I smoke in my home office room which is 20x30 feet. I have a wall mounted 12000btu aircon in the wall, and want to put a window fan, one of those 10 inch high and as wide as the window with twin fans to suck some of the smoke out.

I vaguely remember getting lectured as a kid that running aircon with an open window is like trying to aircon the world. But I just can't remember what exactly the drawback is besides the room not being as cold as it can possibly be? Power bill increase or Damage to aircon or something toxic oozing out of the aircon?

I need to explain this to a non native english speaker who never lived around aircons and Googling for this answer only produces results for ceiling fans, since I guess everybody knows not to do it, so need to write about the reasons...

Thank you in advance.


The main disadvantage is that you are sucking the cold air out of the room. If the aircon currently runs continuously the room will get warmer, but if it gets to your target temperature and the thermostat switches it off, the vent will make that happen less often so your power bill will go up.

How bad that is depends on how hot the replacement air is. The hotter it is the less effective the air conditioner will be, and the more air it sucks out the hotter the room will be. Also, you need to seal around the outside edges of the extractor fan otherwise it will just be sucking air in around the outside and blowing it out through the middle. That makes it less effective at clearing the air (but means you lose less cool).

Ideally you'd have a vent coming into the room from somewhere cool, like under the house, so that the air coming in is as cool as possible. But if you're in an apartment chances are that air is coming in under the front door or leaking in around the windows. There's nothing you can really do to make that air cooler.

Almost all the damage to the aircon will happen from the cigarette smoke. Adding the extractor fan will make the air inside cleaner so there will be less overall damage to the air conditioner. I suggest cleaning the filters in the aircon much more often than the manual suggests because of the smoking. The smoke will make those filters dirty much more quickly than expected. If not, you might get toxic cigarette sludge collecting in the aircon and eventually oozing out. That problem will be reduced if you install an extractor fan.


The more sealed off a room is, the more efficient a AC is going to work. An ideal room would be 100% isolated and the aircon would only have to cool it down once and then it would stay cold. (This is very simplified, obviously that would mean that you don't have any fresh air).

Now in reality there is always some heat leaking in and the AC has to work to compensate that. By introducing a fan into your window, or by opening your window you are making a lot more heat leak out and the AC has to work a lot more to compensate for that leaked air.

So from a energy point of view it is a very bad idea to leave the window open or to install window fans.

If you want to know how much the difference is, I would suggest (provided it is a plug-in AC) to buy a device that can determine how much power is used and compare between a day where the window is open and one day where the window is closed. (At days of similar outside temperature of course). I would guess that you could save quite a substantial amount of money over the year by just going out to smoke.

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