The "checking the door twice now" part is the real story here. That's how it always goes with small mistakes that cost real money, you don't just fix the thing, you build a little routine around it and the routine sticks around way longer than the problem. I've seen it with locks, with ovens, with bank cards. One bad night and suddenly you're the person who checks everything three times. A temp alarm would help for sure, but the bigger thing is that Friday night brain just isn't the same as Saturday morning brain, and no gadget really fixes that.

Man, the part about the thermostat fighting the window unit is exactly the kind of thing I bet happens in a ton of houses and nobody ever knows. Two systems both trying to do the same job in the same space is just money going out the window, literally. What gets me is that you can't see any of this from watching your house, you only see it when the data is laid out in front of you hour by hour. So the next step would be checking if that back room even needs the window unit at all once the main system is running right, because maybe you can drop it and save even more. Or go the other way and just let the window unit handle that room and close the vent, depends which one costs less to run. Either way that July data probably paid for the $400 right there.