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HVAC assist system. It cut our peak electricity bill by 40% year over year under similar conditions, too, with substantially better performance.

But really it's very simple. All I'm doing is improving the effectiveness of very traditional methods of temperature control by being more accurate and much more aggressive about exchanging air in and out when appropriate. Obviously in the middle of a 90-110F heat wave that's not going to matter, so it's more of a northern thing - but it really does a great deal in a lot of climates. (And in spring and autumn in more southern climates, I suppose.)

One of the key elements is that outdoor temperature varies a lot from point to point on the property, so we have air exchange measured at five points around the house, keyed to local indoor vs. local outdoor air temperature. (And air quality and a few other things, of course.) The actual air exchange is a combination of the original air-exchange system plus just opening and closing windows. We overcool at night with air exchange so we're always below ambient outdoor temperature during the day.

It's remarkably effective. We went from... well, it varied a lot, but +7 to +10 F above ambient to -7 to -5 below. (I was doing all this in C internally but F because I was talking about it to Americans.)

Again, I'm in an environment where this is particularly effective, but it costs so little and saves so much money and energy use I have to think it has some general utility many places.

Here are some of my posts about it:

https://solarbird.net/blog/tag/ambient-hvac/