I have searched 35 of 500+ pages and have not seen my answer.
We live in Texas and it has been over 100 almost all summer. My Dad says AC units are built to only cool approx 20 degrees lower than outside temp, so even if we have AC units on 80 they will run.
I feel that we should keep the house at a constant temp. The units will run, but at least we will be cool and not stress the units in the evening when we want a cool house to sleep.
My "Better Half" says that the upstairs AC should be on 87 or 89 (my office is upstairs), while the downstairs is at 78 or 80 ("Better half's" office is downstairs and we both work at home). I can even live with both units on 80 (more than reasonable) and running our fans, but some one else thinks we need to have the units temps wide spread. I feel they will run no matter what when the outside temp is 100, why not have them run at a cool temp rather than be sweating all day and units still running to maintain 89! Actually, when he is out of town I run both of them on 78 or 80 and it feels fine.


At what temp should we have them for best savings, and should we put the thermostats on higher temps during the day? Open entry way to second floor and heat rises and all that.

Please help us figure this out. All I can say is that it's a good thing that our offices are on different levels in the house but it sucks that mine is upstairs!

Thank you!

Short answer: Move your office downstairs.

Myth: It's not 20 degrees from outside. It's usually no more than 20 degrees of temperature difference across the evaporator inside. In other words the discharge air will be no more than 20 degrees cooler than the incoming air from the returns. It's dependent on temperature and humidity. 15-20 deg difference is a good benchmark.

The longer an AC runs the more capable it is to lower the humidity in the house. Short cycling is bad.

There is a concept called the "comfort curve" where temperature, humidity, summer and winter are on the chart and it plots a region where MOST people are "COMFORTABLE". Note, I said MOST.

You have an upside down house with an office upstairs. You cannot keep the upstairs warmer during the day when it's "unoccupied" and cooler at night when you sleep and vice versa for downstairs.

The best you can do, I think, is to run the upstairs at a constant temperature which is whatever is comfy and run the downstairs and a comfy temp for the other half during the day and raise the downstairs temp at night. You probably won't get that much difference in savings because the upstairs air will tend to migrate downstairs.

Fans such as ceiling fans cool because water vapor is leaving your skin. It's called evaporative cooling. So, in theory more air movement makes you "feel" cooler, however the technique doesn't work when the air is humid.

Another problem you have to deal with is that during the day the structure is storing heat from the sun beating on it and during the night that heat from the structure is going into the house. So, shortly after the sun goes down it gets really hot because the AC usually can't keep up.

This is where shades and awnings come in. Shades should keep the sun out during the day, so that means closed for areas that get direct sun.

Now should the shades be open or closed at night? To cool, they should be open. Is the better half going to allow that? Probably not.

Does it make any sense now?