# How can i do the correct sizing for split air conditioner?

## Main Question or Discussion Point

hello everybody,
in fact,im asking for a help in a matter confused me alot,its about how can i select the right (split,mini split,portable,window,etc) air conditioner,which is not central air conditioner.
im working now in a company and they size the air conditioner for the customer according to the floor area,when i asked about it"how to size?",they told me the"3*3*3"m^3 is a size for 1hp air conditioner,that was the 1st confusion,when they said hp,as a unit i thought of it as its the conversion from the BTU to the hp,and when i calculated it it didnt give their hp they work with;),anyway thats not what im asking about for now.

so,i figured out that they divide the BTU by 8,for ex:the 12000btu=1.25hp,the 18000=2.25hp and so on,thats not my point now,my point when i googled how to size i got this table which if i know the floor area in feets i'd get the estimated size of the right air conditioner with the BTU then i'd convert it as they do to hp,thats the link of the table,please take a look, http://fedders.stores.yahoo.net/sizingguide2.html [Broken] .

i found that this table doesnt match at all to the values they working with,and the boss told me thats too much for their sizing for the split air conditioner,of course i converted from feets square to meter squares,as thats the metric units we work with here.
so my question is:

how from the floor area i find the right size for the split air conditioner?

in BTU and i'd estimate it to the hp as they convert it,and would like to find a table like that one but in meters square;),may it would be a bit different estimation?,just confused so please help from anyone here knows about that,or have worked in the air conditioning field before,and especially the small units,the splits,i know if it was central we calculate the cooling load and then we find the capacity as ive studied at school.

thanks alot for ur time,and waiting ur reply.
Gaber.

and got me more confused,

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russ_watters
Mentor
I've never heard of an air conditioner sizing via horsepower, but I guess it isn't fundamentally different from using BTU or kW. Just make sure the guy didn't actually mean kW...

thank you for ur answer russ watters,but ive mentioned it wasnt the point sizing via hp,and yes i thought its the conversion from BTU to KW or hp,but it came different to their estimations,anyway,its not the point again,the point is:
how can i get the corrsct size for split air conditioner by knowing the floor area?,did u see the link ive attached?,it showed a table,if u worked in that field before how would u get the size for the air conditioner which is suitable for the place where it will be installed?
thanks for ur time.

russ_watters
Mentor
Sorry, I saw the post just before I went to work this morning and only scanned it...

I'm an HVAC engineer and I don't use rules of thumb for sizing air conditioning systems: I use software that calculates actual heating and cooling loads. Quite obviously, the load can vary considerably depending on which direction it faces, how much window area there is, how good the insulation is, and what floor it is on. For residential use, the software isn't very expensive and takes only a few minutes per room to do the calculation.

For my area (Philadelphia, USA), the chart you linked gives fairly generous sizing. The closest thing to a rule of thumb we use (a quick cross-check) is .25-1 cfm/sq ft and the chart is based on somewhere around .75. For a room that doesn't have an extremely large window area, a lot of southwest exposure, bad insulation, and a roof over it, that would probably be fine.

But since there is a fair amount of money to be saved in buying smaller equipment and even running a smaller electrical feed, I'd seriously consider getting some load calculation software.

thanks russ for ur answer again,
whats the name of the software u use?
is it for the small units and the central air conditioners?
i'd like really to do the right sizing for the air conditioner according to the actual cooling or heating loads,and there isnt a free software u use or know where i find?
and thanks for telling me that the table is a fairly one to use,but even the table when i showed to the boss he said it was too much area floor with comparison with the size of air conditioner,they do that quick check for the room area,but i couldnt get their rules of sizing,so i googled,and found the table,and when i showed it didnt match their estimations,but no problem,i only wanna know the right way,and i will do it,so im glad to get a help from an experianced hvac engineer.
thanks for ur time.

russ_watters
Mentor
Here's the software I use: http://www.elitesoft.com/
There are commercial and residential versions and demos. It tells you only the load - it shouldn't matter what type of a/c unit you use to provide the btus.

Where are you located - knowing your weather would help a lot.

thank you for ur help russ,ill try to get that software,i took a look at the site,and watched the short videa there,and it sounds great to use,and have too many informations,and so helpful.
thanks for ur time,and for the forum.

russ_watters
Mentor
No problem.

hello russ again,
ive got the software and its installed on my computer already now,but i didnt find anything with it about how to size the split air conditioner according to the area floor,all ive found is a duct work,and even didnt know where can i use it to calculate the actual heating or cooling loads,may you please tell me some details about the steps to follow to get the option to size the split ac?,and to calculate the cooling and herating loads?.
im sorry,but i tried so hard to get that software,and after i got it,i didnt get what i was looking for,so please may you help me now?
thanks.

the icon of the software after installing show a duct,and its name is"ductsize",so is that the same software u use?,or i missed the right one?,i downloaded it as the Elite software,so,how can i get the Rhvac calculations and sizing?,with it?,or this is a different software i should get also?,its the residential ac sizing what im looking for now,to size the small units which is splits,and not central ac,so as u see im a little confused now,so i ask you the help.
thanks.

hello russ,
i got the rhvac software also,its one of the elite software,and used it and till now,i dunno how to use it to calculate the cooling loads,may u help me telling me the steps to get it done?

russ_watters
Mentor
Sorry, I lost this thread for a few days...

When you click the preview report button, it gives you a report of the cooling and heating loads of the zones you created. The "zones" dialog box is where you enter most of the information - floor areas, wall areas, windows, etc. You'll need to enter wall types in the master data dialog and indoor/outdoor design conditions as well. And you create an air handler that picks up all the zones. If this is a single room, there is relatively little to enter for zone data (just one zone). But the other stuff does get a little involved.

I wouldn't worry abou the ductwork.

thank you russ for ur answer,
im doing a new project now with rhvac software,all what i wanna use it for is to size a split air conditioner to be installed in a room,so im giving it the inputs,but it asked me about two things i don understand it which is "latitude"&"elevation",what does that mean?
and also what is the "elevation derating".
that was in the general project data dialouge,
and what about the "duct sizing",what should i put about it in the software while im doing sizing for a single room?
thank you so so much-:).

russ_watters
Mentor
It is asking you where on earth the building is located. Just find the nearest city in the menu option for that. "elevation derating" takes into account the difference in density of the air for higher elevations. Ie, in a city like Denver, at an alt of 5000 feet, you need to move more air to get the same BTU rating out of an air conditioner.

I don't think you need to touch the duct sizing part at all - it isn't used in the btu calculation.

thank you russ,but may i ask you one more thing,being polite?
cant you just tell me what are the steps i should follow to size air conditioner"knowing how much cooling/heating load in BTU"for a single room which is:
length=4m&width=4m&hight=3m,including a glass window which is:width=1m&hight=3m
and a wood dorr which is:width=1m&hight=3m----this room is in appartement which is a bedroom.
what are the steps i should do with the rhvac to get the BTU?
SORRY FOR DISTURBING.
and thanks for ur time.

Sorry, I lost this thread for a few days...

When you click the preview report button, it gives you a report of the cooling and heating loads of the zones you created. The "zones" dialog box is where you enter most of the information - floor areas, wall areas, windows, etc. You'll need to enter wall types in the master data dialog and indoor/outdoor design conditions as well. And you create an air handler that picks up all the zones. If this is a single room, there is relatively little to enter for zone data (just one zone). But the other stuff does get a little involved.

I wouldn't worry abou the ductwork.
It is so helpful.. thank's a lot