Air-con Sizing?

[technocracy]

So I am now sorting out the air-con requirements for my new house - the smaller bedrooms are easy however the two large rooms I would like to get a proper heat gain calculation done. As I walked in a shop here (Laos) and asked what size air-con I'd need for a room 8m x 11m (two rooms with an archway between them) and a room 8m x 8m - I was told instantly 2 x 40,000BTU units. This finger in the air estimate seems completely over top in my opinion - considering a 4m x 4m room we are using the smallest Mitsubishi Mr Slim units (I have one in a room currently 3.5m x 3m and it easily cools that space - and sits on fan half the time rather than cooling).

Since putting a 40,000BTU unit in not only just saps huge amounts of power but also if oversized is total waste of money (and from my reading won't actually cool the space as good as a correctly size one). I just get the feeling it's the sell the biggest most expensive bit of kit they have routine! I am going to visit other shops obviously however I some how doubt anyone will sit down and calculate the BTUs - I have looked for calculations but cannot find any concise enough to trust.

If anyone can help the details of the rooms are:

8m x 11m - One north facing window with 2.5m(w) x 1.5m(h), one east facing window 2m x 1.5m - then the front door and a rear exit door front door is north facing and shaded, the rear is south facing being half glass french door.

8m x 8m (minus 4m2 where the bathroom walls are) - north facing double french doors and a south facing half glass french door

All help appreciated . . Cheers

[quiksilva]

It depends on a number of factors but the guy who just measured up our house told me that the calculation is the size of the room with a multiplier of between 700-800, depending on ceiling height exposure to sun etc. cooler rooms.

(8x8= 64) x 800= 51,200 BTU

Not an expert in the area mind but this seemed to tally with what I have read elsewhere.

[ProThaiExpat]

I would suggest that treating the large room with an arch as one room for airconditioning purposes is problematic. Why not consider to of the smallest units.

[Nawtilus]

Multiplying the square meters by 700 does give you a very reaonsable btu figure.

[technocracy]

Well I understand what you saying with this estimate but it's all a bit vague just multiplying the sq metres by 700-800.

I mean check out this air con sizer:

Air Con - sizing . ..

It takes into consideration windows, location, climate etc.

The house is north north easterly facing so this room gets very little sun throughout the day - lets put it this way come midday to 2pm when the builders stop for lunch they all go for a kip in there as it's the coolest place.

Also I have looked at this one also:

Another air con sizer

On both with measurements, window locations etc and giving an ample 1000 watt of power usage both give roughly 14-15k BTU. In the one with location I have gone for a worse case scenario of El Paso Texas as the location!

[Naam]

giving an ample 1000 watt of power usage both give roughly 14-15k BTU.

to reach that efficiency it might take a few years more of research and development. split units available in Thailand (and elsewhere) draw an average of 1100 watts to deliver 12-13000 btu/h.

[technocracy]

giving an ample 1000 watt of power usage both give roughly 14-15k BTU.

to reach that efficiency it might take a few years more of research and development. split units available in Thailand (and elsewhere) draw an average of 1100 watts to deliver 12-13000 btu/h.

As in 1000 watts of power usage on lighting, TVs, hi-fi draw etc (excluding Air-con)

[elkangorito]

Well I understand what you saying with this estimate but it's all a bit vague just multiplying the sq metres by 700-800.

I mean check out this air con sizer:

Air Con - sizing . ..

It takes into consideration windows, location, climate etc.

The house is north north easterly facing so this room gets very little sun throughout the day - lets put it this way come midday to 2pm when the builders stop for lunch they all go for a kip in there as it's the coolest place.

Also I have looked at this one also:

Another air con sizer

On both with measurements, window locations etc and giving an ample 1000 watt of power usage both give roughly 14-15k BTU. In the one with location I have gone for a worse case scenario of El Paso Texas as the location!

I would certainly use the info you got from the websites as long as it goes into all areas of heat gain, which includes the ambient temperature - for Thailand, 35 degrees Celsius.

Please use kilowatts & not BTU - they're easier to work with. There is not a simple solution to a/c size calculation. Any good refrigeration mechanic will carry out a proper survey & not use a "multiplier" to arrive at the final figure.

