Hi Guys of the SimHQ Encyclopedia...

I figure with the amount of Floridians and similarly heat-challenged Americuns here, this is THE place to ask Excuse the metrics please...

I'm thinking to buy one of these mobile airconditioning units. I know that proper installed split systems are better but that's not an option.

There are roughtly two sizes available - 12000BTU and 7000BTU cooling capacity, double the price for the larger one. Now by looking up some clever charts, I'm finding that 7000BTU are what you would use for approx. 25 square meter rooms, and 12000 can do about 45 sqm.

I'm looking to cool an about 35sqm room, with an open hallway (but doors closed to other rooms). But yet I'm only looking to cool down from about 30 to 25c.

So my basic question is: A slightly undersized AC unit will do what exactly? Will it not manage to get the room down at all... or will it not manage to cool the entire room area evenly... or will it not manage to cool the room down to 20 but get it down to 25 instead?

One part of me argues that if the unit can't expel more heat than is coming in, it will eventually not have an effect at all (i.e. the room will continue to heat up, just at a lower rate).
Another part argues that partial cooling must still partially cool, and in the end bring down temperature by a little, just maybe not as much as expected.

It's quite interesting physics to wrap your head around...

Thanks for all insights!

A consideration is that the smaller unit will run more constantly trying to do the job. This will use more power and be a more constant source of noise. So the smaller unit may actually use more power than the larger unit. As for ability to sufficiently cool the volume to a noticeably cooler temperature, there is no doubt that the larger unit will be better. The smaller unit will be like a fan blowing a cool breeze. It will better than just a fan, but may not achieve what you are looking for. However, based on where you live, a cool breeze may be all you require. I'm thinking that you aren't facing very hot conditions at all. Check to see how much dehumidifying the two units do. That will be almost as important in comfort as actual cooling. You can't over emphasize the effect humidity has on perceived comfort.

If you can swing it, get the larger one. Since doesn't have to work as hard or run as continuously, it will last longer.

Get the bigger one.

Thanks guys, especially OG makes a lot of good points.

I checked another retailer (which I should have done firstplace) and they have a 9500BTU unit for about middle the price of the other two. It's meant for max 80 cubic metres, which comes out at about my floor size for the main room. That one is 300 bucks. I guess a lot also depends on room layout, especially how much wall area gets heated by sunlight, window size and such.

I guess I could still spring the additional 100 for the 12000BTU to have some reserve, but for the 20 or 30 really hot days a year, that extra money doesn't seem worth it. I don't plan to run it unless it gets above 27�C, and then only to bring temp down into 25�C range.

How common are central air conditioning systems in private homes in Austria? I'm guessing not very?

Highly unusual

My apartment actually had a vent for a permanent installed AC in the plans, but the first tennant decided not to use it, so they closed it up. Being a rooftop apartment it does get a lot hotter than the average Austrian home. My parent's house hardly ever reaches 28�C, and up to that point not many people care for AC.

It's becoming increasingly common in the office buildings though.

I'm actually pretty used to 30+, the main reason - as stupid as that may sound - is that my pets suffer for three or four weeks each summer. If I can give them the occasional "cool day" and maybe find easier sleep on the worst days of the year myself that will be worth the 300 bucks.

All the office equipment makes a lot of heat that can't be naturally expelled unless you have no roof.



The Jedi Master

That's it! He's got it! Just remove the roof!

I'm definitely not. 30 Celsius is 86 degrees Fahrenheit. If it was that hot in my house I would have sweat pouring off of me. That's mostly due to the fact that I've always been a "hot bodied" person. I've been told by other people that I always feel like a hot water bottle! And besides, I don't think I would want to be running my PC with a stressed out video card (from gaming) in a house at 86 degrees F.

86 degrees is damn hot. Electronics may start to see issues and I'd be sweating my pooper off.

Then again, in WI 86 degrees generally comes with Vietnam level humidity. I'd be dead in a week.

