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kba4
join:2001-10-23
Canton, OH
 ·RoadRunner Cable
| window AC BTU I hope this is the correct forum. I'm shopping for a window AC unit and have decided to go with Frigidaire, as I've had a pleasant experience with them so far. The thing is, I'm having a difficult time understanding how to calculate necessary BTU.
See, I purchased my first AC unit years ago, 2003 I believe, from Best Buy. It was a Frigidaire and it was rated at like 5200 or 5400BTU. I did some research prior to purchase to determine that my required power was indeed less than 6K BTU, as the room size was less than 200sq-ft. The room had both N and E facing windows, so I chose the N one, as it is the most indirectly-affected by the sun. I installed it and had it on for the entire summer season (except for the night of the great 2003 power outage!). The unit did a poor job of cooling the air in that room and so I replaced it the following year with a new model, stepping up almost 2x in power to 10K BTU. This unit literally blows the other one away in terms of cooling power. It also has a 2-way remote so it can cool more efficiently.
So onto my question: the various 'rules' I've always read say to get sq-ft'age of the room, and that's your base figure. Lighting, electronics, window-facings, etc. all step you up to the next BTU model. But- the rules also say things about not going too high! I am going to get one more AC unit for another room in the house (central air in this old home is horrible). This room is also less than 200sq-ft. This room has both an E and S facing window, and the S one is the only practical place to install the AC, which I know is not recommended. So, taking all the above into consideration, is another identical 10K-BTU AC adequate, or is something higher going to be necessary due to the S-window? This room absolutely must be kept cool, as it is where all the computer and networking equipment for my home/business resides. Ambient temperature during the winter was 75º+, I can only imagine what the summer will bring if I don't head it off!
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SandShark
So it goes
Premium,MVM
join:2000-05-23
Santa Fe, TX
clubs:
 ·Verizon Online DSL
| Re: window AC BTU There are too many variables that you haven't provided to accurately calculate what BTU capacity you'll need for that room. Variables like insulation, wall construction, types of windows, whether or not there is conditioned space above and/or below the room, number of people, etc. I found a pretty decent site where you can enter this information to calculate the BTU capacity you might need. The computer and networking equipment is going to be a large variable in the calculation.
»www.consumerreports.org/cro/appl···rksheet/
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Willy
Premium
join:2000-09-24
USA
 ·Verizon FIOS
·Optimum Online
| Re: window AC BTU said by Rxdoxx  : Question. Would the larger unit be able to effectively run on a much lower setting? I know this is the case for my new dehumidifier, I have the fan speed on low, and it is a lot quieter (and more effective). Wondering if there can be a similar consideration that a larger unit could be effective on a low fan where a smaller unit would have to blast away noisily? Yes that helps if you're dealing with 1 room and there's not to large a difference in the needed size and actual size. If you need a 5000btu you install a 8000btu your probably OK. The larger the units get the greater this problem could become.
I've seen bigger problems when people want to cool a whole floor or apartment with 1 unit.
Calculations say a 16000btu unit in a central location will cover the square footage but of course the calculations don't take into considerations the doorways, the twists and turns of the hallways, etc. so it doesn't cool the whole apartment. Now change that 16,000btu unit to a 25,000btu, even at the lowest settings that center room will probably be uncomfortable, and the rest of the apartment are still hot and humid. | |
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kba4
join:2001-10-23
Canton, OH
·RoadRunner Cable
| great site, thanks! turns out 10K is probably closer than I thought. I'm going to mess around with numbers some more but it looks like for this room 11.9K is the number.
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| Axekick
join:2005-05-01
Saint Louis, MO
·Charter Pipeline
| I cannot imagine a 200 square foot room requiring larger than a 10,000 BTU air conditioner. As noted, going to high will force the air conditioner to kick off too quickly, resulting in uncomfortable humidity levels. Also, the frequent on-off cycle will run your electric bill up.
I would encourage you to seek out current high efficiency window units as they're far superior to older units and cost only slightly more. Check reviews online, as with most products every manufacturer tends to have good and bad models.
I live in the St. Louis area and have a 174 ft^2 room, with one east facing window and one west facing. That room is easily cooled by a 8,000 BTU unit easily in temperatures up to 103 degrees. | |
| 
dgilbert
Good Bye My Friend
Premium,MVM
join:2002-06-15
none
clubs:
| older 6k units would cool circles around new 5.8k units. it is not the .2 difference in BTU, it is the fan speed. manufacturers now use a lower speed fan to get colder air and thus "increase" the BTU rating. this lets them use cheaper junk to make the unit. if you have a lot of electronics or other things causing a lot of heat gain, this slow speed fan just cannot keep up with it.
for example, at one of our tower sites we have a 6x8 building with no windows. the building is wood frame with 6 icnhes of insulation in the wall cavities, a layer of foam insulation onthe outside, and 12" of insulation inthe attic. inside the building is 3 transmitters that run 24/7. they produce enough heat that we have to run the AC even during winter, though we do open the outside air vent on it then and run it on high fan. anyway, for 15 years we used the same old 6k AC unit. never once had an issue with heat buildup in the building. when it finally died we bought a new 5800btu AC and stuck it in. the first thing we noticed was it had low air flow, even on high. sur eenough, even on a 75 degree day it could not keep the building below 90 inside. so we bought a different brand of the same rating. got the same issue. we finally bought a replacement fan for the original unit and put it back it. and here we are now, 20 years on the same window unit and it can keep the inside of the building at 70 degrees even when outside temps are close to or even slightly above 100!
the same thing goes for higher BTU units, we learned thru similar experiences at other sites that when replacing an old unit you have to go up about 25% in BTU rating for the 120v models to get the same cooling ability as an older unit. 240v models don't seem to have this issue in MY EXPERIENCE.
as far as humidity removal, i don't think OH has that bad of a humidity problem for it to be an issue. we stay at 80%+ RH year round here, and everyone oversizes by 1 size to no ill effects. now if you needed only 6k btu and you put in a 24k unit, yes you will have issues. but going up to 8, 10, or maybe even 12 won't be an issue. the smaller the unit the longer it must work to cool, but also the more humidity it will pull out inthe process. so get close to what you need, but go 1 size larger than the specs recommend.
Axekick my room at work is roughly 200 sq ft and it requires 2.5 tons of cooling year round. my shop at home is 330 sq ft and it requires 2 tons, 24k btu. and that is just to keep it tolerable.
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| Raphion
join:2000-10-14
Samsara
| Remember that each Watt of electronics you have in the room will be contributing 3.41 BTU/hr of heat. So a nice desktop computer could produce as much as 1000 BTU/hr easily. And each human in the room contributes roughly 100 Watts or 340 BTU/hr. And don't forget fans and lights, almost anything you have using power in the room will just turn it into heat eventually at the rate of 1 watt to 3.41 BTU. It adds up fast.
Sunshine in a window will bring in obscene amounts of heat. Some metallic window tinting is very helpful for cutting down on that, though it makes a window less of a... window.
Also, A/C BTU rates seem to be measured across a flat temp gradient, meaning same temp inside and out. As it gets hotter outside, it has to pump the heat "uphill", and will be coming up more and more short of that advertised rate as the outside temp goes higher relative to inside. | |
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