There was a situation once (when I was the facilities manager for Sydney Olympic Park) where I had to upgrade the a/c in one of the buildings. I used 2 companies for quotation purposes. One company arrived at a figure, which looked surprisingly small to me, so I asked him how he calculated it. He said that from 'experience' he arrived at his final figure. The other guy used a software package & he came up with a rather large sized unit. I asked him why & he was able to logically explain everything. Naturally, he got the job.

Don't listen to people who 'guess' - see some calculations or figures. There is no such thing as a standard, one step 'multiplication' factor when calculating the size of any a/c unit.

Edited March 16, 2007 by elkangorito

[Gary A]

That sounds like it is way to much capacity for the area. I would think one 40,000 BTU unit would nearly handle it. BUT, I may be inclined to go with the two 40,000 BTU units simply because you may be able to turn one unit off after the area is cooled off. It's better to have a too small unit than a too big of a unit because too big will not efficiently remove moisture from the air because it doesn't run enough. Too big a unit will give you a cold clammy feeling because of excess humidity. Just think of it as having a spare.

I have a 9,000 BTU for my 18 square meter bedroom and it does a great job.

[Naam]

giving an ample 1000 watt of power usage both give roughly 14-15k BTU.

to reach that efficiency it might take a few years more of research and development. split units available in Thailand (and elsewhere) draw an average of 1100 watts to deliver 12-13000 btu/h.

As in 1000 watts of power usage on lighting, TVs, hi-fi draw etc (excluding Air-con)

there is no difference in power usage whether it is lighting or airconditioning. watts are watts and amps are amps. the only difference is (for a split second) the starting amps.

[Naam]

It depends on a number of factors but the guy who just measured up our house told me that the calculation is the size of the room with a multiplier of between 700-800, depending on ceiling height exposure to sun etc. cooler rooms.

(8x8= 64) x 800= 51,200 BTU

Not an expert in the area mind but this seemed to tally with what I have read elsewhere.

(8x8= 64) x 800= 51,200 BTU = ARRRRRGGGGHHHHH!

[technocracy]

giving an ample 1000 watt of power usage both give roughly 14-15k BTU.

to reach that efficiency it might take a few years more of research and development. split units available in Thailand (and elsewhere) draw an average of 1100 watts to deliver 12-13000 btu/h.

As in 1000 watts of power usage on lighting, TVs, hi-fi draw etc (excluding Air-con)

there is no difference in power usage whether it is lighting or airconditioning. watts are watts and amps are amps. the only difference is (for a split second) the starting amps.

I know, I know - a 1000 watts is 1000 watts - the 1000 watts was provided as an example of addition heat what would be in the room .... i.e. this 1000 watts is what the A/C will also be cooling/fighting against, along with the bodies in the room etc. The calculations on those pages take into account what amount of watts usage/heat is generate in the room by the equipment in it.

elkangorito - my thoughts exactly! After thoroughly looking into it I think I'll be purchasing two 15k BTU (sorry 4400 watt ) air cons. Yes the first link takes into account the locale and have just re-done it based on Miami - which looking at average temperatures through the year has pretty much the same climate as Vientiane. Since we get a bit more of a temperature change in the 'Winter' months than say Bangkok and it comes up with 4400 watt - if I change it to Yuma in Arizona (one of the hottest town in the US) it only goes up to 26400 BTU/7850 watts.

Dr Naam - (8x8= 64) x 800= 51,200 BTU = ARRRRRGGGGHHHHH! - Very true!! Anyone would think I am cooling a 200 sq metre office! I really don't understand how anyone could accept such a calculation! The don't actually make a 'home' unit large enough to cool that amount of heat (well Mitsubishi don't anyway!)!!!

But cheers all for the replies it's set my mind at ease that I am not just being a tight arse - but a sensible one at that

[panaipon]

giving an ample 1000 watt of power usage both give roughly 14-15k BTU.

to reach that efficiency it might take a few years more of research and development. split units available in Thailand (and elsewhere) draw an average of 1100 watts to deliver 12-13000 btu/h.

As in 1000 watts of power usage on lighting, TVs, hi-fi draw etc (excluding Air-con)

there is no difference in power usage whether it is lighting or airconditioning. watts are watts and amps are amps. the only difference is (for a split second) the starting amps.

