[velosol]

If the weather is such that you need AC or heat, why would you have the windows open? Add humidity to the mix and open windows become even more problematic.

An oversized AC is going to run more if you have your windows open when it's hot out although with humidity may actually work better as it'll run long enough to pull out some moisture (latent cooling).

If the issue is around fresh air your average pre-2000 house breathes plenty with closed windows and newer homes tend to have active ventilation (e.g. energy recovery ventilators).

I feel like I must be missing something or you live in a warm-temperate climate such that a heat pump heats faster than it cools (heat pumps go way down in heating efficiency as it gets cold out and most work very poorly when it's much below 0°F).

[war1025]

The temperature generally swings something like 20F during the course of a 24 hour day. At night, I am quite content to have the windows open. During the day, I'm not around so why bother cooling the house. Then when I get home from work, it'll take the rest of the evening to cool off the house, at which point I might as well have just opened the windows and let everything cool off naturally.

Contrast that to the winter, where you can quite easily have a whole range of temperatures throughout the day based on being home or away and dropping the temp down at night while you sleep.

Heat pumps I'd guess work quite well over the same delta range that summer AC usually operates in (30F probably, even on the hottest days).

Beyond that, natural gas or propane is going to be much more cost effective.

[velosol]

Ah that makes more sense, thanks for helping me to understand the open windows viewpoint.

For me electricity is less expensive during the day (solar) and my house is well insulated so it takes fairly little to combat the heat gain during the day compared with letting the house heat soak through the day and then try to cool everything off in the evening. There's also often more humidity in summer so I'm trying to dehumidify at the same time and an adjusting AC (e.g. variable refrigerant flow or just a variable speed on the compressor) can run at lower consumption for longer to meet both needs at once.