I am currently getting quotes to add Split Ductless A/C to a Condo. The Condo has 2 Bedrooms and a total of 920 sq ft. I specified to each contractor that I want to cool the main area (kitchen, dining, and living) as well as the master bedroom. I've attached the layout which shows the master bedroom at the end of a hallway.

One contractor has suggested that because of how A/C works that a single 1.5 ton unit located in the main area will cool the entire Condo. He says the entire Condo is like a tub being filled with water. The cool/dry air produced by the A/C unit will drop to the lowest level and spread throughout the Condo and gradually fill up all rooms including the bedrooms down the hall. He says that if we leave all the doors open and run the a/c all day while we are at work, we will come home to a completely cool Condo.

This is a very attractive if true, because his proposal rates to be the least expensive. Others so far seem to be quite worried about cooling the entire Condo. One has even recommended two inside units (one in main area, and one in master bedroom).

Does the "filling a tub" analogy hold water? :-)Can you please evaluate this proposal for effectiveness?