View Full Version : Sizing aircon

07-05-2005, 08:11 PM
Hi anybody got a quick rule of thumb I can use for sizing up a room/conservatory for air-con.

Only need a rough guide based on floor area.

Thanks Morph

07-05-2005, 09:18 PM
I was about to add my tuppance worth, until you said, conservatory!!
Wot.................Conservatory!!!!!! ????

This topic has been covered to death before, if only I can find the darned link!!

take length and width in feet, multiply by 150 , that gives you Kw rating.

Dont ask me how it works, it does somehow. But that rule applies to normal rooms, not conservatories and all those unusual glass cladded south facing places stuffed with a thousand people high on ecstacy and swinging their bodies about.

hope that helps!!


07-05-2005, 09:21 PM
I forgot to say as well............ask B& Q how to do it, ie: size up one of their systems for conservatory

If that dont work, the wholesalers will be only too happy to help.

Good luck

08-05-2005, 01:20 PM
50w/cubic metre
and some common sense.

10-05-2005, 11:04 PM
For a standard room i would use 150w/m2 or 50btu/ft2
for a computer room or PABX room 190w/m2 or 60btu/ft2
However a conservatory isn't so easy as it depends on the buid and orientation. for North and west the 190w/m2 is probably ok but for south/east it is better to give the wholesalers the full details - glass area/ orientation, material ect and get them to size it for you :D

13-05-2005, 04:37 PM
email me at dcoles@powercold.com and I'll send you a simple formula you can use in most cases.

Why not post the formula so that we can all benefit :)

14-05-2005, 05:07 PM
I'm sorry, but that is double dutch (sorry peter) to me. I.ve written it down but it does not make any sense.

Is that an English expression Frank? Never heard of it before.
What does it mean anyway

14-05-2005, 06:53 PM

If you ask thumb rule 10fx10fx10f=1000sq/ft=1ton or around 12000 BTU at 42c ambient at 30% RH , increase
in RH will lead to better efficency of the A/C. Try it out
it will work perfectly in tropical climate.

15-05-2005, 02:21 PM
How are you able to edit my post Peter?

Double dutch is an expression we use to mean " it does not make much sense" - i.e. like trying to understand your mother tonge posts when we don't speak your language :)

15-05-2005, 09:41 PM
Ohhhh.... sorry for that Frank, that was not my intention at all.
These are the privileges of the moderators.
I must have pushed on the edit button instead of the reply,..again...sorry,... this was not my intention.
Can your remember the original post Frank so that it can be corrected again?

17-05-2005, 06:53 PM
Oh dont worry Peter.............it happens.......
wasnt the first and definately wont be the last.....

Frank, dont read too much into the Double Dutch thingy.............chill!!!


Andy W
14-06-2005, 07:17 PM
AC calc by cliksoftware still gets my vote, mine is version 2.8 and still works fine, I was going to update it to the latest copy but I cant see the point, the only thing it does not work out correctly is conservatories but that is what rule of thumb is all about and I have not sized one yet that does not work.

Andy W
14-06-2005, 07:24 PM
Hi anybody got a quick rule of thumb I can use for sizing up a room/conservatory for air-con.

Only need a rough guide based on floor area.

Thanks MorphLength x width x height x 90 watts per cubic metres = required duty or length x width x 300 watts per square metre = required duty, I always work them out both ways and go for the bigger duty, always better to be safe than sorry.

17-12-2005, 03:47 PM
If you are going to do correct calculations you will need ambient conditions for the area before you can do anything else. I will not even try to enter the discussion using tons and BTU's..forgot those years ago.

02-01-2006, 12:51 PM
Panasonic offer a simple(ish) calculator on their .com website

03-01-2006, 06:52 AM
question. Why do people still use IP instead of SI units.
I thought the metric system was introduced to the US 5 yrs ago?? There is a chart I've been trying to get hold of that the old SEQEB had for room sizing.

I've been told .075kw/m2 + .025kw/m2 for each half door window. If higher than 8" ceilings add .025kw/m2 for each meter above.

Seems to be close on small applications. Does not take into consideration insulation etc etc etc.

I think DOE2 programs does the calc?

