Can an 800 sq. ft. Cabin be Cooled with One Window AC Unit?

[forbin]

I know this is a stupid question, and there are many factors that go into a cooling design, way more than we could dig into here, so this is a Hail Mary play. Please be kind even if you think I'm an idiot. The downstairs is 800 ft^2, of which 180 ft^2 is the bedroom. The living room and kitchen are part of one big open area. There is also a 200 ft^2 open loft area. The cabin has an 9/12 pitch metal roof and insulated pine walls. The ceiling is around 20 ft. up, I guess. The question is, hypothetically, is it possible to cool a whole cabin with one downstairs window A/C unit in the 24000 BTU range?

 

[Vigo]

I would say yes. I have a 22k window unit in a 20x25 metal shop building, and another 22k window unit in a ~900sq ft mobile home. Either one is fine in all but the hottest conditions. But, here in San Antonio, that's most of the summer. Daytime highs are 90+ for 3 months, and there have been years when it crested 100 for 3 months straight (didn't happen this year). If you live somewhere easier than this i would say it will be 'ok'. I wouldn't expect the upstairs space in the cabin to be comfortable, though. It takes an absurd amount of downstairs cooling to cool an upper floor, like get the downstairs to 65 so the upstairs can stay below 80. If you want to sleep up there I would install the window unit on the upper floor and it will technically do a bit better all around. Cool air will cool the upstairs then 'fall' downstairs, and the unit will be sucking in the hotter air of the building vs the cooler air. Having the air inlet AND outlet be no more than waist high is a nasty compromise in most window unit installs.

The mobile home is insulated better than the shop, but i wouldn't say 'well'. If i leave the 22k cranked all day i can get the large open area it's in down to 69 even when it's 100 outside. There's not enough air circulation in a long skinny rectangle building to have it cool the whole thing evenly unless i run a bunch of fans to move air around. I have 2 smaller window units (one at each end) to alleviate this, but if it were distributed through good ducting like a central air unit the 22k by itself would be nearly sufficient.

The 20x25 shop is insulated, so not 'just' a metal panel building, but still has an uninsulated 12x12ft roll up door, a couple of windows, etc. Compared to most habitable structures still poorly insulated. That thing I can't get the interior below 80 when it's 100 outside unless i run the unit all day. If i let the interior get up to ~95 and then turn on the ac it won't get it below 80 before the 'sun load' starts to drop off anyway. If i turn it on in the morning and the interior never gets a chance to heat up, it will keep the interior below 80.

 

[sunshine_eggo]

It depends.

It really comes down to insulation. If your insulation inhibits the transfer of heat by 24k BTU/hr, then no problems at all. If you have poor insulation, it will be about how slowly the temperature rises compared to no cooling at all.

How to Calculate Heat Load | Heat Load Calculation Formula

Calculate heat load using this easy guide. Understand what your calculation means and seek the help of an HVAC professional for a more exact number.

www.plumbersstock.com

You'd serve yourself better investing in a minisplit.

 

[robby]

No not unless your living some place that stays relatively cool in the summer. You would have to mount it upstairs and hope that enough cool air flows downstairs. Honestly I think there is only a very small chance this would provide a comfortable experience downstairs.
Also Window AC's are a nightmare to clean and the efficiency sucks. My recommendation is to get two mini Split AC's that are sized for both upstairs and downstairs.

There are several different sized Pioneer units listed on this Amazon page down in the middle
Amazon Mini Split Pioneer

 

[400bird]

I think the answer is yes. It won't be efficient or an even temperature and "cool" is relative.

You'll either have difficulty cooling the loft (AC downstairs) or if you put it upstairs you'd need some fans to move the cool air up top. Either way you'd need fans to move air into the bedroom.

I'd recommend a mini split or 3, or a single with 3 heads.

 

[Doggydogworld]

I know this is a stupid question, and there are many factors that go into a cooling design, way more than we could dig into here, so this is a Hail Mary play. Please be kind even if you think I'm an idiot. The downstairs is 800 ft^2, of which 180 ft^2 is the bedroom. The living room and kitchen are part of one big open area. There is also a 200 ft^2 open loft area. The cabin has an 9/12 pitch metal roof and insulated pine walls. The ceiling is around 20 ft. up, I guess. The question is, hypothetically, is it possible to cool a whole cabin with one downstairs window A/C unit in the 24000 BTU range?

