is my AC BTU sufficient for my room?

[Sambora]

Hello,

 

I recently went for a smaller BTU capacity in my living room as the new AC body is smaller in size and better fits to my low-hanging ceiling. I went for the Mitsubishi MSZ-LN18VF with max. BTU 20.472. My old York 30K(!) BTU I used for the last 5 years died.

 

my living room is 6.4m x 4.4m with 2/3 of my ceiling at a height of 2.75m and 1/3 at ~2.4m (low hanging ceiling.

 

I get direct sunlight into the living room from ~2PM till sunset. I use aluminum blinds in the room. I have light grey floor tiles. I am on a higher floor with floors above and below me.

 

I usually have the AC running at 22 degrees and with a fan speed level 2 (out of 4). That gives me an actual temperature of ~24.5 degrees which is comfortable for me.

 

When I do some search on ‚independent‘ websites, then I am told that my BTU size is well enough. When I look at the Thai AC brand calculators on their websites, it’s too weak. But I guess they want customers to buy the bigger ACs.

 

I don’t actually know if my BTU is suitable for my room. How do I know if it actually is. Based on my fan speed and temp settings above, it’s clearly not running at full capacity, neither in terms of speed nor temperature.

 

Any feedback is appreciated.

[stubuzz]

35 minutes ago, Sambora said:

I usually have the AC running at 22 degrees and with a fan speed level 2 (out of 4). That gives me an actual temperature of ~24.5 degrees which is comfortable for me.

It appears to be ok.

[Crossy]

Using the standard Thai guesstimate of 600-700 BTU / m² gives 17,000 to 20,000 BTU.

 

I reckon you're pretty well on the money.

[bankruatsteve]

Try putting the fan speed to "Auto".  I have 11k BTU cooling a much larger area although no direct sunlight and set to 24C.

[Brierley]

I think the key is the time of day. By 2pm the walls of your house and the steel and concrete tiles have absorbed as much heat as they can and are beginning to radiate heat into the living space, most buildings are the same. Aluminum blinds, light floor tiles etc make no difference when it comes to radiated heat from concrete. Your BTU rating is probably entirely adequate for the size of the room but it is defeated after 2pm when radiated heat comes into play, we have the same problem. One trick is to add more attic insulation to slow the rate of heat penetration from above, another is to shield the outside of the windows  with bamboo curtains to prevent the sun hitting the glass and radiating. Most people compensate by buying excessive BTU units which is overkill outside of those three afternoon hours.

[ExpatOilWorker]

Go for the 20k unit.

It is not the end of the world if you have to live with a slight higher temperature for a few hours on a few sunny afternoons.

Big oversized units give the room a sticky humid feeling.

[Damrongsak]

When it gets below 25C I have to throw a pakama over my shoulders unless I'm working. 

 

[sometimewoodworker]

9 hours ago, Sambora said:

I usually have the AC running at 22 degrees and with a fan speed level 2 (out of 4). That gives me an actual temperature of ~24.5 degrees which is comfortable for me.

It’s doing the job you want..

9 hours ago, Sambora said:

I don’t actually know if my BTU is suitable for my room.

It’s doing the job you want, therefore it is

9 hours ago, Sambora said:

How do I know if it actually is. Based on my fan speed and temp settings above, it’s clearly not running at full capacity, neither in terms of speed nor temperature.

It’s doing the job you want, therefore it is Perfectly ok

[Sambora]

Thank you all for your feedback, much appreciated ????

 

I read on the internet that oversized ACs, probably like my old 30k BTU from York, run at risk of short cycling as they use their excess capacity to cool indoor spaces quickly but at the cost of short cycling that maintains a specific temperature. So their compressor cycles on and off very often.

 

I am wondering if my new Mitsubishi AC is actually more suitable for my room now? Is this cycling on and off only identifiable through noise e.g. when the compressor switches on? Both my AC unit and compressor are pretty much noiseless. ????

Edited May 15, 2021 by Sambora

[bankruatsteve]

4 minutes ago, Sambora said:

I am wondering if my new Mitsubishi AC is actually more suitable for my room now? Is this cycling on and off only identifiable through noise e.g. when the compressor switches on? Both my AC unit and compressor are pretty much noiseless. ????

If it is "inverter", it won't cycle - it will use enough energy to maintain temperature and likely well below the 20k BTU maximum.

Edited May 15, 2021 by bankruatsteve

[Sambora]

34 minutes ago, bankruatsteve said:

If it is "inverter", it won't cycle - it will use enough energy to maintain temperature and likely well below the 20k BTU maximum.

yes, it is an inverter. 

[tlandtday]

32 minutes ago, Sambora said:

yes, it is an inverter. 

Inverter won't be cycling like a tradional.  I think the thai estimate is overkill.  I have a small aircon in my living room which gets hot due to the windows but I only run it at 28 during the day.  I have acclimated to 26-27 in my other rooms and it is no problem.  Another word of advise is clean the aircons at least 2 or 3 times a month for max cooling.

[sometimewoodworker]

4 hours ago, Sambora said:

I read on the internet that oversized ACs, probably like my old 30k BTU from York, run at risk of short cycling as they use their excess capacity to cool indoor spaces quickly but at the cost of short cycling that maintains a specific temperature. So their compressor cycles on and off very often.

That is completely correct for conventional AC units. Your last one was definitely oversized for the space. 
 

If your current new one were a conventional AC unit I would say that it is also oversized. 
 

However as it is an inverter unit the sizing is much less clear and the drawbacks of somewhat oversized units have virtually disappeared. 
 

To be clear with a room that should have a 9,000BTU if you put in a conventional 18,000BTU unit you will have humidity problems along with shorter lifespan. 
But if you put in an 18,000BTU inverter your humidity will almost certainly be OK. 
 

This doesn’t mean that you should go to units that are twice the power that’s correct for the room. 

 

[Crossy]

2 hours ago, sometimewoodworker said:

To be clear with a room that should have a 9,000BTU

 

320 BTU/m² seems a bit low for a room with direct sun and an occupant who likes it at 24C.

 

But as you say, the bigger inverter will throttle its output to suit and provide adequate dehumidification.

 

[sometimewoodworker]

4 hours ago, Crossy said:

 

320 BTU/m² seems a bit low for a room with direct sun and an occupant who likes it at 24C.

 

But as you say, the bigger inverter will throttle its output to suit and provide adequate dehumidification.

 

I think you misunderstood or I wasn’t clear enough. The example I gave was not supposed to relate to or advice about the OPs actual situation with AC sizing, I didn’t do any calculations about his room. 
 

it is just a handy example and the AC sizes work exactly and are available. 
 

as for actual sizing I would probably start at 550BTU/m^{2 ,} he might need a bit more if conventional and could probably use the Thai “freezer” rating of 700~800BTU/m² with an inverter with no problem 

 

 

[Crossy]

8 hours ago, sometimewoodworker said:

I think you misunderstood or I wasn’t clear enough.

 

It was me.

 

You said "To be clear with a room that should have a 9,000BTU".

 

I read "To be clear that room should have a 9,000BTU".

 

Sorry for the misunderstanding, seems I've caught the dreaded "it was a misunderstanding" bug from the Thai government.