air-conditioning
For 2 bedrooms and 1 living room, and assuming you have the flexibility for both options and the total output is the same, is it better to install a Multi-Split 3×1 Air Con System (3 interior units / 1 exterior unit), or to install 3 separate Split Air Con Systems (3 interior units / 3 exterior units)?
Also, which setup is generally more energy-efficient?
Since it sounds like it might be difficult for you to get to a lumberyard to build your own solution, you might be better off using a pre-made product. AC-Safe, makes Universal Air Conditioner Supports that might be useful in your situation.
The system requires you to screw the platform into the window sill (which may or may not be possible in your situation), then simply rests against the outside of the building to support the A\C unit.
I've also seen brackets that mount to the bottom of A\C unit itself, and offer support against the outside of the building (but I was unable to find an example online. I think Thermwell makes them).
Basically you are trying to balance your new ducts with respect to your old ducts by sizing the new ducts. If you make a mistake or if your relative cooling loads (office vs rest of the house) ever changes, your only recourse would be to replace the ducts.
My preference is to size the ducts as large as reasonable possible (without increasing cost too much) and use a damper to tune the airflow ratios. Since you have 12" duct and a place to attach them, just get a damper like this:
http://www.grainger.com/Grainger/DAYTON-Round-Balancing-Damper-2TFY1?Pid=search
An uninsulated unshaded west wall in Texas can probably generate a very high cooling load in the afternoon.
Best Answer
There are three issues here, which system is better, which system is more energy-efficient and which system is more reliable.
The first issue encompasses all aspects and involves a priority ranking which only you can do.
An issue not brought up by anyone else is the installation. Do you have room for 3 outside units? Does it matter if more space is used or would you prefer to use that space for landscaping or something else?
What about electrical power? Will the installer need to run 3 circuits, one for each compressor? Does your existing breaker panel have the room for 3 circuits?
Another installation consideration: which system is easier to run the linesets (tubes which carry the liquid and gas freon) for?
I expect 3 compressors to cost more than one big compressor. Have you gotten any price quotes?
The one big compressor, assuming it is capable of variable speed, will certainly be more significantly more efficient under anything less than full load because the condenser of a single outdoor unit system has three times the cooling area of a single condenser in a 3 unit system which means the compressor motor doesn't have to work as hard.
The major factor in power consumption is turning the freon back into a liquid and that depends on how fast/easily the condenser can cool it off. All else being equal, a bigger condenser is better (removes the heat faster).
The faster you condense the freon, the less back pressure on the compressor and the less work for the motor.
Most of the time you will not be running full load or all indoor units at once. Even during full load, the big unit will never be less efficient.
The last question is reliability and available. One answer correctly claims that a 3 unit system will be more available but neglects to mention that it will also be more prone to failure because you have more units to fail and each unit is more likely to be run at full capacity while the big unit will usually be running well below its full capacity. Imagine the lifetime of two cars, one run at 90 MPH and one run at 30 MPH.