What are the potential problems, if any, of venting a portable A/C unit into a closet or attic?

August 13, 2010 12:07 PM

What are the potential problems, if any, of venting a portable A/C unit into a closet or attic?

I've got a portable A/C unit, but no windows in the room I'm trying to vent. But there's an empty closet, and access to the attic. Could I just vent it into there?

I've let it run for a while, and it seems no moisture is coming out of the vent -- just hot air.

Forgive me if this is a stupid question.

This is the the model I have.

posted by wordsmith to home & garden (14 answers total)

The moisture won't be obvious; it will just be very humid air. Over time, it will cause an enclosed space to mildew. Furthermore, if the heat isn't going outside your house, it's just going to heat up the whole house... the immediate room where the AC is located may stay cool, but everywhere else is going to get hotter.
posted by sonic meat machine at 12:09 PM on August 13, 2010 [1 favorite]

Seconding sonic meat. If I had no choice I'd ventilate into the attic, but it makes more sense to install the AC properly in another room and pipe the cool air through a duct or wide hose using blower fans on each end, as sort of a poor man's central air.
posted by crapmatic at 12:15 PM on August 13, 2010 [1 favorite]

You need to vent A/C outside the house. That's what an air conditioner does. Or is supposed to do. Freestanding "air conditioners" aren't really air conditioners in any real sense. They just blow hot air from one room to another.
posted by dfriedman at 12:18 PM on August 13, 2010 [1 favorite]

Venting humid air into an attic seems like a Very Bad Idea
posted by KokuRyu at 12:18 PM on August 13, 2010 [2 favorites]

The second law of thermodynamics means that you will make your house hotter this way in addition to causing moisture problems wherever you vent it.
posted by TedW at 12:23 PM on August 13, 2010 [2 favorites]

Venting humid air into an attic seems like a Very Bad Idea

Totally agree... I should clarify by saying it CAN be done to get the room cool, but you risk initiating a mildew and mold problem in the attic.
posted by crapmatic at 12:24 PM on August 13, 2010

I've let it run for a while, and it seems no moisture is coming out of the vent -- just hot air.

It seems like that unit has a dehumidifier function with a drip tray. Is it collecting water?
posted by mr_roboto at 12:32 PM on August 13, 2010

It helps if you think of an air conditioner as a device that can only move heat from one place to another, because that's what it is. It cannot create coldness or destroy heat, just shuffle it from inside your house to outside (ideally.) The same is true of your refrigerator: it moves heat from inside the box to the surrounding room, which is why the coils at the back are warm.
posted by Rhomboid at 12:40 PM on August 13, 2010

That unit has an Auto Evaporation Function which is marketing speak for dumping the moisture that condenses on the evaporator into the vented air stream. Hooking this up to your typical attic would be bad. Exhausting it into a closet would be bad and wouldn't give you any net cooling (in fact you'd heat the system up from electricity consumption).

If you aren't renting (or you have an understanding landlord) you could run rigid metal vent pipe into the attic and then up and trough the roof capping it with a vent cap. You want to use rigid metal vent to reduce friction. Essentially you would be installing a bathroom fan without the bathroom fan. If your room has an outside wall but no window you could do the same thing with a wall mount exhaust kit.
posted by Mitheral at 12:48 PM on August 13, 2010

I can attest to venting a portable AC anywhere in the house other than outside will just make the house hotter from experience. One of my roommates in our apartment has a loft bedroom without any outside windows and the landlord decided to "help" out the situation by installing a portable AC...then cutting a hole in the loft bedroom's wall to vent the hot air out into the living room.

We're already on the top floor of the building, which makes it hot when the sun is out and blazing. However, the hot air pouring into the living room makes it worse. Even on the hottest days, on some merciful evenings roommate #2 and I have bedrooms with windows so we can open our doors for a bit of cross breeze and get some relief without running our ACs in our rooms. Unfortunately roommate #1 does not have that option, so even when it might be a breezy evening out after a scorcher of a day, he has turn his AC on to alleviate the stuffy heat trapped in the room, which in turn means roommate #1 has to turn on his AC, which starts a chain reaction of roommate #2 and I having to close our doors to keep the heat from coming in our room, which then means we have to turn on our ACs. (Not that we're annoyed by this, because seriously, dude got no windows. He needs *something*, I just find it weird the landlord thought this was a good idea).

The temperature difference is pretty noticeable. Whenever I leave my room to grab water or go to the bathroom, it is hot. Not just, oh, it's a bit warmer than usual. It's a stuffy hot. Like imagine how it would feel in your house if you were running a heater in summer. Yea, it's exactly like how you'd imagine because that's basically what is happening.