The bedroom is the problem. You need a lot of airflow in and out to equalize the temperatures. Maybe get a separate 5k BTU unit for it. The open space should be OK with a ceiling fan. This assumes the ceiling is insulated.

Window units get a bad rap, but if they only run 15-20 days a year the inefficiency doesn't really bite you.

 

[forbin]

I would say yes. I have a 22k window unit in a 20x25 metal shop building, and another 22k window unit in a ~900sq ft mobile home. Either one is fine in all but the hottest conditions. But, here in San Antonio, that's most of the summer. Daytime highs are 90+ for 3 months, and there have been years when it crested 100 for 3 months straight (didn't happen this year). If you live somewhere easier than this i would say it will be 'ok'. I wouldn't expect the upstairs space in the cabin to be comfortable, though. It takes an absurd amount of downstairs cooling to cool an upper floor, like get the downstairs to 65 so the upstairs can stay below 80. If you want to sleep up there I would install the window unit on the upper floor and it will technically do a bit better all around. Cool air will cool the upstairs then 'fall' downstairs, and the unit will be sucking in the hotter air of the building vs the cooler air. Having the air inlet AND outlet be no more than waist high is a nasty compromise in most window unit installs.

The mobile home is insulated better than the shop, but i wouldn't say 'well'. If i leave the 22k cranked all day i can get the large open area it's in down to 69 even when it's 100 outside. There's not enough air circulation in a long skinny rectangle building to have it cool the whole thing evenly unless i run a bunch of fans to move air around. I have 2 smaller window units (one at each end) to alleviate this, but if it were distributed through good ducting like a central air unit the 22k by itself would be nearly sufficient.

The 20x25 shop is insulated, so not 'just' a metal panel building, but still has an uninsulated 12x12ft roll up door, a couple of windows, etc. Compared to most habitable structures still poorly insulated. That thing I can't get the interior below 80 when it's 100 outside unless i run the unit all day. If i let the interior get up to ~95 and then turn on the ac it won't get it below 80 before the 'sun load' starts to drop off anyway. If i turn it on in the morning and the interior never gets a chance to heat up, it will keep the interior below 80.

Thanks for the reply. It sounds like your metal shop building is a lot smaller than our cabin. How high is the ceiling? And does your A/C run continuously, or does it cycle? My limiting factor is the amount of available solar energy. I can only generate about 24 kWhr/day with my panels. If I have to run the A/C continuously, it will eat that up and there won't be anything left to power the rest of the cabin. :-(

 

[Maitake]

My house is open concept 2400sf.
A one ton mitsu mini split keeps it 15f below ambient.
85 and dry is very very comfortable when its 100 outside.

 

[forbin]

It depends.

It really comes down to insulation. If your insulation inhibits the transfer of heat by 24k BTU/hr, then no problems at all. If you have poor insulation, it will be about how slowly the temperature rises compared to no cooling at all.

How to Calculate Heat Load | Heat Load Calculation Formula

Calculate heat load using this easy guide. Understand what your calculation means and seek the help of an HVAC professional for a more exact number.

www.plumbersstock.com

You'd serve yourself better investing in a minisplit.

That's what I'm afraid of. The ceiling is beautiful from the inside, with whitewashed pine boards and natural-color exposed rafters, but I don't think it is insulated hardly at all. As far as I can tell, it's a layer of boards covered with tar paper and then a metal roof. It's possible that the layer of black material between the boards and the metal is not just tar paper, but it's not very thick in any case. I may have to forego the beauty and add insulation to the visible underside of the ceiling.

I've read a lot about the benefits of a mini-split, most notably better efficiency. However, every time I compare them to window A/C units of similar capacity (24,000 BTU) it seems like the wattage consumption is about the same, and the window units are about a fifth of the price. :-/

 

[Vigo]

Ceiling is single pitch 12' tall on one end and 10' tall on the other.

If i let the building heat up and then turn it on it will run continuously for hours. If i start it in the morning before the building has heated up it will cycle normally. My particular units (they're identical) pull 2000-2500w with compressor running on high fan speed.

I second the recommendation to go mini split instead, btw. My situation is a fluke because I bought one 22k to put in the shop, and got another one basically free when the first one broke, but i kept it and repaired it. Since I had it, i decided to punt on my own mini split plan for the mobile home and stick the 'free' one in there.

