How Much Btu (?) Needed For 18Sqm Bedroom?

[THAIPHUKET]

The title say it all, more info I don't have

[MJCM]

Maybe this helps ?

http://www.ukofficedirect.co.uk/calculating_what_air_conditioner_cp.aspx

[THAIPHUKET]

If everything in life went as promptly and perfectly , heaven would be comparatively speaking hell :-)

[sotsira]

12000btu should be just fine.

[Crossy]

The standard Thai 'calculation' of 600 BTU / m² gives us about 11k BTU, if you like it arctic go for a 12k otherwise 10k is going to be more than adequate for a bedroom.

[tatom]

In the US, I have a small house (50 sq mt.), and a single unit of 5,000 BTU cools the whole house! In the summer, its just as hot as it is here, so I can only imagine that its because the house is well insulated.

Perhaps it would pay to insulate a house here?

[lopburi3]

That should make the normal 9k models sold here fine for most people (that do not require temp below 25). We prefer 28 degrees but low humidity and use 9k units for two 25 square meter bedrooms without issues.

[lopburi3]

In the US, I have a small house (50 sq mt.), and a single unit of 5,000 BTU cools the whole house! In the summer, its just as hot as it is here, so I can only imagine that its because the house is well insulated.

Perhaps it would pay to insulate a house here?

It becomes very hard to insulate after build as homes are cement here so other than ceiling not much that can be done without large expense (which is not the same as when you have heating in winter offset costs of cold areas).

[MartinL]

Which direction does your bedroom face? Is there a large area of glass in the walls (ie. big windows)? Are your walls/windows shaded to minimise sunlight impingement on them? Wall colour affects things too, a reason why most Thai houses are pale colours.

A west-facing room will gain a lot of heat in the afternoon as the sun sinks and shines directly onto walls & windows and deep into the room and will retain a lot of that heat well into the night. It'll only cool-down once the ambient air temperature drops. On a hot night it won't cool a lot at all.

An east-facing room gains heat in the morning and has time to cool during the day.

A south-facing room will have surprisingly small heat gain from the sun because the sun's overhead & heats the roof space and comparatively little heat is transmitted into the room.

North-facing rooms are least influenced by solar gain.

Building construction materials also have an influence on how much heat builds-up in the walls and how quickly that heat is released at night.

Heat output from people & electrical equipment can also contribute but this is a bedroom.

Air temperature is only part of the problem - a very significant contribution to the temperature you feel in a room is radiant heat from the building fabric, something you can't measure easily. Your A.C. thermostat can only react to air temperature not the radiant temperature.

As your A.C. cools the air in the room, heat is released from the warm walls to further heat the air so it has to remove not only heat in the air but also from the building fabric if it's to provide a comfortable environment.

The size of the A.C. unit influences the speed at which heat is removed from the room but, given enough time, any unit can cool a room to the required temperature if there is no heat gain - 26C is the same temperature whether your unit is large or small. A small unit might run non-stop in an effort to cool the air yet not achieve it, a large unit might run for short periods followed by long periods when it is off - I'm talking about the refrigerant side here, not the fan which might run constantly – so you’d be paying for A.C. capacity that’s not necessary.

In our unshaded west-facing 16 sq. m. (4x4x3 high) bedroom with one outside wall and 20% glass in that wall, we have a 13,000 BTU/hr. A.C. unit. I've measured the outside wall temperature and I've found it can reach 55C if not more. Mrs. L. likes to set a temperature of 26C and that is quite easily achieved if the A.C. is switched on around an hour before bedtime and left running all night. My preference for 24C or less is much more difficult to achieve so we go for 26C. Personally, I’d go for a 15,000 BTU/hr. unit because we’re west-facing although 13,000 is OK but I feel any less would be too small.

If I'd been involved in the design of this house, I'd have put all the bedrooms on the east or north side.

So use the “rule of thumb” 600 BTU/hr/sq. m. (176 W/sq. m.) floor area that Crossy mentioned by all means but remember it’s just an average and will be more than adequate in some circumstances, totally inadequate in others.

Sorry if this is a bit involved when talking about a bedroom but it’s just part of the exercise I often had to go through in my professional life, where we had to design to meet strict energy consumption targets, and it’s sometimes useful to have a bit of background info. when making a decision. Also, it's a rare opportunity to remember what I used to do for a living in UK.

Incidentally, why do we use BTU/hr. here when these units are rated in kW, the internationally-accepted unit? I hadn't used BTU/hr. for many, many years in UK. Is it a common unit in USA?

[Crossy]

Incidentally, why do we use BTU/hr. here when these units are rated in kW, the internationally-accepted unit? I hadn't used BTU/hr. for many, many years in UK. Is it a common unit in USA?

The US use 'Tons' of A/C just to confuse everyone

1 Ton = 12,000 BTU/h = 3.516 kW

[kyrre]

I found one example on how to calculate the BTU needs. How accurate it is regading to thailand i don't know. But today's unit with inverter compressors it does nothing wrong if the unit is bigger than needed except for the cost, what is maybe more impotent is how low the cooling can go. The lower it can go the more stable temperature you get as the unit will deliver cooling all the way instead of turning off.

Step 1

Find the volume of your room in cubic feet. This is done by measuring the length, width and height of the room in feet and multiply all the three dimensions together.

Volume = Width X Length X Height (cubic feet)

Step 2

Multiply this volume by 6.

C1 = Volume X 6

Step 3

Estimate the number of people (N) that will usually occupy this room. Each person produces about 500 BTU/hr of heat for normal office-related activity. Multiply this two figures together.

C2 = N x 500 BTU/hr

Step 4

Add C1 and C2 together and you will get a very simplified cooling capacity needed for the room.

Estimated Cooling Capacity needed = C1 + C2 (BTU/hr)