After my last post, I really think the answer as to why the house will start to get warmer on its own lies in residual heat in the walls, floors and other elements of the house. I don’t think this effect was as large in our older house because the layout of that house was more conducive to having air pass over floors and walls, and our new house is a one story ranch so I think there’s less air that directly passes across the lower parts of the house. (especially floors and lower parts of walls) This means that these elements retain a bit of heat that gets released back into the house and warms the house up, even just a couple of degrees.
In my research on this issue, I found a recommendation to run the whole house fan at high around bed time to quickly replace all the warm/stale inside air with fresh/cool outside air, then run it all night long at low to keep replacing air and give things a chance to cool down. Then in the morning, when it is still cool out, run it again at high for a while before turning it off and closing all the windows. This allows you to “trap” all that cool air inside and keep the house cooler for longer. So far this has been how we’ve started running the fan, and yesterday was the hottest day of the year so far and we didn’t need to run A/C until about 7pm.
I’m all about that kind of energy efficiency.