What should the settings of each be for efficiency in the summer or winter if you have downstairs and upstairs ac central air units?

First I would like to say that a study by Carrier ac company resulted in the following results. The best temperature to set an indoor climate at is 72. F. people at rest and people active will be most comfortable @ this temperature. There are exceptions to this general rule and humidity plays into the equasion also. 72.F. and 50% humidity is an optinium number if you are shooting to get the fewist amount of complaints if it is your responsibility to control the temperature in a large public facility as it is mine. next I will discuss how heat travels. there are three ways that heat travels. Heat is the only part of temperature that is worth discussing. cold is simply the lack of heat. Heat conducts, radiates,and convects. these are the ways that the energy of heat travels. conduction and radiation probably play a very small part in your application. most likely you should be concerned with convection. This is the charictoristic of heat travel that states Heat rises. In the summer the heat in the envelope of the house exterior walls will rise to the highest part of your home. The air with the heat removed will fall to the lowest part of your home. The job of a central AC unit is to remove heat from air and the rest of the fixtures and structures inside this envevope. the greatest amount of this heat is at the highest places of your home and therefore the upstairs equipment will have the greatest amount of heat to move outside. because mechanical equipment doesn't operate at 100% efficencuies and the unit with the most product available to it {Heat}will surely move more of it per unit if electricity than a unit with operating to remove something that there is very little of. That said, it is my poinion that using the upstairs equipment to do the majiority of the work in the summer to move heat outside is probably the best way to achieve comfort in your application. In the winter this reverses because of convection. Heat should be supplied at the lowest point in the home and working with convection the upstairs unit probably will not cycle or operate very often. Of corse there are alot of varibles such as insulation, Windows, sun exposures, open floor plans, width of stairwells for currents of air to rise and fall. To name a few. also the size of the equipment is a huge factor in this. to try to simplify what all I,m trying to tell you is, if you can be confortable operating only one piece of your equipment per application then you will save the most electricity and heating fuel. remember heat from downstairs and cool from upstairs. If you erxperance comfort problems you might try running the equipment fans in the on mode to circulate the air in the areas that the actual heating or cooling are not working. this will use less energy than if the equipment were performing both functions of circulation and heat transfer. one other thing comes to mind when I consider your concern is that you might just offset the thermostats by 1 or 2 degrees to take into account for the days that are extreemly hot or cold to assist the working unit {depending on season}. remember in summer set the upstairs unit @ 72.F. and the downstairs @73.F. or even 74.F. or what ever your prefernce. and in the winter reverse the offest. Work with convection not aganst it!