Great question! Not sure I have enough information to fully respond but I will try my best. Youcan always post a follow-up comment below if I totally missed the intent of your question.
Heat gain is the critical issue
The air conditioning load isn't based so much on number of rooms as it is based on a heat loss/gain analysis.
- If a home has a good thermal envelope (ceiling and wall insulation with energyefficient windows) a portable air conditioner and/or window air conditioning unit can keep a fiveroom home very comfortable.
- My first home was a 900 square feet ranch and we used a windowshaker for years with great results.
- A portable air conditioner can be effective as long as it issized properly for the heat gain of the home.
Ductless air conditioning
Mitsubishi makes a variety of energy efficient, ductless air conditioning systems that would workin your brother's home.Basically the condenser is located outdoors and one or more wall unitshang in designated areas within the thermal envelop.
- Refrigerant is pumped to the evaporatorwall unit to provide cooling (some units also provide heat) to rooms that require cooling (orheating).
- Some models allow you to connect up to eight wall units to a single outdoor condenser.
Conventional A/C system
You also could opt to have a split coil, high pressure conventional air conditioning systeminstalled in the home.
Basically you have a condenser outdoors and an evaporator in the atticwith small diameter ducts that distribute conditioned air through ceiling grills in assorted roomsthroughout the house.
Return air is brought back to the evaporator from a central location. Thissystem is used often in larger homes with in-floor or radiant heating systems.
A recommendation and word of caution
Based on a home I suspect is under 1000 square feet the Mitsubishi ductless split system maymake the most economic and performance sense for your brother.
One word of caution - make certain that an installing contractor does not install a system thatis oversized for the home.
- The only way to know what size system is required is to performheat loss/gain calculations on the home based on room by room analysis.
- Properly sized airconditioners will operate at peak efficiency and remove excess humidity from the air.