But, i will be installing at least one mini split on the mobile home before winter (for heat pump heating function), and eventually installing that second 22k unit into the shop. At that point the shop will be 500sqft with 44k btu/hr of cooling, but since i work on cars and the shop is tiny, i'm sometimes bringing in fully heat-soaked vehicles that are too long to close the bay door on, and if i can have cold air blowing on me for the hour or two im working on that hot engine or whatever, that's more important to me than cooling the entire space, or any sort of efficiency concern. That shop is grid powered. My mobile home is 100% off grid, so running 5kw of ac just to blow cold air out a 12' wide hole in the wall is not practical. ?

BTW, if you decide to stick with the window unit plan, there are some inverter-driven window units that function similarly to mini splits and i would recommend you seek that out IF you don't just go with a mini split in the first place.

 

[forbin]

My house is open concept 2400sf.
A one ton mitsu mini split keeps it 15f below ambient.
85 and dry is very very comfortable when its 100 outside.

By open concept, I assume you still have separate bedrooms, bathrooms, etc.? Are you using a single-zone or multi-zone unit?

 

[Vigo]

I've read a lot about the benefits of a mini-split, most notably better efficiency. However, every time I compare them to window A/C units of similar capacity (24,000 BTU) it seems like the wattage consumption is about the same, and the window units are about a fifth of the price. :-/

Whoops, didn't see you posted this while i was writing my last one.

Mini splits are not really more efficient at their max settings than a 'conventional' AC because they have more conversion losses. However, they can 'throttle back' to much lower power consumption and become near-silent, while also never having a huge compressor startup surge. Because they tend to run nearly continuously while at lower settings rather than turning on and off, they tend to keep the temperature much more stable in a space.

Between the noise, temp/humidity stability, overall efficiency gain when not running on max settings, and lack of humongous inrush currents to account for, they are a way more pleasant experience for off-grid living.
From what i have seen, 24k window units would be $6-800 on the low end, while 24k mini splits would be maybe $1800-3k. This is from pretty casual observation so i may be wrong.

 

[forbin]

No not unless your living some place that stays relatively cool in the summer. You would have to mount it upstairs and hope that enough cool air flows downstairs. Honestly I think there is only a very small chance this would provide a comfortable experience downstairs.
Also Window AC's are a nightmare to clean and the efficiency sucks. My recommendation is to get two mini Split AC's that are sized for both upstairs and downstairs.

There are several different sized Pioneer units listed on this Amazon page down in the middle
Amazon Mini Split Pioneer

Thanks for the reply. I've been reading a lot about the better efficiency of mini-splits, but it seems like every time I compare them to window A/C units of similar capacity, the wattage consumption comes out about the same, and the window units are about 20% of the price. It's confusing. My limitation is the amount of solar power I can generate during the day. I'm limited to about 24 kWh. A/C eats that up fast.

My wife and I are pretty hardy. We went through the past two Oklahoma summers with many days exceeding 100F. The loft where I work on my PC is usually about 5F hotter than downstairs. All we had were box fans and we did okay, but not great. We can probably get by with a less than optimal experience.

 

[sunshine_eggo]

That's what I'm afraid of. The ceiling is beautiful from the inside, with whitewashed pine boards and natural-color exposed rafters, but I don't think it is insulated hardly at all. As far as I can tell, it's a layer of boards covered with tar paper and then a metal roof. It's possible that the layer of black material between the boards and the metal is not just tar paper, but it's not very thick in any case. I may have to forego the beauty and add insulation to the visible underside of the ceiling.

I've read a lot about the benefits of a mini-split, most notably better efficiency. However, every time I compare them to window A/C units of similar capacity (24,000 BTU) it seems like the wattage consumption is about the same, and the window units are about a fifth of the price. :-/

Data plates list max. This is where SEER matters.

A 24,000 BTU minisplit with a 20SEER rating will have a typical 24000/20 = 1200W consumption, and it will have ZERO surge. It will also run at lower power settings if that's all that's needed to maintain the temp. They also tend to be much quieter than window units.

A 24,000 BTU window A/C unit typically doesn't list a SEER, and unless they're inverter-based motors (they typically aren't since they're cheap), they may have a MASSIVE surge 5X the run current. Your inverter might not even start it. These things run full blast or are off.