I know, I know - a 1000 watts is 1000 watts - the 1000 watts was provided as an example of addition heat what would be in the room .... i.e. this 1000 watts is what the A/C will also be cooling/fighting against, along with the bodies in the room etc. The calculations on those pages take into account what amount of watts usage/heat is generate in the room by the equipment in it.

elkangorito - my thoughts exactly! After thoroughly looking into it I think I'll be purchasing two 15k BTU (sorry 4400 watt ) air cons. Yes the first link takes into account the locale and have just re-done it based on Miami - which looking at average temperatures through the year has pretty much the same climate as Vientiane. Since we get a bit more of a temperature change in the 'Winter' months than say Bangkok and it comes up with 4400 watt - if I change it to Yuma in Arizona (one of the hottest town in the US) it only goes up to 26400 BTU/7850 watts.

Dr Naam - (8x8= 64) x 800= 51,200 BTU = ARRRRRGGGGHHHHH! - Very true!! Anyone would think I am cooling a 200 sq metre office! I really don't understand how anyone could accept such a calculation! The don't actually make a 'home' unit large enough to cool that amount of heat (well Mitsubishi don't anyway!)!!!

But cheers all for the replies it's set my mind at ease that I am not just being a tight arse - but a sensible one at that

technocracy

I think you should be careful about sizing of the A/C. I do agree that 51k is a little large.

It depends on what that room will be used for. If you plan to use it as living room, then I

believe that you shouldn't go below 48k BTU myself. If you size the A/C too small, the compressor will run 100% all the time. That shorten livespan of the compressor and more important, it will consume lots of power. In the long run I think you should go for a bit large than a bit too small A/C. I am not sure exactly how the layout of the room looks like, but I think there should be more than 1 A/C unit there.

You can send me the layout and I can place the A/C for you. My email is [email protected]

Nat

[Naam]

If you plan to use it as living room, then I

believe that you shouldn't go below 48k BTU myself. If you size the A/C too small, the compressor will run 100% all the time. That shorten livespan of the compressor and more important, it will consume lots of power. In the long run I think you should go for a bit large than a bit too small A/C.

AAAAARRRRRRRGGGGGGHHHHHH!!!!

GOOD LORD BUDDAH... why do you let it happen that people who have quite obviously no freaking idea are allowed to give technical advice?

:D

[elkangorito]

If you plan to use it as living room, then I

believe that you shouldn't go below 48k BTU myself. If you size the A/C too small, the compressor will run 100% all the time. That shorten livespan of the compressor and more important, it will consume lots of power. In the long run I think you should go for a bit large than a bit too small A/C.

AAAAARRRRRRRGGGGGGHHHHHH!!!!

GOOD LORD BUDDAH... why do you let it happen that people who have quite obviously no freaking idea are allowed to give technical advice?

:D

Maybe these people are 'experts', ex being an unknown quantity & spurt being a drip under pressure?

[Naam]

for the umpteenth time some facts about sizing an airconditioning unit:

-dozens of factors (adding up to the total heat load) play a role.

-too big a unit (than needed) generates WORSE effects than a unit which is too small.

-a continuously running small compressor lasts LONGER than an off/on off/on off/on big compressor.

-longer running time of a small compressor does not equal higher power consumption.

-smaller compressors (a/c units) -running longer- are dehumidfying much more than big compressors running a short time only thus adding to personal comfort due to lower relative humidity.

-in the case of the OP seeking advice TWO units (albeit more expensive initially) are the solution. the units should be placed in a 90º angle.

-capacity of the units depends mainly WHAT temperature (delta t to outside temperature) and relative humidity the OP is trying to achieve in the area.

and... and... and...

[technocracy]

-in the case of the OP seeking advice TWO units (albeit more expensive initially) are the solution. the units should be placed in a 90º angle.

With you on everything . . . but not quite sure on the two unit idea - the two rooms downstairs which I am treating as a single room are joined by a 2.5m wide arch which is nearly 3 metres tall. The sizer on the first link I gave says:

'If the room is connected to another by a permanently open door or archway more than 5 feet wide you should consider it as one room; skip this step and go on to step 6. Otherwise, enter the width of the door or archway into the box.'

