Aircon Selection Charts

[jombom]

Looking at a number of aircon selection charts recently, the units seem to be massively oversized.

Typically the room is calculated in M2 (sq metre) and then multiplied by a magic factor of approx 800 to get the Btu size.

Perplexed by the magic 800 number, I did a few calcs of my own, and discovered the 800 is made up as follows.

Sq metre to sq ft factor = 10.76

Ceiling height 15 ft

Assumed temp differential requirement of 25C giving us a very old ''rule of thumb'' factor of 5 ---- this number dates back to post WW2 in Europe, and maybe even longer. The 5 is Btu per cubic ft of airspace.

Result 10.76 x 15 x 5 = 800 approx

Problem 1; -- 9 ft of ceiling is a common height so 9/15 is 60%

Problem 2; -- In Thailand the max ambient is 35 to 40 and most people are happy at 25, -- so 40-25 = 15

But charts based on 25 difference so 15/25 is 60%

Solution ; -- 60% x 60% = 36% implying aircon selection charts are 3 times the requirement approx.

Contrary view.

1 --- if aircons are run too close to the load, the compressor will tend to get very hot, resulting in shorter service life.

2 --- there is not a huge difference in the capital cost of different sized units, so we are really looking at power consumption.

Perhaps the experts have a valuable opinion.

[Crossy]

I'm sure Naam will be along shortly

The chart used by HomePro in the Bangkok area seems to be in the region of 'floor area x 600 = BTU'.

The problem is that you are comparing one rule of thumb with another (5BTU per cubic foot and 600/800 BTU per m2). Rules of thumb, whilst very useful, tend to work properly over a relatively small range of inputs (in this case the major variable is ambient temperature), if they were 100% reliable we engineers would be using them all the time, we don't, we do the sums. One size most definitely does not fit all.

A more comprehensive calculator estimator can be found here http://wiki.answers....oner_for_a_room but it still uses rules of thumb, just more of them. Another one here http://www.fairair.c...lator.Size.aspx and here http://oee.nrcan.gc....oms-ac/size.cfm these are just a few of the 2 million plus Google hits from 'estimate aircon size'.

I've not tried them all, but will lay odds that they all come out with vastly different answers because they are based upon conditions prevailing in different locations.

Of course, even if we do the sums and calculate to the last BTU, we then find that the range of A/C sizes offered has pretty big steps so we have to use the nearest match anyway

[Naam]

-rule of thumb might apply or might not.

-estimate based on room volume depends on room height and height of indoor unit installation, especially when room heights are as mentioned 15' (>4.50m).

-600 btu/h per m² based on ambient max temp 35ºC and targeted room temp 25ºC is fair if no excessive radiation on outside walls and glass panes.

-capacity steps are indeed quite big, usually 9,000 / 12,000 / 18,000 / 24,000 / 36,000 but pricewise acceptable.

[jombom]

Thank you for your replies. They are quite informative.

However, it still appears to me that many of the selection charts, are 300% over-rated.

So, this brings me back to duty cycle. I accept Naam's point about the capital cost, not being dramatically different.

So what is the best or better duty cycle ? Obviously an over-sized unit will have a low duty cycle, and a unit working at close to max demand, will have a high duty cycle. What is the perceived view of the optimum condition??

[weka]

is it a case of "if in doubt go for the larger BTU units?"

[harrry]

is it a case of "if in doubt go for the larger BTU units?"

I do not think so. this will do nothing to control the humidity. Best is one that is bug enogh and runs most of the time.

[Crossy]

is it a case of "if in doubt go for the larger BTU units?"

Not necessarily. A unit that is vastly over-sized will cool the room very rapidly but will not have time to produce any de-humidifying effect, the result will be that the room feels cold, clammy and uncomfortable. Most of us feel comfortable at warmer temperatures if we can get the humidity down to sensible levels. This will save energy as we don't need to be so cold.

You should use whatever size guestimator you feel comfortable with and apply a bit of common sense, if you estimate 9,500 BTU then a 9,000 will do the job just fine whereas a 12,000 may be over sized.

[Langsuan Man]

Here is a sizing chart I obtained from Panasonic, tried to get one from Daikin with no luck

CS-S9NKT Room Size : 9-13 SQ.M

CS-S13NKT Room Size : 13-18 SQ.M

CS-S18NKT Room Size : 18-24 SQ.M

CS-S24NKT Room Size : 25-28 SQ.M

CS-PS9NKT Room Size : 9-13 SQ.M

CS-PS13NKT Room Size : 13-18 SQ.M

CS-PS18NKT Room Size : 18-24 SQ.M

CS-PS24NKT Room Size : 25-28 SQ.M

CS-C9NKT Room Size : 9-14 SQ.M

CS-C12NKT Room Size : 14-22 SQ.M

CS-C18NKT Room Size : 22-30 SQ.M

CS-C24NKT Room Size : 30-36 SQ.M

CS-PC9NKT Room Size : 9-14 SQ.M

CS-PC12NKT Room Size : 14-22 SQ.M

CS-PC18NKT Room Size : 22-30 SQ.M

CS-PC24NKT Room Size : 30-36 SQ.M

Panasonic AC chart.pdf

[weka]

Thank you for your help folks

[KunMatt]

I was going to start a new topic but hopefully this one will suffice and get some replies.

After doing my homework on aircons a bit too late, I bought a Samsung 18,000 BTU unit a few months ago that I now think is far too big for my living room. My room is about 25m2, 3m high ceiling.

According to most websites and calculators an aircon of about 6,000 BTU would have been sufficient for my room. However, the two main bedrooms are adjoined to the living room and sometimes I have been using the single aircon to cool all 3 rooms in the evening. Each bedroom is about 20m2.

I now come to understand that an important purpose of an aircon in this country is not just cooling the air but also dehumidifying it, and with my huge aircon unit I only need to run it for about 5 minutes to bring the living room down to a comfortable 28 degrees but I find if I turn it off once it has reached the set temperature then I will need to run it again soon as the room heats back up and feels uncomfortable again. (For the record, I am going to begin improving the insulation of the ceiling and adding some attic and roof vents/fans this week which I hope will keep the temperature down for longer).

My AC has a Dry mode which from what I gather (can only find a manual in Thai), this is a dehumidify mode.

So, can anyone give me any advice? Is it worth changing the AC to a smaller one? Obviously it will be much cheaper to run and, more importantly, I hope make my house more comfortable.

Or would it be better to make do with the AC I have by using the Cool and Dry modes in a smart method? If so, would the best way be to use Cool first to get down to the desired temperature then just leave it on Dry for a period, or use the modes the other way around?

I assume the Dry mode would use much less power than the cooling mode too?

Sorry for so many questions, the "experts" who sold this aircon to me were of no help whatsoever. They tried to convince me to buy a 24,000 BTU unit at the time!

[lopburi3]

The Thai charts seem to be designed to sell more expensive products and meat locker cooling requirements directly under a hot tin roof. Most of us will find one size smaller than above listing adequate and able to lower humidity much better I believe. I have an 18k and a 9k in groundfloor bedroom of 25 square meters (used all day as computer is there) and have not had any day this year the 9k was not able to cope. Last year there were a couple days ran the 18k but could have done without. Had only expected the 9k to be an emergency backup in case of 18k downtime but found it was fine - if using just one again would not buy more than 13k. Understand conditions vary greatly so my experience will not be everyone's but in general expect most will not need the 'recommended' size normally listed here.