I live in a three story townhouse in Orlando, FL. I have two 2 ton heat pumps. One for the first two floors and one for the third floor where the bedrooms are.
In attempts to save money we've been setting the temp back to 85 during the day when we're away from the house (We usually keep it around 78 when we're home). When we get home in the evening we'll leave the upstairs system set to 85 and just cool the first two floors where we'll be spending most of our time. At bedtime we'll set the downstairs thermostat to 85 and cool the upstairs during the night. This seems to work well and hopefully saves a bit of energy.
Occasionally, however, we wind up spending all or most of the day upstairs... if someone is home sick, or recently, we were redecorating one of the bedrooms. In that case, we had been leaving the downstairs thermostat set to 85 and just letting the upstairs system run. What I noticed, however, is that even with just the upstairs system running, the downstairs usually stays pretty cool. I'm sure that this is because the nicely cooled upstairs air tends to fall down the stairs and the warmer air from downstairs rises and gets cooled by the AC running upstairs.
Now, my question is this... do you think that we should also run the downstairs AC during the times we are home, but only upstairs? I'm thinking that the upstairs system may be working harder and longer than it would with both systems running and I wonder if it might be using more power than I would use by running both systems. Or maybe I just have way too much time on my hands and it doesn't really matter