Advice on AC size?

June 18, 2024 1:14 PM

I'm in AC decision-making hell. I'm going to put a window AC in my small home office. I thought I'd found my ideal model, but it's a strange shape and turn's out it won't fit. So now I'm back to square one. The model I'm now leaning toward bc it's quiet is 8000 BTUs. That's the smallest it comes. But the room is maybe like 150 sqft, so is 8000 BTUs just too big? Are there reasons why having a too big AC is a bad idea?
posted by gigondas to Home & Garden (12 answers total)

Are there reasons why having a too big AC is a bad idea?

Yes, if it is too overpowered it can "short cycle" which is energy inefficient, bad for the ac, and also doesn't manage humidity well (because it doesn't ever run long enough do do much conditioning). You can google that term to read more about it.
posted by advil at 1:24 PM on June 18 [1 favorite]

Since you say it's quiet-- is it an inverter type? From what I've read there's less of an issue with those being oversized because they are able to efficiently run well below their full capacity.
posted by ambulanceambiance at 1:30 PM on June 18 [1 favorite]

Probably should have added this caveat: I wasn't really intending to weigh in on the "is 8000 btu too much" part of it. That is, I don't know if 150 sq ft vs 8000 btu is a crazy mismatch, especially if you have tall ceilings or a warm climate, or can just run it on eco mode or the like; hopefully someone else has more concrete responses about that.
posted by advil at 1:40 PM on June 18

I ran an 8000 btu Midea u-shaped air conditioner for my small home office that's about that size, volume-wise maybe even smaller because it is a converted attic space so the ceiling is angled and 2 of the 4 walls are only 3' high. It never had any problems, it kept me cool and comfortable and I didn't have any humidity issues.

This is for a south-facing room that got a lot of radiant and conductive heat because the ceiling was the roof (with only the space between the rafters separating them).

I now have a mini split so I don't use it anymore but I never had any issues.
posted by misskaz at 2:02 PM on June 18

Response by poster: Misskaz, that's the model I'm looking at because it's supposed to be quiet. But, upon further thought but short of actually measuring, I think my office is more like 120 square feet. It gets really hot in July and August. And probably later this week.
posted by gigondas at 5:05 PM on June 18

I had same a/c unit in a basement apartment, 125 sqft room, low ceilings and it was a charm. Buy it.
posted by sandmanwv at 5:10 PM on June 18

"is 8000 BTU's too much" isn't a super answerable question because, as others have stated, it depends on the humidity you're battling, facing of the room, story the room is on, etc

HOWEVER, I just popped mine back in the windows today; we use one in each of our bedrooms. They're both within the same range of your office (one is actually 120 sq ft, and the other is 150), and they work very well; all the bedrooms are on the afternoon 'hot side' of the house. They are very quiet, work well for the space, and can handle rather high temperatures (for us at least; easily into the low hundreds). They are dope as hell, and were drastic improvements over 'regular' style window AC units. PNW summers are starting to get a BIT WARM and they are handling it very well, and will serve us until we are able to install a whole house heat pump.

I will say, the first installation is a bit of a pain in the ass. Watch the installation videos they, and other third parties offer. If you're going to remove it, it would behoove you to sharpie up all the 'settings' of the installation bracket.
posted by furnace.heart at 6:42 PM on June 18

Per Hitachi a "rule of thumb" for a window unit/room type a/c is "120BTU per square foot to get the minimum BTU of air conditioner you should buy".

So 125 sq ft * 120 BTU/sq ft = 15,000 BTU.

I don't know anything more about it than that - and maybe that page isn't even talking about the same type of unit you're looking at - but it is, at least, a starting point.
posted by flug at 6:56 PM on June 18

Response by poster: I'm in NYC, so humid enough. I was all set to buy the Midea unit but have now been reading that more than a few people experience a mold issue, so I'm now rethinking it. Ugh!
posted by gigondas at 6:58 PM on June 18

I have a 8000btu midea Ushape unit I got cheap in a sunny nyc bedroom (100 sq ft, but 13 ft ceilings) and while a little oversized, it’s fine. We run it either on dry or on eco mode, and haven’t had an issue with mold at all.

Obvi a smaller unit is probably cheaper, but the newer efficient modern ACs seem to be less problematic than prior ACs.
posted by larthegreat at 7:14 PM on June 18

My strategy for the midea is actually not to use eco mode--to keep air flowing at all times by either having it on Cool, or on Fan, so nothing is ever stagnant (can turn off after several hours of Fan). This seems to be working well so far.
posted by rivenwanderer at 8:57 PM on June 18

That Midea is an inverter model, so there is not as much concern with it being oversized. I have one and like it a lot.
posted by ambulanceambiance at 6:29 AM on June 19 [1 favorite]