The house in question is a 1400 Square foot, two story house with forced air heating. It has a single return on the first floor and also has a very large duct to the outside on the return side. Fortunately for heating costs, the house is located in southern California where it generally doesn’t get too cold.

It does get hot in the summer, however, so am considering central air conditioning. It is a relatively simple matter to add additional return ducting to the first floor. (The heater, and thus the central air, will be located below the first floor.) Putting a return on the second floor would be extremely difficult.

The single return on the first floor seems to work acceptably, although certainly it would clearly be better to have returns in each room. My question: how much would the proposed air conditioning be compromised by not having any return on the second floor? I recognize the hot air rises (or conversly cold air sinks); will cold air sinking leave me with a refrigerated first floor and a still too hot second floor. Can I compensate by redcing the flow to the first floor? Is the proposed installation, with just a return from the first floor, worth a try?

Is there any air flow modelling software that might be useful for analyzing this?

Richard