The background: We moved into a home that is two stories tall with a central air conditioner (actually a heat pump) for downstairs and a second central A/C (also a H/P) for upstairs. We are beginning our first summer in this house. At night, the tempurature gets down to the mid 70’s (F) this time of year. In several weeks, the temp will stay in the high 80’s all night.
The problem: At night, I turn the thermostat on the downstairs unit up to around 85 degrees. All the bedrooms are upstairs, so I figure there is no need to run the A/C when no one is around. I set the upstairs unit at a temperature we’re comfortable with and we all go to bed. Often, around 4:00AM or so, my wife or I will wake up because of uncomfortable heat. We usually lower the upstairs thermostat and go back to bed. Another detail: the door to the master bedroom where we sleep is right next to the top of the stairs.
The question: What is the optimal way (in both comfort and economy) to set the thermostats in this situation? Am I better of leaving the downstairs area cool all night, even though no one is there? I suspect that, since heat rises, the downstairs warms the upstairs area and the upstairs unit has trouble keeping up.
So what do y’all think? Thanks for your attention.