There is a thermostat upstairs and downstairs. How is the most efficient way to set the thermostats? When the one downstairs is turned off it stays comfortable but is that overworking the system by using only the upstairs?
We need much more information than that.. What kind of HVAC system do you have?
I don't fully understand your question. Like any machine, the more you use it, the more stress is out on it. However, having idle units is not ideal either. I think good discernment should govern that for anything.
However, if you could be more specific, that would be better!