How to calculate BTU for aircon

as above, want to find out if current is undersize or contractor don't oversize me too much.

Go see your PAP MP lor.

as above, want to find out if current is undersize or contractor don't oversize me too much.

http://www.hearth.co...-bin/btucalc.pl

You can assume 150W/m2 or 500BTU/m2. But it all depends on whether you need the AC in the day or night, orientation of room, how much heat generating stuff you have...

Simple way / a rough idea, is to get the square area of the room in M2 and multiply by 550
or if you're more a imperial person, Feet2 x 60, assuming a standard 2.5M 0r 8.5 Feet ceiling height.

But it's always better to oversize it a little, considering the heat here . . . . .. 2-3,000 more is OK.

Hope this helps.

just go best denki at amk & get their air-con they include free home install & confirm honest, i got my portable aircon for $800, my room is 4x4x4 m

1 BTU = 1.05506kJ

Assume room size is 4mL X4mB X 3mH =48cubic meter of air space.
Assume to cool room temp from 33degC to 23degC = 10degC change.
Density of Air = 1.293kg/m3
Specific heat capacity of air = 1.0035kJ/kg.C

So 48 x 1.239 = 59.5kg of air needed to be cooled.

Now 59.5kg x 1.0035kJ/kg.C x 10degC = 597kJ (or ~ 565BTU)

This is purely for a super insulated room. (no heat generating computers, humans, or sunshine or hot walls)

Now it gets more compliacted....
Specific heat capacity of human body is =3470kJ/kg.C (average)
Assume human body must maintain body temperature at 37degC.....so it will generate 3470kJ of heat to maintain it in the aircon room!!

Add it to the figure above....597kJ + 3470kJ = 4067kJ (or 3854BTU)


Do note that the initial 597kJ is just to cool the temperature down by 10degC, once the room is cooled, this part of the energy required will be zero.

The human body is cotinuously need to be be heated up to counter the coolness from the air con, so that 3470kJ is the basic power the aircon need to cool a room with a human inside!!

For your computer, TV, vibrator, PS3, etc....take the wattage figure. Say your electrical equipment is 50watt.
1BTU = 0.293watt-hour
So your electrical equipment will be equivalent to 50/0.293 = 170BTU (or 180kJ) (continuous as long as the TV,computer is turned on in the room)

Some of you will ask about the concrete walls.
Specific heat capacity of concrete wall = 0.8kj/kg.C
Density of concrete is 2000kg/m3.
Assume one 3mL X 3mH X 0.1mThk wall is under direct sunlight of 33degC, the other 3walls are not heated by sun.

Energy need to cool down this wall = 0.8 x (3x3x0.1) x 2000 = 1440kJ per degC

In conclusion, the aircon power need to cool down the human, wall and electrical equipment.
3470(human) + 1440(wall) + 180(equipment) = 5090kJ or 4823BTU

If need to cool the wall by a difference of 10deg....then you will need a HUGE air con power of 14,400kJ...but that is a one off initial heating needs. Once cooled down, it will only need around 1440kJ to maintain it plus minus 1degC (like the human body).


So you need 4823BTU for a typical room!

To refine the above calculation...once the inside wall is cooled down to 23degC....using 14,400BTU of air con power.
Then after, the aircon will only need to maintain the wall temp with regard to the other side temperature.

Concrete thermal conductivity is 1.7W/m.C

so assume all six side is concrete wall (assume the window side is also concrete for easy understanding and calculation)
Asuume the wall is 3mW X 3mL x 3mH = 27m2
Six walls will be 27 x6 = 162m2.
Assume the other side temperature is 5degC higher than the room temp.
Assume wall thickness is 100mm or 0.1m

using the formula q = k.A.dT/s
q= 1.7 X 162 X 5 /0.1 = 13,770watt (or 49573J or 49.573kJ or 47BTU)

So you see again...to maintain the 6 walls temperature needs only very very little 47BTU.

The bulk of the heating energy is used to cool down the initial hot 33deg walls due to sunlight (14,440kJ).
Once the wall is cooled down....then minimum energy of 47BTU is required to maintain it.

All the above complex calculation translates to industrial rule of thumb - about 0.2 kw/sqm or 700 Btu/sqm (very conservative). Lol.

so should I get Daikin or Mitsubishi?

so should I get Daikin or Mitsubishi?

Mitsubishi, Panasonic or Samsung.

Daikin unless you get from dealer confirmed new in box with warranty.

Alot of refub daikins(with new case and box) around nowadays and alot of smaller to mid-sized dealers/contractors/IDs would chop u.


You need at least 80KBtus with the fur and winter activities.

so should I get Daikin or Mitsubishi?

vouch for toshiba inverter. been using for 5 years, going strong.

recently went to browse gain city. hitachi's 4 ticks EER looks interesting but do not know anyone using it yet.

some 4 ticks are more efficient. but key is still how many BTU can the compressor support. get your 5 year warranty. mine comes with parts and labour.

Edited by Sabian, 24 May 2011 - 05:13 PM.

vouch for toshiba inverter. been using for 5 years, going strong.

recently went to browse gain city. hitachi's 4 ticks EER looks interesting but do not know anyone using it yet.

some 4 ticks are more efficient. but key is still how many BTU can the compressor support. get your 5 year warranty. mine comes with parts and labour.

so 6 years coming, no more warranty liao...touch wood

just go best denki at amk & get their air-con they include free home install & confirm honest, i got my portable aircon for $800, my room is 4x4x4 m

Nowaday all the big stores selling also hv free installation/dismantling/extended warranty. Yr portable aircon is those tat can move ard izzit? Not exactly same topic lah.

Mitsubishi, Panasonic or Samsung.

Daikin unless you get from dealer confirmed new in box with warranty.

Alot of refub daikins(with new case and box) around nowadays and alot of smaller to mid-sized dealers/contractors/IDs would chop u.


You need at least 80KBtus with the fur and winter activities.

Note your avatar is furless

so 6 years coming, no more warranty liao...touch wood

Yes, touch wood. 5 yr insurance is max offered.

for my house past & present.. i always only use 9000btu units for every room be it a 3.5m x 6.5m or 4m x 7m.. even living room also use 9000btu.

18000btu cools down a larger area faster, but it cost more and also consumes more electricity.

I dun mind taking longer time to cool down a room so its always 9000 btu.

Actually i feel it down't take too long to cool also.

vouch for toshiba inverter. been using for 5 years, going strong.

recently went to browse gain city. hitachi's 4 ticks EER looks interesting but do not know anyone using it yet.

some 4 ticks are more efficient. but key is still how many BTU can the compressor support. get your 5 year warranty. mine comes with parts and labour.

I agree with you on toshiba.. Compressor is made in japan.

My previous house uses a Made in Korea LG inverter system-4 for 6 years, due to poor maintenance on my part, compressor died. Then i switch to Toshiba invertor system-4.. same 9000 BTU units but Toshiba is quiter and more powerful.

current house using Panasonic Inverter, 2x system-1 & 1x system-2.. not very powerful... misses the Toshiba system.

so should I get Daikin or Mitsubishi?

Both brands are good... installer more important. So google for reputable shops that can help you plan the aircon ducts layouts, and also good installers will lay nicely for you.

easiest way is square feet x 60

unless inside alot people staying wif heavy electrical stuffs then x 70-80