The situation: We live in a one bedroom + den (rental). We're using the den as a second bedroom, as we're a family of four and that's what we can afford right now. The den has no windows, however, and so gets very warm.

The project: We have a small 8,000 btu air conditioner that I would like to vent out the window in the living room. We're in a long-term rental situation, and our landlords are fine with us doing minor projects. What I'm thinking is cutting a hole in the ceiling of the den, and using flexible vinyl ducting run through the ceiling cavity (which I have access to). The cavity is just slightly too small to crawl through, so I'm wondering a couple things:

  1. Is there a problem venting an air conditioner through roughly 30' of ducting?

  2. Would would be the easiest way of fishing the ducting through the cavity?

  3. Are there any major problems that I'm not considering, or any simpler solutions? It's not possible to vent to the roof, and I'm wary about venting into the ceiling cavity. It's a straight shot from the den to the window.

A couple pictures: the tubing I'm hoping to use

the ceiling cavity - it's a straight shot to the window from the bedroom

Thanks for any help you can offer!

  • Does your A/C have duct connections? It's not typical. If not, how would you get the air into the vent? Are you planning to build some kind of enclosure around the back side of the A/C to collect it? Where will it get air - are you going to install another duct to bring cool air to the inlet? If not, it's going to blow cool room air through the coil and out the vent.
    – Mark
    Commented Oct 1, 2016 at 13:29

1 Answer 1


I suspect the owner's manual for your AC would spell out the duct length capacity but I strongly suspect that 30' is much too long, especially with that vinyl duct which is, quite frankly, garbage. Corrugated duct in general is not a great idea for long runs because it creates so much more air resistance but if you have to use it, look for the metal type which looks like a Slinky wrapped in tin foil. Not to mention it will last much longer. Better still, if you can figure a way to route it, is rigid sheet metal duct which you can get at a big box store and perhaps fasten smaller sections together as you push it ahead in the overhead space, sealing the seams with foil duct tape. That would give you the best chance at a satisfactory outcome but again, I think you will find 30' is too far for a small unit regardless...

BTW, do not under any circumstances vent into the ceiling space. Utter disaster in the making ;)

  • Thanks for the comment! What would be the side effect of a run that's 'too long'? I suspect you're correct, but I'm wondering what would happen.
    – user57169
    Commented Aug 1, 2016 at 4:42
  • @user57169 The AC will essentially stop working as it's unable to efficiently vent the heat it produces. The coil will also ice up, which could potentially damage it.
    – PaulBinCT2
    Commented Aug 1, 2016 at 11:49

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