Room Air Conditioner BTU Size FAQs

Questions & answers on choosing the size for a room air conditioner

A/C Cooling Square Feet Capacity Estimate Variation FAQs

Why do A/C estimates of number of square feet covered vary from one manufacturer or site or table to another? Which do I believe?

In searching for a 14,000 BTU portable air conditioner, I find the square feet they claim to cool varies from 400 to 700 depending on brand. What am I to believe? - On 2017-07-02 by Barbara

Reply by (mod) - two different air conditioners, both rated at 14,000 BTUh may indeed have different effective cooling capacity.

Cooling capacity will vary by equipment design and features and still more be individual building features like solar gain, Windows, air leaks, insulation.

So two different air conditioners, both rated at 14,000 BTUh may indeed have different effective cooling capacity.

Besides variation in the building and its local climate, something as simple as a different supply air fan or air filtering system can affect the unit's cooling capacity, or its engineers may have simply reached a different conclusion from those at another company.

Bottom line: rely on the manufacturer's recommended cooling capacity for the equipment that they manufacture.

Portable A/C vs Window A/C Unit Capacity FAQs

Choosing an A/C unit for a very hot apartment: Portable vs Window A/C Unit

I having a really hard time solving the btu question for two situations in an extremely hot apartment, upstairs and down.

I'm buying a/c with consideration for just the hottest days, 94-105 degrees (but we've gotten as high as 109). 1)-Tiny (100 sq ft or less) upstairs bedroom, with 6ft window that gets full, brutal sun.

I have a 12,000 btu dual-hose portable in this room, which is still inadequate when it gets over 93-94.

On a 98 degree day, it will still be almost 80 in the room, which is too high for a family member with a health condition. I'm told that a window unit will cool much better than a portable, but the manufacturer warned me not to go over 6000 btu for this room, which to me sounds unrealistic, given that the 12,000 btu in the portable is already inadequate.

This room is at the end of a hallway. So, two questions for this room: I'd like to know if it's possible to cool the hallway and bathroom with the help of fans from this room.

(Cold is what I'm looking for, so it's okay if that room is cold). And also, I'm confused as to whether I should size down for a window unit, to stay in accordance with the charts (which would be 6-7500, which doesn't seem realistic to me).

Would a 10,000 btu window unit cool better than the 12,000 portable? If so, if I got a 10 (or even 12,000) btu window unit, and it turns out to be too big for the room, would leaving the door open to cool the hall and bath prevent the problem of cycling off too soon?

Second problem is downstairs; 8 ft ceilings, approx. 450 sq ft., living/ dining area, kitchen, small bath, poorly fitting doors, window and sliding glass door that get full sun, and a gas stove that's always warm, with continuously burning pilot lights, plus an additional mini fridge that gets warm.

Other charts I see recommend 10-12,000 btu for this space, although I see yours says up to 13,500.

This, again seems too low to me, given the problem upstairs, but my perspective may be skewed by lesser efficiency of portables(?). I'm confused as to whether I should be adding 4000 btu for the small kitchen if there's not a not of cooking going on (even though that oven stays warm).

If I take the figures from your chart for that space, and add for the kitchen, sliding glass door, and window that get full sun, that puts me at 19000-20,5000 for the 450 sq ft, which seems realistic to me, given how hot the apartment is, but again, I'm seeing a lot of other charts that say this is way oversized, and the phone rep at frigidaire said this is way too big.

Any perspective you could give would be appreciated.

Just to add a clarification for my question below, for the window and door downstairs, they are on the sunny side, but they don't get quite the pounding sun that the upstairs window gets, due to eaves and an overhang, but there is afternoon sun, especially through the door.

So I'm not sure if I should be adding 1500 btus for them, or not. - by Heather

Reply by (mod) -

Heather

The problem is not just BTU size but how effectively the air conditioner can cycle room air through itself and move air to the outdoors.

I'd ask for a 30 day return privelege, then try what the manufacturer recommends for the sized room (and climate) where you live.

by Heather - Translate BTUs between Portable Unit & Window Unit?

Could I ask, then, if you have any experience translating btus from portable to window units?

Manufacturer recommendation is the same for both, but the 12,000 dual-hose portable isn't sufficient for the 100 sq ft space, for which they recommend no more than 6000. I'm wondering if I need to size down significantly with a window unit or not.

My problem is that I have to pay a lot for installation, and will not be able to pay for a mistake, even if the store will exchange. I've just had someone put in a 230v outlet for downstairs, thinking I needed at least 18000 btu, based on some charts, but then was told that's way too high by the manufacturer.

Although by your chart, it would be ok. It will be a total of $440 just for the outlet if I have to switch again, and over $600, as it is, just for one installation.

I'm sorry if I wasn't clear. The 12,000 btu portable is rated for up to 400 sq. ft., just as a window unit of that size would be.

All of the industry charts (and manufacturer recommendations, across brands) say I should have no more than a 6 or 7 btu unit (either portable or window) in that space, yet the 12,000 btu, while it blows cold and stays on continuously (no cycling off), is not adequate for the small space in extreme heat waves, which are my primary concern. (It's fine when temps are in the 80s).

I have read multiple reviews in which buyers said smaller btu window units ran circles around higher btu portables, which is why I had the question about the difference between the two, since my experience with two different portables is that they don't cool adequately even half the area for which they're rated.

