commented Jun 17, 2016 at 20:15

8 Answers 8


It's possible but it will be creating a situation where your a/c is fighting against itself, because you'll simply be redistributing the heat inside your house, instead of putting it on the outside.

Why? Because the warm air will creep back into the room you were cooling, so the a/c will be acting like a big do-nothing machine - to a degree anyway.

Mold isn't going to be too concerning, however, because you won't be taking moisture out of the one room and putting it into the other - the moisture in the air in the warm room may increase a little (because warm air can hold more water vapor than cold air, so if moisture is there, it'll get picked up) but not to the degree it would if you were, say, venting a dryer into that room, where you'd be introducing lots of NEW moisture.

All that said - I do not recommend this idea. Better to create a vent/opening (properly, of course, but that's a different question) in the outside wall to allow the warm air to go where it is supposed to.

  • 4
    fighting against itself A good summary of the situation. You get cold air out one side and heat energy from the condensor representing what was removed from the other room plus heat from the energy required to compress the freon, plus heat from the compressor motor going out the other for a net gain of thermal energy within the house envelope. TINSTAAFL. Commented Aug 25, 2012 at 16:46
  • The single vent portable AC units are already fighting against themselves by blowing the hot air out and then sucking in cool air from the room to replace it, which sucks outside air in the building. So doing this would just make it fight against itself more but it would still work making the other room hotter. Commented Jun 20, 2021 at 4:38

Moisture is a problem. Air conditioners actively remove moisture from the cooled space and deliver it into the warmed space. One of the main features of air conditioning is dehumidification. The warmer air in the discharge room may already be heavily saturated with moisture, and the even hotter air coming from the discharge chute will add to that moisture. Prolonged use could lead to mold problems.

Further, interior walls of houses are rarely insulated, so the walls between the cooled room and now super-heated room will transmit the heat back to the cooled room. Also, air leaks will filter some warm air back. This will not totally overcome the cooling effect (we often air-condition one room and not another) but it will diminish the value and efficientcy, since the now heated room is much warmer than it normally would be.

Overall it is a bad idea. It is possible to create a small panel captive in a window that can hold the discharge chute of the air conditioner to vent outside.

  • 5
    The dehumidification by AC systems is from condensation on the cold side that goes out the condensation drain. Coolant lines are a closed system, no moisture is transfered from the cold to the hot side of the AC coils.
    – BMitch
    Commented Aug 25, 2012 at 15:17

Yes, it will work in terms of cooling the room, but it won't be as efficient as an external vent.

If you have the door to the cooled room closed and as air tight as possible, the adjacent room being warm is like the outside of the house being warm. Yes, there will be some unwanted heat transfer but there will still be a temperature differential. The best thing is to try it and see if it is cool enough for you and if the cost in power is worth it.

As for your question about moisture, the answer is "it depends". A typical full house AC has no moisture connection between the cool side and the warm side. All water taken out of the cooled air goes into a drip tray and then a drain.

The more modern window AC are designed so that the water from the drip tray can hit the condenser coils to help cool them off, improving their SEER. This is a moisture path to the exhaust.

Some portable AC have a drain hose and that means no added moisture in the exhaust.

DeLonghi portable air conditioners exhaust warm moisture-filled air out their exhaust hoses, so there is less condensate to empty manually. But this means the exhaust has the extra moisture removed from the cooled air.

However, if the cooled room does not have a constant source of additional moisture, this may not be a problem for the room receiving the exhaust. It would be best if that room had an open window or two.


How about keeping the portable air conditioner outside the window and vent in only the cool air in. Just the reverse of conventional portable ducting. This will keep all the noise and heat outside the window and only cool air is allowed to enter through duct.

  • The problem is the OP wants to cool a room without a window. They'd have to cut a hole in the wall for the duct with your solution.
    – Doresoom
    Commented Dec 3, 2014 at 15:12
  • Or put the AC unit in the other room and make a vent that blows the cold air in the room that they want to cool. Over all a 10,000 BTU unit will draw about 1000 Watts and act like a 1000W heater in the building. One room will be colder but on average everything will be hotter. Commented Jun 20, 2021 at 4:40

No, then it will not be operating as an AC. Why would you want to? No window? Trying to cool a large space beyond the ACs listed capacity? Or simply lacking a duct extension that will reach an external opening?

  • The OP explains that he wants to use it in a room without a window.
    – Niall C.
    Commented Oct 23, 2012 at 14:24
  • Point taken. The operative response to any reason for off-standard use of the floor unit was the "No" part (The OP's was but one of several "convenience" reasons cited for AC-ing AC-ed air). From that is readily apparent, that a vent will need to be found or fabricated, inside the room; or somewhere outside the room--even if purchase of supplemental ducting is required...but then the room is not closed off to itself. Thanks again.
    – Lex
    Commented Oct 25, 2012 at 4:29

I have the same situation, only a sliding glass door. Next to it is a floor vent where the heat comes out during the winter.

To fix the problem I removed the vent cap, and connected the hot air flow vent to the floor to blow into my duct work. The air actually cools off before it gets to any other room.

I set the AC on a boot pan, and ran a round drainage hose out the sliding glass door. The door remains open about an inch, and I put a strip of foam down the door. Then I cut a piece of broomstick to wedge the door closed so it will not open for security purposes.

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    AC's don't create cold, they move heat, and in the process the motor and other moving parts create some waste heat. By venting inside, you're heating your home to cool a room.
    – BMitch
    Commented Aug 4, 2015 at 19:19
  • The whole concept of a single vent portable AC unit is flawed to begin with. It sucks hot outside air in the building to replace the air blown out the vent pipe. This is the same flaw but it's even worse now! But if it is venting in to a cool basement where the concrete floor can cool the air it might actually work out as well as venting it outside. So +1. It may be stupid but it can work. Commented Jun 20, 2021 at 4:42

Put the vent hose in a bucket filled with water, the air heats the water not the room. The water will evaporate creating humidity for the air conditioner. Just add water as needed. That's what I do for my room. Although I don't vent mine out the window for security reasons.

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    -1 since AC's don't create cold, they just move heat, venting it inside just adds a heater to the room from the friction in the pumps and fans. This is quite the opposite of what you want to do. At best, you may have created a very expensive swamp cooler, but that only works in dry climates where the extra humidity is welcome in the summer.
    – BMitch
    Commented Feb 19, 2016 at 1:42

Vent it into the attic space making sure to cover the output hose with window screening.

  • This is a horrible idea. Commented Jun 7, 2016 at 18:15
  • If the roof is vented as it should be and the ceiling is sealed so very hot attic air won't get sucked back down, then it isn't any worse than venting it outdoors. Commented Jun 20, 2021 at 4:45

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