When you compare EER ratings between window units and mini-splits, the mini-splits tend to be about 25% more efficient in test laboratory controlled conditions.

 

[noenegdod]

Thanks for the reply. I've been reading a lot about the better efficiency of mini-splits, but it seems like every time I compare them to window A/C units of similar capacity, the wattage consumption comes out about the same, and the window units are about 20% of the price. It's confusing. My limitation is the amount of solar power I can generate during the day. I'm limited to about 24 kWh. A/C eats that up fast.

My wife and I are pretty hardy. We went through the past two Oklahoma summers with many days exceeding 100F. The loft where I work on my PC is usually about 5F hotter than downstairs. All we had were box fans and we did okay, but not great. We can probably get by with a less than optimal experience.

Based on this, grab a window ac and give it a try. It will be a lot better than what you have been doing.

Try installing a ceiling fan to help the up stairs. The unfortunate part is that it sounds like cooling is only available while the sun is up. If you had power to let it pull the building down a little further in the cooler part of the morning that would help a lot with keeping the building cooler throughout the day and also help a lot with the COP.

 

[Vigo]

My limitation is the amount of solar power I can generate during the day. I'm limited to about 24 kWh. A/C eats that up fast.

I can tell you that I have 10kw of panel and inverter and i dislike operating that single 22k window unit on my system because of its 'all or nothing' nature requiring me to manually keep track of when to turn it on and off so it doesn't drag my batteries way down. During the middle of the day it has to be very cloudy for me to worry about it, but even slight changes in solar conditions can change the time of day when it starts to drain my batteries by a huge amount, so it becomes sort of an irritant factor around 430-630pm. Just something I have to think about that i don't really want to.

For comparison, the inverter driven unit in my bedroom that operates more like a mini split, maxes out at 800-1000w but will never hit it unless i turn the unit off and let the space heat way up before turning it back on. If i leave it running it will maintain the temp set point while usually remaining under 400w except in the hottest part of the day, and by evening it has throttled back into the 200s W and a little after the sun has gone down it settles to ~150w and stays like that all night.

If my 22k window unit were a mini split it would function the same way with a proportionately larger watt amount, and it's possible that if i swapped my 22k window unit for a 22k mini split right now, i could just leave it turned on 24/7, or on a timer switch from 8a-10pm or something like that, and never have to worry about what it's doing again because it would NEVER hit the 2500w my window unit does and would probably spend most of its time under 1000w. It's also possible i could get away with a mini split smaller than 22k btu because it would stay running nearly all the time and it would never have to play 'catch up' with the heat load.

 

[Maitake]

By open concept, I assume you still have separate bedrooms, bathrooms, etc.? Are you using a single-zone or multi-zone unit?

Separate yes, but all off the main area. One head unit in the main area.

 

[robby]

I don't know if this helps you but I can give you some numbers for my Panasonic Mini Splits in the Master Bedroom
It's about 500Sq/ft room with a 15ft ceiling. The AC is 18,000 BTU Inverter unit. It's 240V (BTW Never use 120V AC's) it starts up drawing about 200W then in 10 seconds ramps up to 900W and stays there for about 30 minutes until the room drops from 88deg to about 79 deg then it drops to about 500W for another 20 minutes until the room is at about 76deg.
then it drops down in consumption to about 250-300W and stays that was for most of the night keeping the room at 76deg. If you open the door or a window and the temp rises it will ramp up again to 500W.

 

[TGPB]

We have been cooling a 2000 sq ft home for the last 20 years with a single window a/c for the last 20 years. It gets to 100F here in the summer. We have it mounted near the ceiling upstairs. We have an open floorplan upstairs.

 

[Steve_S]

Buying an AC ONlY is so 1980's ! and wasteful in reality.
Everyone needs cooling AND heating and energy efficiency is always a win. A Heat Pump is the best solution and best Bang per Buck !

Check out https://senville.ca/?gclid=CjwKCAjw...U9Q2fow-RIBdsfw_YbLLKlLwEBWjXXxRoCxNcQAvD_BwE FYI, Cheaper in the States.
There are 120VAC & 240VAC units

Also check this out from Will Prowse which he just updated !

Solar Friendly Air Conditioners

If you wish to add air conditioning to a structure while powering it with solar power, this is what you need: High Efficiency Mini-Split DC Inverter Heat Pump System . SEER Rating of 18-27 is...

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