What temperature should I set my up and downstairs AC/Heat units in the winter and in the summer? For the most efficient, should up and down be set at the same temperature or not. What about summer, what about winter?

Hello Calban

What about the temperature you require?
If you feel happy with 20C/68F than set on 20C/68F.
Set each thermostat to what you feel comfortable with, and leave it at that.
Why change in summer or winter time?

:)

Well actually since comfort is a combination of temperture and humidity and season, ther is reason to have different temperatures.

The highest and lowest temperature you and/or guests feel comfortable at for summer and winter respectively saves the most energy.

With programmable thermostats, you can save even more energy by making it slightly warmer or cooler when your not home. Like upstairs is occupied at night only. Downstairs is occupied in the evening assuming you work during the day.

Heat pump systems may require more time to recover.