Sizing an Air Conditioning System

[Encid]

I thought I'd post this topic as I am now in the market for buying some AC units for our Guest House which is currently under construction.

 

Lots of useful comments already other old topics in this forum, but with inverter technology being commonplace now, the best thing about them is that they still perform well even when oversized.

 

So how to size an AC unit?

 

Our Guest House consists of a single bedroom (sized 21 sq.m.) and an open living/dining/kitchen area (sized 36 sq.m.) and the ceilings will be 3m high.

 

The floor plan is below (North is to the right of the image):

 

 

I used several online BTU calculators... Carrier Thailand, Calculator.net, Mitsubishi Heavy Industries, and TheWeatherAir, as well as the often referred to local standards of somewhere between 700-900 BTU/sq.m.

 

I then put them all into a table so I could compare the results.

 

The table:

 

 

As you can see, the Carrier BTU calculator returns the highest BTU result, and Calculator.net returns the lowest BTU result.

 

I think it's safe to say that the local metric of 800 BTU/sq.m. is pretty close to the norm.

 

So oversizing slightly, I will be looking at buying an 18,000 BTU wall mounted inverter split system for the bedroom, and a 30,000 BTU ceiling mounted cassette inverter split system for the Living/Dining/Kitchen area.

 

Comments/thoughts?

 

[OneMoreFarang]

I had a similar problem - and no real solution.

I looked at several calculators. Some want to know the size of the windows, location of the windows (direct sunlight), etc. That obviously makes sense. But I didn't see any AC calculator which asks about the desired room temperature.

I.e. do you want in the living room 26 degrees Celsius? Or 24? Or cooler? Obviously the AC has to work harder if someone wants 22 degrees inside temperature while the sun is shining in the windows in the afternoon.

 

So I think one question you have to ask yourself is how cold should it be. If you have visitors from Siberia in your guest house who think 22 degrees is warm then better buy a bigger AC.

Or maybe you don't need a big AC if the family from Thailand stays there and is happy with 26-degree room temperature. 

[Woof999]

You've already mentioned the only piece of wisdom that I can give, but it's worth repeating anyway.

 

Slight oversize is well worth the additional initial cost. It can help cool a room faster, but can also mean the compressor works at less capacity, possibly benefiting in longevity, but definitely helping with noise. One of my compressors is on the outside wall just behind the chair I spend most of my life on. Since I replaced and oversized the AC in this room it has been noticeably quieter.

 

Finally, as I'm in a single position in this room for most of the time, angling the fan vertically and horizontally so that it points continuously towards my desk means I can increase the AC temp from around 23C to more like 24 or 25C. It feels noticeably cooler while saving money. Not directly related to the OPs question I know.

[KhunLA]

If you use their charts, you'll be oversized, if GH is a quality build.

 

We have a 13k in the bedroom (22.8m²), ceiling same, and could have easily went with a 9k BTU.  

 

Main room, same as your GH, open area, but larger (83.35m²), basically huge eat in kitchen & entrance way to house.   The 24k BTU handles that with no problem, and we do a lot of home cooking.

[unheard]

29 minutes ago, KhunLA said:

If you use their charts, you'll be oversized, if GH is a quality build.

????????

[steven100]

4 hours ago, Encid said:

18,000 BTU wall mounted inverter split system for the bedroom, and a 30,000 BTU

what an overkill ...    18,000 for a 21sqm ??  that's way to big .....      and 30,000 BTU for 36sqm  ??    that's way to big.   I bought a 9000 btu for a 24sqm ....  and it works fine. 

[stubuzz]

I have a Mitsubishi 9000 BTU inverter in a 16m2 room and it cools effectively.

[steven100]

I think you'll find an 11,000BTU and an 18,000BTU will do the job fine.    just my opinion.

[lopburi3]

I really can not read the plan too well (old eyes) but appears there is a patio type door?  But without knowing construction and insulation planned and how cold cool has to be a bit hard to estimate.  My BR with basic fiberglass insulation is 26sm and 9k units cools to my 26/27c comfort temp with no issues.  Red brick walls with cement cover and no insulation but no outside doors or windows.

[bunnydrops]

I was told once by a AC guy that if anything it is better to be a little undersized. He said that being over sized, the room will get cooler to quick for the dehumidifier to do its job and the air will feel clammy.

[Yellowtail]

I'm with OneMoreFarang, a lot depends on how cool you want it.

 

I like it cool. I have a 36K in 36m^2 kitchen and "living room" and it's been great. Cool & quiet all day. 

 

Unlike traditional units, when sizing inverters it does not make sense to buy a smaller unit. 

 

I would bite the bullet and go with cassettes in both rooms, so much better than wall units. 

 

When you compare the specs, be sure to compare the condenser physical sizes. A 30K unit that uses the same condenser as a 24K unit has to work a lot harder. 

 

Make sure whatever you're getting has copper coils....

[lopburi3]

12 minutes ago, bunnydrops said:

I was told once by a AC guy that if anything it is better to be a little undersized. He said that being over sized, the room will get cooler to quick for the dehumidifier to do its job and the air will feel clammy.

