Better to have high ceilings or low ceilings to save on utilities?

[KatrynS]

I'm sure it has an impact, but not sure how much. I think you can use vaulted/high ceilings to your advantage in some situations (cool sinks), but it does increase the total cooling load. I think other things, like surface area of roof, windows, how many windows face S or W are much bigger factors!

In a 2-story, I think a much bigger effect is having lots of space open the 2 floors with big lofts. Our last house in CO had that with wall of windows facing S to boot and it was impossible to cool the upstairs without over cooling the downstairs (cold air from upstairs just fell down to 1sr floor), or heat downstairs without roasting upstairs. Dual zone HVAC helps, but I'm glad our new house has limited open areas between the levels! Everywhere is the temp is should be and we keep the upstairs 3F hotter during the day and the downstairs 2F hotter during the night (thermodynamically, won't go more than 2F hotter than upstairs no matter what the AC is set to(in our house)).

[observer53]

True. That's why I said that you have to look at exposures, windows, etc. which really are more impactful, particularly on cooling costs, than ceiling height. Trees and other shade outside are other factors. Two story homes, especially with a single unit, present their own set of issues, especially with the full open areas as you describe.

[AzDreamer]

Try this simple experiment. On a hot day measure the temperature at the top of your high ceilings. Now turn off your air conditioner for a day and measure the temperature again. If the temperature is the same both days then you aren't spending money to cool the upper portion of your room.

[KatrynS]

Originally Posted by AzDreamer

Try this simple experiment. On a hot day measure the temperature at the top of your high ceilings. Now turn off your air conditioner for a day and measure the temperature again. If the temperature is the same both days then you aren't spending money to cool the upper portion of your room.

Actually, that's a flawed experiment since even if there was no heating cost the temp the second time would be higher because the air below it would be hotter, which would conduct heat into the higher air. Heat doesn't travel up and down the same, so that's not equivalent to cooling. You're not isolating one factor.

Think of how your hot water heater works (cold water flows in the bottom, as water is heated it rises and floats on top where it is syphoned off, a stratified fluid). Of course air doesn't stratify as well as a liquid, and fans and AC vents cause mixing. If I reversed the hot and cold pipes, I'd have a totally different heat transfer problem!

In my case, I have a 2-story with a tall entry, high ceilings downstairs and reg upstairs, but otherwise 2 sep levels with sep HVAC. During the day, I don't cool the upstairs much, so the upstairs and the high ceilings act as a thermal insulator from the HOT attic and roof. Very little of the exterior is exposed to S or W solar radiant heating, so the extra load from having taller walls is negligible. In the evening, I don't run the downstairs AC much, but it will suck some cooling from upstairs, but that should be independent of the height of the downstairs ceilings. So, without running numbers it looks to me like the extra cooling load IN MY CASE is probably pretty small.

BTW, generally programmable HVAC controls save $$, but I won't let DH upgrade our up/down manual ones since we're ALWAYS home. I just change the settings when I get up/get ready for bed. Just another example of how it's tough to generalize.
Of course I'm just tweaking your tail, but the truth is I'm too lazy/busy to run the calculations for my own home let alone come up with some generalized truths. There's definitely a cooling cost, but how significant? I think it is much less than other factors.