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We live in a two-story home in Florida, about 3,000 sqft. We have an HVAC for downstairs, and one for upstairs.

We typically will leave downstairs pretty warm at night (~80), and when we're ready for bed have the upstairs unit at ~74. Since warm air rises, is this actually making our upstairs unit work longer than it should?

I think the answer has to be "yes", but my question then becomes, is this any less energy efficient than keeping both units at the same temperature? It seems likely that my upstairs unit probably only has to compensate for the amount of warm air that makes it to the 2nd floor, where leaving both on would have both units accounting for the warm air throughout the entire house. Is this correct?

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Let's change our nomenclature around to say your house has two zones and each zone has two states, occupied and unoccupied. Being able to adjust each zone so that it is only running while occupied is exactly what you are trying to achieve in Building Management System because there could be substantial savings to be gained.

This is just a fancy way of telling you that you are doing the right thing. If you're thinking about leaving them both on to compensate for warm air, it is not generally considered better than having them set as you originally stated. In my opinion, the important part would be to have your zone set at 80 to come on and cool down just before dawn during the cool part of the day rather to have it come on and try to cool when outside temperature is at say 100 and rising.

I don't think anyone can really put a fine point on this. Since there are a lot of factors to consider while trying to get the best efficiency from your system. It would be quite hard without a survey of your residence.

  • +1 @Retired Master Electrician, but I believe you can indeed put a fine point on it with the law of conservation of energy and the inherent rule that with energy conversion the output is always lower than the input, the loss is in itself heat. Thus, if he is running TWO A/C units, he is spending (for swag purposes) two times the amount of energy he is spending when running one, and in fact, is realizing the LOSS of both units since they are not perfectly efficient. Further, if one unit performs the job satisfactorily for the goal he wants to achieve, he is only losing ONE efficiency factor. – noybman Sep 2 '17 at 0:03
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Warm Air rises - even when you cool the lower floor - the warm air will rise. There is a temperature coefficient for how hot your 1st floor ceiling is versus the temperature setting of the upper floor. In well designed systems your cooling air from upstairs will draw down to the lower floor - so in effect the upper AC unit would be cooling that air too to some degree.

One effect is this : Your upper AC unit was sized for that floor - it will run to satisfy the upper temperature setting.

While saving energy is good your upper unit will be working harder.

What the trade-off point is (1st Floor Temp vs 2nd Floor temp) vs Ambient outside temp along with the Air circulation flow of the home from 2nd floor to first floor will all factor in to the threshold of energy savings vs A/C unit life.

Now all of that said knowing the actual values to plug in - most are easily obtainable - air flow 1st floor to 2nd floor is more difficult to know.

Then you would need to do some calculations based on that data to determine the optimum 1st floor Thermostat value vs 2nd floor Thermostat value.

Given your variance of 6 degrees - I think you are probably very close to a good setting if Air flow 2nd to 1st floor is ok and the units are sized appropriately - for example if your 2nd floor unit is not undersized given the space (Volume of Air to be cooled).

Just as an FYI : Your 1st Floor unit will also run - just not as much, depending on how much airflow makes it to the second floor or heats up the 2nd floor - flooring will determine how much the upper unit runs. This Air Flow equation is your big unknown value. Hopefully you have continuous soffits with a continuous ridge vents..

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    Also +1 on the note about home efficiency vs. just the A/C. Still, if one unit works hard, vs. two units working hard, there is certainly a cost from a unit working "at all" – noybman Sep 2 '17 at 0:06
  • @noybman you are correct two units running - how often that 1st floor unit cycles to run (rather how much you save by it not running) and how much extra the second floor unit will cycle to make up for it (how much more energy will the second floor unit use). 2nd Floor Added Costs -1st Floor Savings = ?? – Ken Sep 2 '17 at 9:39

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