We have a two story house with central heating/cooling units for each floor. At night my hubby sets the thermostat really high downstairs and turns the thermostat upstairs really low...usually a 15-20 degree difference between the two. He says he's doing this to save electricity. Heat rises, right? So if it is warmer downstairs the heat is going to come up the stairwell and make the upstairs unit run more, right? So how can doing this save energy?