Also depend what is your temperature setting. To me is better oversize than undersize. Size like mine using 1HP is over work. Just like a heavy car with 2L engine drink more fuel than a 3L same car, and the power is there when you need it.

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Added on April 2, 2012, 10:52 pm

2 and 1.5hp piping is the same. And some installer told me even better to use 2.5 piping because if not enough power can still upgrade without doing piping. In a hot day, 2hp will cool the room faster, and than switch to lower the compressor. 1.5hp may take slightly longer to cool the room to desire temperature.

Heat transfer is not on the copper piping. Is on the Compressor, when you install larger piping, the a/c installer just need to top the gas to ensure pressure is there.

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Added on April 3, 2012, 9:30 am

Your link do not take account of outside temperature. To actually calculate correct size you need to know the average temperature of the weather during the operation time.

Try use this

http://www.calculator.net/btu-calculator.html 

http://papamy.panasonic.com.my/papamy/horse.html
And from York Malaysia <http://www.york.com.my/main.asp?tpage=faq>
It is not easy to estimate the actual heat load of a room without going through some tedious calculation. You are advised to consult air conditioner contractors before you decide on the equipment. Bring along the plan and measurement of your room and be specific what is the function of the room, is it a bedroom or a living room?

There is however, a rule of thumb that could give you a rough estimation of the required cooling capacity. This can be achieved by doing a simple calculation.

First, multiply the volume of your room in cubic feet with a factor of 6. Determine the number of person using the room and multiply with 500Btu/hr, as each person produces 500Btu/hr of heat for normal activity. Add these two together, and you get the estimated cooling capacity.

The calculation just now is for bedroom only. For other applications, the factor has to be changed. This rule of thumb calculation is just for estimation purposes. It is advised to reconfirm with your air conditioner contractors again before you place your order.

just to add: not just outside temp is important. you should also consider how well insulated your room is, how many people usually occupy it and what desired temp you want to chill down the room to.

in a well insulated room, a 1 HP aircon can easily handle a 25 sqm room. however if there is lot of traffic through the room (i.e. moving a lot of chilled air out) or a lot of people inside the room then you might need more power. same goes for equipment in the room that generates heat (PCs, lights, fridge, whatnot).

unless you know precisely how the room will be utilised it will be hard to give a good recommendation for the size of AC required.

FYI: 7,5 HP of a/C capacity *can* easily cool 550-650 cubicmetres of air to actual 25°C in a 3-5 person household if the rooms are insulated and windows ducted in normal malaysian climate (also in hot season!).

very specific question. what's sure: R22 is beeing phased out because it harms the ozone layer. that for me is enough reason no to use ACs with R22 and i dont.

now to your question: R401A tends to be more energy efficient than R22. if i remember correctly by 10-15% but i really dont have the supporting figure at hand anymore. i looked into this around 4 years ago when i changed my ACs to 401A inverters. i found an interesting read on the issue here:

https://docs.google.com/viewer?a=v&q=cache:...zusFVDI_EZB-DUg