(I previously tried the recommended 7,000 btu portable in there; it was little better than a fan).

I don't know much about what could be unusual about the space, other than it's the hottest building we've ever lived in, by far. I have the heat vent in the room sealed off, and foam core and insulated curtains on the windows.

Unfortunately, I will have to have a frame built to install the upstairs a/c in a wide horizonatally sliding window, so that there will be some window left to get air when it's not in use, and for downstairs, a special plywood "window" made to fit the unit into a french door, with the a/c on a cart of some sort, as there's no window downstairs wide enough to accommodate an a/c.

While the downstairs is cooler than up, the inadequacy of the 12,000 btus upstairs is why I'm also questioning what's needed for the 4x larger space downstairs--and whether or not I should be ignoring the charts with window units, as I have to do with the portables, or if they are more accurate.

If I use your 12-13,500 figure for 450 sq ft multi-rooms for downstairs (it's a horseshoe-shaped space) and add for a kitchen and windows, 18,000 ought not be too much (and would work with my outlet). It was my reading reviews of 15,000 btu window units, in which people are cooling 1000+ sq ft areas with that smaller size that led me to believe I had made a mistake in calculations, since I have no experience with window A/Cs.

If I'm basing the decision on what it takes to cool a tiny room upstairs with a portable, that's a different story.

While the frigidaire phone rep did say not to go bigger than the charts, she also said a window unit could not cool lower than 17-20 degrees lower than the outside temperature, no matter what, which as I understand it, is not true.

The electrician told me I could not use the 230v outlet with a 115v unit, so from what I'm seeing at retailers, I have to buy at least an 18000 btus unit to use that new downstairs outlet.

Reply by (mod) - translating btus from portable to window units?

Heather I'd like to help but I don't understand any difference between BTUs from one machine or another. Manufacturers rate the cooling capacity of their equipment in BTUs per Hour of cooling capacity and offer general advice about the room size that each unit should be able to handle.

Neither portable AC unit nor window A/C unit ought to be difficult to install since it's basically a "set it in place and plug it in" procedure - especially if you've already got a 220V receptacle where it's needed.

If a 12,000 BTUh portable cooling unit was not able to cool the room you're concerned with, I too and cautious about how a 6,000 BTUh window unit would do the job.

There may be differences in effectiveness between the two approaches to cooling, particulary depending on how the portable unit is set-up, but that's a very large difference.

Are you telling me that the manufacturer of the portable unit says you needed 12,000 BTUh for the same room size that the window unit manufacturer says takes a 6000 BTUh unit? If so that alone is interesting.

I'd also look at the heat gain, drafts, leaks, heat sources for the area of concern to see if there's something unusual about that space.

Reply by (mod) - Differences in room cooling efficiency and effectiveness of free-standing portable units vs. window units.

Thanks Heather, that tells us some important data about the differences in room cooling efficiency and effectiveness of free-standing portable units vs. window units.

A window A/C has an easier time getting rid of the heat it's drawn from the room air as well as heat generated by its own compressor motor as those are hanging outside in the outdoor air. It suffers the disadvantage that it has to draw room air to itself at its window location.

A free-standing portable A/C unit has to push outdoors not only the heat it has removed from room air but also the heat generated by its own motors. It has the advantage of being able to be placed in the center of a room but I have never seen anyone live with that placement as it's a bit awkward.

The fact that your portable unit never shut off tells us that it was not able to handle the cooling load required.

YOu do not want a unit that is too big either as then it will cool but it won't dehumidify the space.

Unless your building has some unusual heat gain source or air leak source or internal heat generating source, and provided you've got typical 8 ft. ceiling heights, I'd go with the manufacturer's recommendations.

If you're going to install a through-wall unit rather than a window unit, you can frame the opening large enough to take a larger sized unit, but still install a smaller (recommended) sized unit (along with appropriate weather sealing). If you're cooling multiple rooms, indeed a larger capacity through-wall unit can make sense.

It IS indeed very difficult to cool a horseshoe shaped space from a single point as there's not going to be good air circulation from the remote leg. You might, depending on home layout, improve the performance of the unit by some box fans to move house air around.

The Frigidaire phone rep is being cautious so as to avoid a future dispute, I suspect, since the absolute cooling ability of any A/C unit is going to depend also on the size, shape, ceiling height, insulation, heat gain, and other features of the specific space.

The electrician is dead right but it certainly should be possible for him to give you 115VAC at the wall where needed. Or both.

Do not get a way oversized unit.

by Heather - BTU charts are probably more accurate for window A/C units than for portable A/C units

Thank you so much for your help. If I understand correctly, then, understand that you the charts are more accurate for window units, and I need to size down from the 12,000 I'm using in the portable. If I could trouble you with just two more questions, then:

Would it be unadvisable to instead stay with 10-12,000 btu for that small, end of the hallway room upstairs, with the plan of leaving the door open to cool the rest of the upstairs (using fans)? My landlord has refused to allow me to install a through-the-wall unit.

And do you feel then,that an 18000 btu for the 450 ft horseshoe-shaped space downstairs would be way oversized (going by your multiple room chart, including a kitchen, it would be 16-17,500 btu, not including sunny windows)?

It will be another $200 to change the outlet to 115v, and if a 15 btu a/c turns out to be too small, another $200 to change it back again.