4,037 posts, read 8,939,249 times Moving into a two story house, I'm trying to prevent the electricity bill from exceeding my mortgage payment (kidding...kind of). We have two AC units, one that controls the downstairs and one for upstairs. In my head, it seems to be less costly to only run one at a time (or at least set the other one significantly higher) depending on where we are in the house; lower unit during the day and and the upper at night. Will running the lower unit when we're upstairs keep hot air from rising as much or will it push the hot air up to the bedrooms? I'm struggling to understand the relationship between the two. Anyone want to do their good deed for the day and help me wrap my brain around this? 16,450 posts, read 27,949,097 times 4,575 posts, read 7,333,295 times Good to know, thanks. Do you think it's using more energy by having to bring the upstairs temp down from a higher temp in the evening than it would if we simply kept the temperature more constant?

I hope that at least makes sense. Originally Posted by DetroitN8V It absolutely does use more energy to cool the upstairs from a higher temperature, but you're probably on a "time of use" plan, where the energy is much cheaper at night.. (my plan starts off peak at 7pm), so instead of $.25 per kw daytime/on peak, im paying $.05 per kw/off peak.. running from 7pm to midnight continuously costs the same as running only one hour from noon-seven pm..I guess the true determining factor for how I set the thermostats will be if the upstairs AC can kick on at 7pm and have the upstairs cool enough by the time I go to bed. I just stopped by the house and it was 95 degrees upstairs! 311 posts, read 315,749 times You might not want to turn the upstairs off completely, I don't think you want it going up to 95! With a programmable thermostat I would just set the daytime temp to 80-85 so that you can still use the upstairs if needed without boiling, then set it for a cooler temp for nighttime.