Also the middle room (one attached by the arch) is totally shaded from sunlight all day long being NNE facing and having balconies to the front and rear. Also it's not a living area as such rather a entry hall - putting this together makes me reluctant to install an addition unit for this space alone.

The upstairs bedroom is 100% certain to be a 4400watt (15k Btu) unit.

[Naam]

With you on everything . . . but not quite sure

where do you live Techno? i bring the wine, we take a look at the rooms, you give me a few minutes and then we open and empty the bottle. what say you?

[Gary A]

Well Dr. Naam, you told it how it is and gave good advice. Unfortunately some people like to post and even though they have no idea what they are talking about they will represent themselves to be experts. Sometimes they are able to totally confuse the issue.

Edited March 18, 2007 by Gary A

[panaipon]

for the umpteenth time some facts about sizing an airconditioning unit:

-dozens of factors (adding up to the total heat load) play a role.

-too big a unit (than needed) generates WORSE effects than a unit which is too small.

-a continuously running small compressor lasts LONGER than an off/on off/on off/on big compressor.

-longer running time of a small compressor does not equal higher power consumption.

-smaller compressors (a/c units) -running longer- are dehumidfying much more than big compressors running a short time only thus adding to personal comfort due to lower relative humidity.

-in the case of the OP seeking advice TWO units (albeit more expensive initially) are the solution. the units should be placed in a 90º angle.

-capacity of the units depends mainly WHAT temperature (delta t to outside temperature) and relative humidity the OP is trying to achieve in the area.

and... and... and...

I agree with you to certain extent. We are not designing a coldstorage which we have to consider lots of factors ie. workers, machines etc. This is just a house in Laos and I assume that the temperature are not much differ from Thailand. We can just use a rule of thumb to calculate the heat load. 700-800 BTU/Hr/m2 is a good estimate.

Btw. we never place a/c 90 deg angle coz of the air throw. We need as much air throw as possible. By having cross wind, the air-throw can be reduced.

[panaipon]

Well Dr. Naam, you told it how it is and gave good advice. Unfortunately some people like to post and even though they have no idea what they are talking about they will represent themselves to be experts. Sometimes they are able to totally confuse the issue.

Sorry to hear this Gary.

I was just giving what I think was right.

I never consider myself as expert.

Just an ex-YORK guy.

[Naam]

The upstairs bedroom is 100% certain to be a 4400watt (15k Btu) unit.

that depends on the "usage". both you and your wife going to bed at the same time (no pun intended!)? what is your desired temperature for sleeping? when do you get up? when does your wife get up? you have to take individual needs into consideration.

example: our master bedroom is approx. 38m² with considerable heat load (three outside walls - south and west with direct sun).

i am going to bed at 11.30, prefer a temperature of 18ºC when i go to bed but cannot stand a temperature below 26ºC a couple of hours later.

my wife (usually) goes to bed when i get up and that is each and every day at 04.00hrs. she would like to sleep at an ambient temperature of MINUS 25ºC and if possible having a couple of standing fans besides the ceiling fan on full blast.

bedroom warms up during daytime outside temp minus 7-8ºC.

solution: two units each 13.000btu/h switched on full blast around 22.00hrs = 18-20ºC @ 11.30hrs depending on outside temperature. before i settle down and listen to my pillow, i switch off one unit and adjust the other at lowest fan speed to 26ºC. if the nights are not too hot the compressor of the running unit will not start till i get up, respectively my wife goes to bed. the remote controls of both units as well as the control of the ceiling fan are placed between our pillows. my wife then can feel free to abuse the remotes as she pleases

[rgs2001uk]

where do you live Techno? i bring the wine, we take a look at the rooms, you give me a few minutes and then we open and empty the bottle. what say you?

DrNaam

i hear you, i feel your pain.

normally us mere mortals bow down in the presence of your superior knowledge.

however, may i have the audacity to humbly beg a slightly different approach.

you take a look at the room, Techno supplies the wine, you empty the bottle with or without his help.

[technocracy]

<deleted> - To clarify this thread I fully understand that Air-con estimation (the important word is 'estimation') is based upon many different requirements and dependancies. My original request was for someone who had a fully detailed calculation to check my requirements. Although I have found a seemingly decent online resource for this - but I don't believe everything I read! Hence I was after someone with some actual knowledge of performing detailed calculation.

