View Full Version : Air Conditioning room sizing calculations

15-03-2014, 12:33 PM
I am wondering if anyone knows of a standard way to work out the required system size for a room, and the calculations you would use to work this out.

I have looked online for a calculation to work this out and it varies a lot between each way of doing it...

so far I have found: multiplying the length of the room by its width to find the square footage and from this a table showing where that number falls and the amount of btus for that size room...it also says to add 10% to capacity for heavily sunny lit rooms and decrease by 10% for heavily shaded rooms...add 600 btus for every extra person if more than 2 regularly occupy the room...and add 4000 btus if its a kitchen or on second floor.

I have also found one: find the square footage and multiply by 25 to convert it into btus for that size room... then add x amount for heavily lit rooms etc same as above.

another I found was length x width x 337 and then add x amount for same variables as other 2 above.

But I have also tried calculators on other sites where you add the length width and height and then it calculates the total for you.

All of these methods create a vast difference in the total result, the top three methods are quite similarly matched but the other method adding the height of the room gives you virtually double the other 3 ways??

Is there an exact calculation which will give good accurate results every time.

The reason I am asking is I don't usually deal with room sizing as the place I work does it all and are reluctant to divulge it with me. My dads friend is having a new building built and wants an ac unit in the basement and was told to get a 7.6kw unit by the person hes buying it from on ebay but asked if I could confirm it for him, the methods I used above give different results and so am unsure what to suggest for him.

The basement is going to be 10m x 6m x 2.5m high.

I figured if anyone knows a calculation which is reliable it is something I can make note of for possible future use.

The size I got with the top method worked out at 3.7kw so use a 4.1kw unit.

The second came in at 4.7kw so use a 5.3kw unit.

so as you can see there is quite a big difference just between those 2, the calculators where you add the height of the room basically add about another 2kw to the total size. :confused: :confused:

install monkey
15-03-2014, 12:43 PM
a basement should have no solar heat gain- typically the ground externally should be 10-15 deg- therefore im assuming its only going to be used for heating- go for a 5 kw unit- heat outputs are higher than cooling outputs- as the room is too big for a wall mount - i would recommend an underceiling unit or

15-03-2014, 01:09 PM
When I worked it out I just did a standard calculation with nothing added for heat gain as it is all going to be below ground. Im not 100% sure whether the cooling side will be used, I think it will be mainly heating because as you say it will be cool with being below ground but they are looking at keeping cats down there to breed them. I think the unit he was looking at buying was a 7.6kw mitsi highwall and I think it is being left as an open plan area so airflow down the room would be good.

When he told me it was a 7.6kw unit he was looking to buy and had been recommended I thought it sounded very big especially with it being below ground, but like I said in first post I don't deal with room sizing.

I don't suppose you know of a calculation I could have to hand for future reference do you?

asking online etc is only way I pick stuff like this up because at our place no one shares information...im guessing they wouldn't tell me because they'll be thinking im doing loads of jobs on the side...but im not going to see my dads best mate pay 30 - 35ph for something I can install for him for a couple of beers....just a case of making sure the unit is adequate for the job.

install monkey
15-03-2014, 01:29 PM
i use area of room length x width x 110w/m2 or 150w/m2 for a conservatory
typically a wall mount will give an air throw of 4 mtr- and the ceiling is low at 2.5m- if theyre breeding cats then just stick a radiator down there;)

15-03-2014, 01:52 PM
Ha ha, well I must admit I thought the same thing with regards the radiator but I think hes got it in his head he wants an ac unit in there so hell stick to his guns no doubt. I think he just wants to make sure it has something in there to keep the air circulating as well as providing some temperature control as well.

Looking at it then the second calculation is actually pretty accurate isn't it, I actually cant believe there are so many different ways to work it out. I was expecting there to be maybe one or two variations to work it out and them all to be very similar in results but there is quite a difference between them.

I guess its more just a case of finding what works best for you and sticking with it.

Thanks for your help, much appreciated.

install monkey
15-03-2014, 03:23 PM
you can go into wal properties, glazing types and roof construction, which orientation , lighting loads- people loads- just more room to balls up the calculation

15-03-2014, 06:36 PM
i use area of room length x width x 110w/m2 or 150w/m2 for a conservatory
typically a wall mount will give an air throw of 4 mtr- and the ceiling is low at 2.5m- if theyre breeding cats then just stick a radiator down there;)

10 x 6 x .110= 6.6kw.

that's why he wants a 7kw unit I guess...

install monkey
15-03-2014, 07:00 PM
cats have fur- apart from them weird ones:p

10 x 6 x .110= 6.6kw.

that's why he wants a 7kw unit I guess...

15-03-2014, 08:52 PM
He's also going to need fresh air ventilation with corresponding extract system.