That should not be an issue with modern inverter models as they cool as needed (power varies) rather then turning off and on.

[digbeth]

had an installer refuses to install an oversized invertor unit stating that the ballpark figure the sales guy did was wrong and oversold us, claiming that being oversized means it will run 'throttled' all the time and will not feel as cold as what we're used to, this was a Mitsubishi dealer that only sold one brand, they took everything back and refunded the difference and deliver us the proper sized unit later.

 

apparently they had spades of complaint and return due to the invertor unit 'not feeling as cold' as the normal units

[Yellowtail]

Yeah, if you want cold air blowing on you all the time, buy an undersized conventional unit. 

[lopburi3]

4 minutes ago, digbeth said:

had an installer refuses to install an oversized invertor unit stating that the ballpark figure the sales guy did was wrong and oversold us, claiming that being oversized means it will run 'throttled' all the time and will not feel as cold as what we're used to, this was a Mitsubishi dealer that only sold one brand, they took everything back and refunded the difference and deliver us the proper sized unit later.

 

apparently they had spades of complaint and return due to the invertor unit 'not feeling as cold' as the normal units

That is the name of the game - it cools the room - it is not a fan for personal cooling.  But it does take getting used to - you can always set the temp lower for a time if need to feel the cool.  But it is something to consider - big is not always better - even for inverters.

[unheard]

As it's been mentioned already, the A/C sizing process is mostly just a guessing game. The common sizing charts are not particularly useful either.

 

There are many important to A/C sizing factors that are unique to each particular house - quality of wall/ceiling insulation, shading of walls, thermal qualities of windows/doors and even floors, house ventilation and air leaks, geographical location and local climate. Just to name a few.

 

It's also common to neglect (or not to be aware of) the A/Cs SEER rating which is an important metric.

Over the years the highly SEER rated systems will save a significant amount on energy costs, e.g. Fujitsu IMAX (obviously not a factor when on solar).

It's a very efficient model made by a high quality brand but for some reason Fujitsu A/Cs are not even mentioned on this board.

 

But It's a different story in the U.S. and Australia where Fujitsu units are very popular and being cross-shopped among other high quality brands such as Daikin and Mitsubishi.

 

In addition the Fujitsu IMAX model is generally over specced in relation to the advertised BTU cooling capacity, especially on the lowest capacity units in the model's lineup.

Edited March 23, 2023 by unheard

[Encid]

20 hours ago, OneMoreFarang said:

So I think one question you have to ask yourself is how cold should it be.

I think that is a very pertinent point... my wife and I like our living/dining area in the evening to be around 25°C, but we like our bedroom a touch cooler at 24°C.

When my brother and his wife visit (from Melbourne) they find it very warm here, and set their sleeping temperature down to 20°C.

When my Thai in-laws stay with us, the AC in their bedroom is usually set at 26-27°C. 

 

So... quite a variance.

[Encid]

17 hours ago, KhunLA said:

If you use their charts, you'll be oversized, if GH is a quality build.

The main bedroom in the Guest House will be double walled 7.5cm AAC blocks with a 5cm air gap between.

The only windows there are facing North and will be 2.7m high double glazed with uPVC frames.

 

The rest of the GH will be single wall 7.5cm AAC blocks.

All the windows and sliding doors face North except for one which faces East.

All will be 2.7m high double glazed with uPVC frames.

The East face will have a 3m wide eave over it and some ornamental slats with will reduce the direct sunlight somewhat.

The South face will be completely shaded - zero direct sunlight. 

The West face will be  double walled 7.5cm AAC blocks with a 5cm air gap between.

The only window on the West side will be the bathroom window, which will also be double glazed with a uPVC frame.

The bathroom will also be fitted with a ceiling mounted exhaust fan, so I am not expecting that window to be used very often.

 

The roof will be made from BlueScope steel with a 50mm PU foam cladding, and we intend to use SCG 6" Stay Cool thermal insulation in the ceiling cavity.

 

It should be a "quality build" with "reasonable" insulation. 

[Encid]

16 hours ago, bunnydrops said:

I was told once by a AC guy that if anything it is better to be a little undersized. He said that being over sized, the room will get cooler to quick for the dehumidifier to do its job and the air will feel clammy.

I think that applies to pre-inverter technology.

They still perform well even when oversized.

[Encid]

16 hours ago, Yellowtail said:

I would bite the bullet and go with cassettes in both rooms, so much better than wall units.

That was my original plan, however both my wife and my builder said that it is not good (according to Thai "feng shui") to have a cassette unit in the bedroom, as you cannot control the amount of air that is blowing at your face and "you can get sick".

 

Thai "feng shui" is important in building a house here... I'm not brave enough to go against the grain... we will be installing a wall mounted split system in the bedroom. 

[Encid]

16 hours ago, digbeth said:

apparently they had spades of complaint and return due to the invertor unit 'not feeling as cold' as the normal units

We had a similar complaint when we replaced our old 18K Mr Slim unit with a new 18K inverter Mr Slim unit a couple of years ago in our current house... we didn't feel as cool as we did with the old AC unit... and a thermometer proved us correct.

