Two story house, two AC units...most efficient use? (mortgage, deed)

[DetroitN8V]

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. Is this counterintuitive? 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? Thanks in advance.

[observer53]

I don't think the first floor unit pushes air upstairs, hot or otherwise. People I know who have the dual units use them pretty much as you describe, but you will probably have to start running the upper one earlier in the evening, if you've kept it from coming on all day.

[Zippyman]

You're on the right track - when we lived in a two-story, we just ran the upstairs at night, and the downstairs when we were at home. The bills werent as terrible as you might imagine. We picked up a couple of programmable thermostats on ebay so we didnt have to remember to shut downstairs off and "pre-cool" the upstairs.

[DetroitN8V]

Originally Posted by Zippyman

You're on the right track - when we lived in a two-story, we just ran the upstairs at night, and the downstairs when we were at home. The bills werent as terrible as you might imagine. We picked up a couple of programmable thermostats on ebay so we didnt have to remember to shut downstairs off and "pre-cool" the upstairs.

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.

[Zippyman]

Originally Posted by DetroitN8V

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.

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..

[DetroitN8V]

Originally Posted by Zippyman

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..

Yeah good point. 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!

[phx1205]

Originally Posted by DetroitN8V

Yeah good point. 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!

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. Just remember if you set it to be 76 at 9pm the unit is going to turn on earlier than that so it's at the desired temp at that time. It's going to take a couple hours for it to get from the 80s down into a comfortable tempersture. Probably will take some trial and error to figure out the desired temp and time so the unit doesn't run on before 7pm.

[DetroitN8V]

Originally Posted by phx1205

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. Just remember if you set it to be 76 at 9pm the unit is going to turn on earlier than that so it's at the desired temp at that time. It's going to take a couple hours for it to get from the 80s down into a comfortable tempersture. Probably will take some trial and error to figure out the desired temp and time so the unit doesn't run on before 7pm.

You can turn off "recovery mode" on every thermostat I've had. That makes it so it turns on at the set on time, rather than turning on early to get to your desired temp at the start time. Good advice though, thanks.

[johnp292]

I have a 2 story with the same set up you do, and we are on the time of day plan. I have the upstairs go up to 99 at 8am and back down to 77 at 7pm. Downstairs goes up to 90 I think at 11pm and down to 77 at 7am. In my case, the returns for both units are at the top of the stairs, and upstairs rarely goes over 85. I'm guessing that's because the return is drawing some of the cooler air up from the open living space below. We also keep all the bedroom doors closed during the day. It does get pretty warm in the bedrooms but nobody's ever in them. One thing I have found it necessary to do is to turn the fan upstairs on "on" rather than "auto" at night. If I don't do that the bedrooms get too warm before the compressor kicks on ( the thermostat is in the hall). That means I have to remember to switch it back in the morning or the fan runs all day. My bill is manageable, actually not much worse than my bills were in PA.

[burkie0213]

Does keeping the upstairs AC of completely make the one downstairs overwork? I turned off the upstairs one but my bill went up instead of down. It is just 5 steps to get upstairs. It is a long 3300 sq foot house..