In a 2-story home with open 2-story entry in AZ, what is the best way to set the 2 thermostats (up/down) for efficiency? Same temp?
Hot air rises, so setting main-level thermostat higher seems to cause the upstairs unit to run much more. A full and a half-size unit are on this house; full is main level. There is an open "gallery" around the upstairs with bedroom doors off of it. House is 2800 square feet total, but large open entry has small open loft adjacent. Setting the thermostat higher during absence causes units to run far more than 15 minutes to return the house to comfortable. We currently have them both set to 80 degrees 24/7 and use ceiling fans to supplement. We would like to cool bedrooms more at night for sleeping, but not if it means doubling our power bill. This is an energy efficient home with sunscreens on all windows.
A two-story home in Arizona with an open gallery is the worst possible situation. If the walls are poorly insulated and there are cheap windows (which is normal in the southwest), it can be difficult to manage temperature.
I would recommend the following:
First, set the temperature on the second floor 7-8 degrees lower than the first floor at night. The idea is that the second-floor unit will run double time and the cool air will "fall." The lower-level unit will not run.
Second, verify each bedroom has return air vents and keep the bedroom doors closed at all times. If there are no return air vents, make sure there is a 1 to 2-inch gap under the bedroom doors. You need the same amount of return vents or gap under the doors as the supply vents (in square inches); otherwise the rooms will pressurize and will stop cooling. When the doors are closed, see which rooms get hotter and try to adjust the supply vents to even out the upstairs temperature.
Third, have the A/C units checked for refrigeration levels. I have seen systems get low even though they are not leaking. If the systems are not charged, you will never cool the house, but you'll spend tons of money.
Fourth, if all else fails, consider purchasing a portable room A/C for the bedroom you sleep in. The idea here is just cool the space you need to.