I installed a smart thermostat on my 2nd floor since I ended up tweaking it often to reduce my electricity bill. I left the 1st floor as a dumb thermostat since I've always left it off. My upstairs has a huge open connection with the downstairs, so I'm worried this is potentially straining the HVAC system. Particularly in the summer when it can get to 105 degrees F outside, I worry the heat rising from downstairs might be costing more in re-cooling upstairs than if I kept the downstairs cool through the day.
Would it be better to install a smart thermostat downstairs as well and keep it on, or is it more energy efficient to keep the downstairs thermostat off if I'm happy with its temperature?