It is a willy waving excercise of who knows the most about air con - I am just after considered thoughts as I want to get the sizing right. Is that too much to ask?

As for our temperature requirements - 25C is plenty cool enough for both me and the wife, I ain't into this walk in cold store feeling some folks like!

[ThailandTony]

I have a 78 m2 living room, with a 3m high ceiling and average wall/ceiling insulation. I have 2 units, 1 x 18k & 1 x 24k positioned at opposite ends of the 13 x 6 room. They eat up my a/c requirements for breakfast. 2x 40k is super overkill and v. expensive, Your gut feeling was correct, they are trying to oversell you, and a/c systems increase in cost exponentially so they are really spanking your monkey. If you want to play safe go for 2 x 24k split units and you'll never look back. Also if 1 goes wrong you'll still have the other as back-up. The secret is a high ceiling and lower FCU's. Mine are at +2.2m and ceiling height is +3.0m so they deliver the cold air below the warmest air and from either end so they balance the temperature across the room. 6 months of the year I only need to use 1 or no units so I am happier not having a very expensive a/c not working half of the year. Good luck.

[Naam]

The secret is a high ceiling and lower FCU's. Mine are at +2.2m and ceiling height is +3.0m so they deliver the cold air below the warmest air and from either end so they balance the temperature across the room.

BINGO! high ceilings are something that a/c "experts" worldwide misinterpret when considering volume in capacity calculations. volume counts of course when central airconditioning or ceiling units are installed. but high ceilings (= >volume) with rather low mounted split units help to save energy. all my rooms are 3.50m high and the a/c inside units are mounted at about 2.50m.

[Naam]

If you want to play safe go for 2 x 24k split units and you'll never look back. Also if 1 goes wrong you'll still have the other as back-up.

i would consider even that as overdimensioned Tony but agree with you on two units. my living room is open to entrance hall, dining room, library and tv-room. total area approximately 150m² with 5 units, each 13k btu/h, installed. until now it was never necessary to switch on more than 2 units to keep the whole area at ~26-27ºC.

one caveat though... i moved into the home in august last year and have not yet experienced the real hot months april and may. also worthwhile to mention is that i made great efforts as far as outside walls (20cm superblock), windows (reflective gold coating as in hotels and office buildings), ceiling insulation and attic ventilation is concerned.

[Naam]

It is a willy waving excercise of who knows the most about air con - I am just after considered thoughts as I want to get the sizing right. Is that too much to ask?

As for our temperature requirements - 25C is plenty cool enough for both me and the wife, I ain't into this walk in cold store feeling some folks like!

it is still a difficult task Techno. something like asking a surgeon over the phone how he would treat a patient

shooting from the hip i'd say that two 12-13k units are sufficient to achieve a temperature of 27º with a comfortable low level of humidity. but if you aim for 25º when outside temperatures are 35º or above you should go for two 18k units.

[Naam]

where do you live Techno? i bring the wine, we take a look at the rooms, you give me a few minutes and then we open and empty the bottle. what say you?

DrNaam

i hear you, i feel your pain.

normally us mere mortals bow down in the presence of your superior knowledge.

however, may i have the audacity to humbly beg a slightly different approach.

you take a look at the room, Techno supplies the wine, you empty the bottle with or without his help.

is that negotiable RGS? what about Techno AND me supplying wine?

[elkangorito]

I agree with you to certain extent. We are not designing a coldstorage which we have to consider lots of factors ie. workers, machines etc. This is just a house in Laos and I assume that the temperature are not much differ from Thailand. We can just use a rule of thumb to calculate the heat load. 700-800 BTU/Hr/m2 is a good estimate.

Btw. we never place a/c 90 deg angle coz of the air throw. We need as much air throw as possible. By having cross wind, the air-throw can be reduced.

Does your 'rule of thumb' include thermal ceiling insulation? Type of wall/ceiling construction material? etc etc. I supposed you could go either way with the rule of thumb approach - too big or too small.

On the other hand, survey the site correctly & your chances of having an economical & efficient system increase dramatically.

Technocracy, I'll email a mate of my'n in Oz & hopefully he will send me some a/c survey documents, then we can all move away from speculation.