In Florida EVERY temp comes with high humidity. When it's 40F it seeps into your bones, when it's 90F you apply antiperspirant to your entire body.



The Jedi Master

Room length x width x 35 = proper ac size.
Or so we were told back in the day when I worked at a tv/appliance store in college.
Of course everyone wanted the 5000btu sale a/c, but then brought it back because it didn't cool down the house. NO KIDDING! LOL

You don't want the thing running constantly. Go too small and you'll still lose money due to the power bill and still won't have cooled things down how you want.
Here, you've got to either have a/c or somewhere to go when it gets hot. People die from that, especially the elderly.

Thanks Kryp. Measured the room up, including every nock and cranny it comes to about 30sqm, which is 320sqf and so about 11000BTU by your formula. That one here multiplies by 25
http://electrical.about.com/od/heatingairconditioning/qt/airconditionersizing.htm

That's a pretty cool website too:
http://www.energystar.gov/?c=roomac.pr_properly_sized

Of course I'm tempted to try my luck with the 9500BTU size unit and just keep the room shaded as well as I can (window size and how many outside/sunny walls must count somewhere...)

I'll see tomorrow. I'm not worried about power usage as much, because it will be running about 5 hours a day, max 10, for three months top. That's 300 hours max, I expect less (not using it on 27�C or lower), if it needs an additional 1KW per hour at 20 Cent it's sixty Euros.

The problem with an "oversized" AC unit is unless you're really wanting to convert the room in question into a refrigerator it'll cycle on and off excessively.

I won't even go into all the problems/annoyances that can produce.

Perhaps, it will if one doesn't understand the controls. The modern units of decent design and manufacture don't have this problem if run correctly.

Also, check if any of those A/C they're selling there is an inverter.

If you have an under powered unit and try to
crank it up to get more out of it, you can
(depending on the type) cause it to freeze,
which will either result in it tripping off,
or soldiering on with the fan thwarted by
ice, either rendering it pretty much useless.
And how hard it has to work is corresponds
directly to the humidity.

Water has a huge effect on temperature. Here,
water is cheap and relatively cool all year
round, so an alternative to AC in a small
apartment is a bathtub full of cold water.
(This also works in the reverse in winter
if heat is inadequate while hot water is
available.)

An AC unit that is too small will be running pretty much constantly and will result in a bigger electricity bill and insufficient cooling.

The bigger unit may possibly end up cycling, which also isn't good...

If you can try to get a unit that is better matched to the space you want to cool.

Other than that the only advice I can give is make sure doors and windows are closed when the AC is running, regularly check the evaporator for ice (never try and break or chip it off though... switch the AC off and let it melt) and clean any dust off the condensor. Make sure the guy who installs the AC does a good job (i.e. the liquid line between the condenser and expansion device is properly insulated, the fan on the condensor has a good unrestricted airflow, any brazing on the pipework is properly done, water condensation drainage on the evaporator is sufficient) and ask him what vacuum he's pulling before he charges the system with gas... if his answer is anything other than "less than 5 Torr" sack him and get someone who knows what they're doing. Most problems with AC (and refrigeration in general) are caused by either systems that aren't properly matched to the demand, haven't been properly installed, or weren't vacced down properly when they were charged with gas.

I have a 2 speed central air unit. It blows like mad when I turn the temp down, but for maintenance of that temp it runs much more efficiently.

My electric bill dropped a good $30-50/ month after we replaced the previous unit with it.




The Jedi Master

If you oversized the unit, as mentioned it will cycle frequently and may cool the room, but its capacity to remove humidity will be severally limited due to the short run times.

I have no idea what the humidity is like where you live, but if the unit is sized appropriately it will do a much better job of removing humidity with longer run times, which will in turn make you much more comfortable if relative humidity is kept in the 50-55% range. You would probably want it to cycle on/off no more than 2-3 times per hour.