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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?

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You need to consider the entire 1st floor, not just the huge connection. Keeping the downstairs unit off will hurt the air circulation in the rest of the downstairs area. I'd be thinking about setting that thermostat right at your comfort level. The upstairs is always going to be hotter than the lower floors so expect that unit to run more. The only thing that will strain an AC unit is a dirty filter so make sure you change them often. If the upstairs is used primarily for sleeping, you can set it's thermostat a little during the day and then drop it at night.

Right now, I'm not a big fan of Smart thermostats. I've heard a lot of complaints of them really acting up and have had a few requests to replace them with good old programmable ones. People have come to rely too much on technology to do everything. If you're hot, lower the thermostat, if you're cold, raise it. Sorry, I'm ranting now.

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If the upstairs system maintains a temperature you are happy with, it's not straining. Depending on age and technology, most heat and AC works exactly as hard any time it's on; some "stage" and can work more or less hard in steps; and the newest, most efficient variable speed inverter drives can operate at different speeds to match the load. They are designed to run all the time, if they are properly designed at all.

If you are concerned with saving electricity on cooling, you should concentrate on finding ways to insulate your house better. That's where you can really save on heating and cooling costs for minimal investment. When your HVAC system needs replacement, you should also shop for the most efficient replacement you can find, as there is a wide range of efficiency and a lot of it isn't very good. That generally does not pay off when the old system is still working, as it's a considerable investment.

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Speaking about A/C only (not heat). As you say, the cold upstairs air will settle downstairs.

Thinking only about energy usage, if you close some of the downstairs room doors, so the upstairs A/C isn't cooling the entire downstairs, you're better off running the upstairs zone only and keeping the downstairs one off as you do now. If you keep all the downstairs doors open you should run both systems. The upstairs one is not designed (sized) to cool the entire house.

Thinking about comfort

  • If you allow the house to warm up at times, such as when you go out, the upstairs will cool back down to a comfortable temperature much faster if you run both systems. Otherwise the upstairs A/C has to cool the downstairs before it can start cooling the upstairs.
  • If the upstairs bedroom doors are closed, the thermostat is in the upstairs hall, and the return duct for upstairs is also centrally located in the hall (three conditions) then the upstairs bedrooms will become cold and clammy at night, because the thermostat feels warm air from downstairs and keeps running the system until the upstairs hall is cold, by which time the bedrooms are too cold. This can be fixed in several ways: 1) Run the downstairs A/C at night to establish a bed of cold air at the open stairs 2) Move the thermostat to the master bedroom 3) sleep with bedroom doors open 4) Install return ducts in the bedrooms. Any one of those might help.
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  • Many installed systems are oversized for the actual load due to a variety of influences.
    – Ecnerwal
    May 30, 2022 at 12:48

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