 

We simply set the thermostat a couple of degrees lower, and have been fine ever since.

 

BIG savings on the electric bill... wish we had done it earlier!

[Encid]

16 minutes ago, unheard said:

It's also common to neglect (or not to be aware of) the A/Cs SEER rating which is an important metric.

Over the years the highly SEER rated systems will save a significant amount on energy costs, e.g. Fujitsu IMAX (obviously not a factor when on solar).

It's a very efficient model made by a high quality brand but for some reason Fujitsu A/Cs are not even mentioned on this board.

Fujitsu models are not carried by the big main hardware stores (Global House, Boonthavorn, Thai Watsadu, HomePro etc.) which is probably why they are not mentioned on the forum.

 

I know that they have an R&D and Engineering department in SriRacha (near Laem Chabang port) but I have never seen any retail outlets.

 

Servicing an AC unit on a regular basis is important to keep both your AC and yourself heathy, and I don't think we'd find any Fujitsu-familiar service techs near where we are building, so no thanks... we will go with a well known manufacturer instead.

 

Good point about the SEER rating system though...

[unheard]

19 minutes ago, Encid said:

I know that they have an R&D and Engineering department in SriRacha (near Laem Chabang port) but I have never seen any retail outlets.

 

Servicing an AC unit on a regular basis is important to keep both your AC and yourself heathy,

They manufacture in Thailand for the local market and for export.

How often do you plan to service it?

Do you mean the yearly "tune-ups" which are normally not necessary in properly installed units or the regular clean ups?

Edited March 23, 2023 by unheard

[lopburi3]

1 hour ago, Encid said:

I think that applies to pre-inverter technology.

They still perform well even when oversized.

But as pointed out above - oversize will result in much less feeling of coolness as units will not be blowing very cool air much of the time.  Better to have right size regardless of type.  The issue is if you want a cool room at 26 or a meat locker at 20 can be a major factor on hot days.

[Yellowtail]

1 hour ago, Encid said:

Servicing an AC unit on a regular basis is important to keep both your AC and yourself heathy, and I don't think we'd find any Fujitsu-familiar service techs near where we are building, so no thanks... we will go with a well known manufacturer instead.

Unlike wall units, the cassette units (in my experience) typically do not need any service beyond occasionally clearing the filter, because they have a propeller fan that stays dry all the time, as opposed to a wheel/squirrel-cage fan that stays wet most of the time. 

 

Most split units marketed in the US have to both heat and cool, so comparing Thai models to them makes little sense. 

 

Some cassette models allow you to open one, two, three or all four directions. 

 

[Encid]

44 minutes ago, lopburi3 said:

But as pointed out above - oversize will result in much less feeling of coolness as units will not be blowing very cool air much of the time.  Better to have right size regardless of type.  The issue is if you want a cool room at 26 or a meat locker at 20 can be a major factor on hot days.

Funny you should mention that... 

I used to work at an abattoir in my younger days... on the evening shift actually, on the cleanup crew.

What a messy job.. especially when the meat workers went out on strike and left everything behind!

The refrigerated rooms were kept between 5-8°C.

We had to take a 1 hour break after 4 hours exposure working in those conditions... union rules. 

Edited March 23, 2023 by Encid

[Encid]

58 minutes ago, unheard said:

They manufacture in Thailand for the local market and for export.

So where are their retail outlets? I wouldn't mind taking a look.

 

59 minutes ago, unheard said:

How often do you plan to service it?

Do you mean the yearly "tune-ups" which are normally not necessary in properly installed units or the regular clean ups?

I was considering only the regular maintenance cleanups, although if an unfortunate gecko managed to get inside and fry both himself and a circuit board, then emergency maintenance also needs to be considered.

[Encid]

Just now, Yellowtail said:

Some cassette models allow you to open one, two, three or all four directions.

Thanks for that... I will look into it some more.

Hopefully that would overcome the negative Thai "feng shui" aspects. 

[Yellowtail]

12 minutes ago, Encid said:

Thanks for that... I will look into it some more.

Hopefully that would overcome the negative Thai "feng shui" aspects. 

Good luck with the wife. 

 

I worked in HVAC for twenty years in Thailand and my wife still believes the kid at Home-Pro over me...

[OneMoreFarang]

4 hours ago, Encid said:

I think that is a very pertinent point... my wife and I like our living/dining area in the evening to be around 25°C, but we like our bedroom a touch cooler at 24°C.

When my brother and his wife visit (from Melbourne) they find it very warm here, and set their sleeping temperature down to 20°C.

When my Thai in-laws stay with us, the AC in their bedroom is usually set at 26-27°C. 

 

So... quite a variance.

I also like to sleep with a lower temperature compared to the living room. Normally that does not require a bigger AC. Because the size of the AC depends on the temperature difference (at least the way I understand it).

Daytime outside 35 degrees and inside 25 degrees is the same amount or work for the AC like nighttime 30 degrees outside and 20 degrees inside.

20 inside with 35 outside, that is the challenge.