I can't speak for what happens with attics since I don't have much experience with houses, but in a closet? Definitely don't do that.
posted by kkokkodalk at 1:08 PM on August 13, 2010

The reason you don't vent into the attic is because, when the attic is colder than the vent air, moisture will condense inside the attic space causing mold and other problems; this can be a serious problem in cold weather if the living space is not properly sealed off from the attic.

In practice, if you are venting an air conditioner during hot weather, the roof space is likely to be far too hot for condensation, and if the space is reasonably well vented with soffit and ridge vents there is a good path for exit of any excess humidity. It's probably still against code though, and you will have to get through the moisture barrier and insulation in the roof and then put everything back together and sealed up before the weather turns colder... really not that good an idea.

Though if you have full access to the attic, you could probably install a duct to vent outside -- just like a bathroom or dryer vent. It will need a suitable weather proof vent to the exterior which might mean a trip up a ladder. Done right it would get you the exact result you want without too much expense or complexity (flexible ducts and external vents are cheap and readily available from Home Depot etc.). You will need to seal it up before the weather turns cold to stop heat escaping/cold air flowing back down the pipe during the winter.
posted by Quinbus Flestrin at 2:03 PM on August 13, 2010

set it up in a different room and use portable fans to help circulate air. If you vent anywhere but to the outside you are just asking for issues
posted by zombieApoc at 4:52 PM on August 13, 2010

I recently had an HVAC guy give me an opinion on this very question. (I had him over for some other work and asked him about installing a permanent vent for my portable AC, because I can't get the window-vent things to seal properly without leaking air.) His solution was going to be a run of 4" duct into the attic via the bedroom closet.

The big question is apparently how the attic itself is vented. His questions to me were: "do you have gable vents?" and "is the attic well-insulated?" Answer to both in my case were 'yes.' I think his plan was just to run the vent up to somewhere near the gable vent and terminate it there without actually taking it outside the house. He said he'd give me some sort of plug to go into the bottom end of it during the winter to keep warm air from flowing out.

He wanted about $300 to do it though, which I thought was a bit steep, so I haven't had it done. But he wasn't a fly-by-night guy and seemed more than happy to do it. No idea whether it would violate code or anything, but if I'd been willing to put out the three bills he was standing by with the hole saw.

He also said that he could do it like a traditional dryer vent and take it out through the wall of the house, put a 'squirrel baffle' on the outside, etc., but this of course leaves the vent there forever and wouldn't have been hidden in the closet. The attic vent is easier to remove and patch over since it's really only busting out a few inches of drywall.

The dryer-vent method is probably the "right" way to do it, since it gets the humidity totally out of the house and out of the attic ... I don't think that condensation in the attic is really a problem (in my area, the attic is so hot it's almost certainly warmer near the peak than the exhaust air from the AC) but in some areas or if you were using the AC to cool a particularly hot room, like a server closet, on a cool day, it very well might be a problem.

BTW, if you are open to purchasing a new unit, you might want to consider a "split system" rather than a portable AC with air exhaust. These aren't portable from room to room, but they're much more efficient — on par with a traditional whole-house AC — and require only a small hole bored in an exterior wall. They're pretty neat.
posted by Kadin2048 at 9:24 PM on August 13, 2010

Kadin2048 i writes "I think his plan was just to run the vent up to somewhere near the gable vent and terminate it there without actually taking it outside the house."

This will work mostly if the end of the pipe is right up against the vent and you also have soffit vents as the air flow at the gable is out taking the moisture away from the attic. If you only have gable vents (or gable and ridge vents) then the air flow is into the attic at that point which is bad (but probably still better than just dumping it into the attic.

Kadin2048 i writes "I don't think that condensation in the attic is really a problem (in my area, the attic is so hot it's almost certainly warmer near the peak than the exhaust air from the AC) but in some areas or if you were using the AC to cool a particularly hot room, like a server closet, on a cool day, it very well might be a problem."

Assuming you don't have rigid ceiling insulation then temperature of the attic doesn't really matter. If you dump the exhaust from the A/C into the attic the moisture in that exhaust is going to spread throughout the attic. This moisture will penetrate your fibreglass/cellulose/wool/whatever insulation (if it didn't we wouldn't need vapour barriers in our homes) The insulation directly next to the finished ceiling is going to be at near enough room temperature as to make no difference. So is the bottom chord of your truss or ceiling joist. That moisture laden air is going to condense on those surfaces potentially causing rot and mould.
posted by Mitheral at 10:57 PM on August 13, 2010