Air Con Sizing Opinions

[bangkokpoppys]

I need some help with sizing correctly for an upstairs top floor room sized 5 m x 4m with a high ceiling. It has large sized windows facing South.

 

I'd like to get it down to around 23-24C for comfortable working.

 

What size do you think I should target in an A/C unit?

 

TIA

[wpcoe]

Since it's a top floor, one important factor to consider is the the heat load from above:  Is the ceiling insulated, and is the attic space (if there is one?) adequately ventilated?

 

The standard back-of-a-napkin calculation uses 600 x sq.m of the space to be cooled.  In your case, that is 5m x 4m = 20sq.m, so 600 x 20sq.m = 12,000 BTU.  But the heat load from the ceiling can alter that significantly.

 

Also, are the windows double-paned and/or coated with an infrared reflection film?  Is the south-facing wall a cavity wall or otherwise insulated?  Are the other walls exposed to the outside?

Edited January 10, 2018 by wpcoe

[Pib]

Would agree with a 12K BTU A/C.   I would not go larger than that.   The calculator at this website allows you to enter windows sizes facing certain directions and equipment wattage/heating used in the room also.  For your room size I used some 4 sq/mtr windows size facing south, 1000W equipment heating (i.e., computer, TV, etcz) and 50W lighting heating....got a BTU size recommendation of 12.7K BTU.    

 

Now based on my own personal experience here in my Bangkok home with 7 A/Cs of various sizes in various sized rooms with plenty of and most windows facing north or south I've always found these BTU calculators to over estimate the BTU requirement by 10 to 20% for around a 25 to 26C desired temperature which I use in my home.  But you want a little lower temperature so the BTU requirement would be a little higher.

 

For your room you are looking at a 9-12K BTU requirement based on my experience.  A 12K BTU would be a safe choice.

Now  you just need to choose between a fixed speed (standard) A/C or inverter A/C.  I would go inverter.

 

Yeap, I think wpcoe's rule of thumb of 600 BTUs per square meter of space is a good rule of thumb for use in Thailand. 

[bangkokpoppys]

excellent notes and calcs - thanks so much.

 

for the south facing - all very cheap construction with no film/single pane/solid wall etc.. 

 

i'd say the roof unlikely to be insulated also.

 

will update here how it works out, thanks !

 

[BEVUP]

As the buildings are crap I would be going straight out & buy a 18 btu Previously had a 12 BTU

BTU is worked out on the perfect house

4 x 5 is pretty much a bedroom these days & I put a 12 BTU in & had to make sure I / we slept under it - The roof was not Ins. / Sun rise coming through double glass door / & sun on the Sth wall 

In the end I moved the 12 BTU to a smaller room (works great ) & put a new 18 BTU in

 

As a laugh I said to the Aircon man I will also put Whirly Birds in for better results - His reply ... No need you now have a bigger aircon (no idea about efficient housing )

[Khun Jean]

9000 Would even work. 12000 is pretty large for that size. I don't think there is something in between.

A high ceiling is on your advantage as the hot air will rise and cool air where you live is only needed for the first 150-180 cm from the floor.

Prevent leakage of the cold air under doors or through windows.

A bigger size is often not better as it does not run long enough. It will just start and stop to many times. Another part of airconditioning is removing the water from the air. A bigger capacity aircon will not do that resulting in higher humidity that feels warmer.

I have bought a few aircons in Thailand and i much prefer the 'inverter' ones. They run much sdmoother and not unimportant cheaper.

I use 9000BTU in a 30m2 room and it works just fine. It is a condo with no walls exposed to direct sunlight so that is a big difference. The advice was to use a minimum of 13000BTU, best ignored advice ever. :)

 

 

 

[Pib]

Like I mentioned earlier I think most of these A/C size calculators end of "over-sizing" A/Cs for many people....not all people...just many.   Especially for those folks who want the temp around 25-26C vs around 21-23C.   I'm a 25-26C type person.  But the calculators get the person in the ballpark on a properly sized A/Cs which is the goal of the calculators.

 

Here in my Bangkok home I have a 20K BTU fixed speed in my home's master bedroom which is a  little over 40m2 in size...lets just call it a 6.5m X 6.5m sized room for A/C calculator entry which gives 42.25m2.   This A/C was sized/picked by the home builder around 10 years ago when I bought home new.  This room has BIG/lots windows facing north and south side and few windows on the east side...but there are good curtains on all these windows which are mostly closed during the day.

 

Now when I use the calculator in the weblink I gave in my earlier post it says I need a 20K BTU A/C.  See snapshot of results below.  Wow...perfect match with what the home builder installed. 

 

But I'm here to tell you an A/C around 15K BTU would have been a better choice from a less KwH/lower electric bill standpoint  since the room is little occupied/used during the day...predominately used at night for sleeping after the sun has went down, less cooling needed, etc.  And even 15K BTU would have really been good enough during the day for occasional say use/taking an nap/watching a few hours of TV on the bed assuming you were happy with a "25-26C" temperature. 

 

If wanting a 21-23C temperature then I doubt the 15K BTU A/C could have reached that room temperature "during the day" in the summer...or it's compressor would have been running 100% of the time eating a lot of electricity and putting full stress on the compressor.  Reaching 21-23C during the HOT summer at night would probably be doable but would have taken an hour or two to get to that temp I estimate.  During the less hot time of the year it could have probably reached 21-23C in less than an hour.   But I don't do 21-23C....I do 25-26C...even 27C at night.

 

Picking the right-size A/C is part science, part art, but most of the A/C sizing calculators don't use the "part art" factor because it can vary some much from person to person so this is why I think most A/C calculators end up over-sizing an A/C for many people.    Being a little over-sized is definitely better than being under-sized....and being too much over-sized is bad as it probably going to not make the room feel comfortable to you and/or eat too much electricity. 

 

Yea, part science and part art in right-sizing an A/C to get that happy ground between good cooling and lowest Kwh usage.

 

 

[bangkokpoppys]

Thanks everyone, installing an 18k BTU - will let you know how it goes!

[sometimewoodworker]

On 05/04/2018 at 5:48 PM, bangkokpoppys said:

Thanks everyone, installing an 18k BTU - will let you know how it goes!

You have decided to install an AC that is way oversized so unless it's an inverter then it's not going to run long enough to effectively drop the humidity.

[bangkokpoppys]

On 01/05/2018 at 4:17 PM, sometimewoodworker said:

You have decided to install an AC that is way oversized so unless it's an inverter then it's not going to run long enough to effectively drop the humidity.

Hi, after a month of operation turns out it's well sized, perhaps a little under. One side of the room is open so we lose a lot of cool air, and the windows on one size let in more heat than I imagined. But all good. 

[mogandave]

Glad it worked out for you.

[sometimewoodworker]

On 5/12/2018 at 1:41 PM, bangkokpoppys said:

On 5/1/2018 at 4:17 PM, sometimewoodworker said:

You have decided to install an AC that is way oversized so unless it's an inverter then it's not going to run long enough to effectively drop the humidity.

Hi, after a month of operation turns out it's well sized, perhaps a little under. One side of the room is open so we lose a lot of cool air, and the windows on one size let in more heat than I imagined. But all good. 

All advice and sizing was based on your stated room size of 5 X 4 (rooms can be assumed to have 4 walls)

You now say you have 5 X a lot bigger or 4 X a lot bigger, so without that information any recommendations would automatically be wrong.

 

However good to know that your purchase wasn't bad.