If anyone knows where I can get the table please email me at refrigerationmagnet@lycos.com

04-01-2006, 11:44 PM
The Australian energy rating department recomends 125watts/m2 in living areas and 80watts/m2 in bedrooms.

Question. is there any posible way of working out KW cooling/heating cap when only given a Horsepower rating? I know the conversion but that is only motor kw not capacity no?

06-11-2006, 04:41 AM
Not sure about Australian Energy standards but those specs won't cut it in Queensland,

155- 200 watts per square metre depending on heatload.

Sure those standards will save power but they won't cool the rooms unless it's night time. :D

12-11-2006, 05:32 AM
Hi anybody got a quick rule of thumb I can use for sizing up a room/conservatory for air-con.

Only need a rough guide based on floor area.

Thanks Morph

Hi Morph,

Please take note that RULE OF THUMB FIGURES should never be used for huge places with so many factors that may affect the cooling load e.g. glass walls, glass windows, skylight, building walls, ventilation/infiltration load, etc.

Please check this one:


I hope i have shared something that could be valuable. . . :)


11-01-2007, 10:35 PM
Whenever someone asks me for a unit for a conservatory I first ask why they think they one one, most say to stop it getting too hot whilst they are out! Then I tell them how much it would cost to run the largest wall mount we can get (8kw) and ask if they are still interested, most say they only want to spend less than 500.00 so it's off to B&Q to buy a waste of energy portable.

20-08-2007, 08:24 PM
Well I don't know about anyone else, but I've done several of the suggested calcs on a standard model & have come up with wildly varying results!
The standard model I'm using is my appartment in the Algarve, which is roughly 8m x 6m with an average ceiling height of 3m open plan. Max ambient mid 30's.
I'm gonna bung a 7-8 kw inverter in it. Anyone see any problem with that? :confused:

28-08-2007, 02:22 PM
Try using this great little app for converting, Area, Units, Power, Ect I use it alot its a very usefull program and its free.



I dont use this for making the calculation for what size unit I should be using its not for that but I tend to use:

1) rooms that dont generate to much heat 120w * m2

2) rooms that can get quite warm 150w * m2

3) rooms that get very warm 190w * m2

Then use, Versaverter to convert into BTUs, KW's, Kcal, HP, Ect.

Also you need a little comonsense as well, after doing this for a while you can normally tell as soon as walk into a room and asking a few questions. Large rooms though def do more work on and make sure its correct. beter to go slightly over size than under, be safe not sorry.

Sid Jones
02-09-2007, 12:57 AM
Hi anybody got a quick rule of thumb I can use for sizing up a room/conservatory for air-con.

Only need a rough guide based on floor area.

Thanks Morph Try 120 watts per square meter.

Sid Jones
02-09-2007, 01:07 AM
For your normal apartment requirements 120 watts per square metre should do the trick. I am practicing in South Africa and it works for me.

Good luck regards.
Sid Jones.

02-09-2007, 06:34 AM
Shouldn't we be discouraging rules of thumb? Shouldn't we be pushing doing it the correct way. Seems like a bad idea to get something like that wrong in such a big application. Just my opinion.

03-09-2007, 09:33 PM
Cheers Sid, 6Kw sounds reasonable to me!

05-09-2007, 08:48 PM
Hi anybody got a quick rule of thumb I can use for sizing up a room/conservatory for air-con.

Only need a rough guide based on floor area.

Thanks Morph

Here we use 400 sq ft per ton

08-10-2007, 09:31 AM
in the philippines common conversion of capacity per area is : 550 ~ 650 Btu / m2 depending on the application.

09-10-2007, 10:51 PM
Hi all

Said this before but, I dont generally dont do rule of thumb when sizing, but when i do i never go below 125w per square meter in an office / domestic install and between 250w and 400w per square meter for conservatories depending on which way they face.

If in doubt always ask some one who knows, most suppliers will size for you on the back of maybe getting an order.

I tend to normally advise a 5kw inverter heat pump as they are normally on manufacturers smaller chasis, neater looking and lower price bracket, which would provide ample heating in winter and cooling for 95% of the UK summer, even on the hottest day it would cool the conservatory to an aceptable temperature to sit in.

Thats my thoughts any way

C Bagtas
16-10-2007, 11:20 PM
For me 12000 Btu is enough coz we have always low ambient temperature.Correct me if im wrong.