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Comment by userbinator | Hacker News Reader

Power is cheaper than things failing earlier than they could have due to thermal cycling.

Do you have any citation for thermal cycling through the range of an AC inside a house affecting the failure rate of any household goods? Based on my experience this does not seem likely to be the case: thermal effects tend to be exponential or high powers of temperature, and most thermal cycling that people worry about is on the order of 100C or more. 10/20C of variation seems unlikely to be relevant.

(There is a case where if your house is well insulated enough, then it can be worth avoiding cycling your AC during the day, because it is both less efficient and will wear more while pulling the temperature down and this can be worse than just leaving it running in a sufficiently insulated house. It still is probably worth turning it off if you're away for a week, though)

But with most houses, the AC will cycle on and off throughout the day anyway - raising the temp while you're gone will often result in less cycling. Especially if you also increase the hysteresis range while out. So you get a triple efficiency win, from better AC efficiency, less AC motor wear, and less heat loss to the outside.

> because it is both less efficient and will wear more while pulling the temperature down

That's just not true. Aircon is a heat pump. It pumps heat more efficiently when more heat is available to be pumped.

Warmer indoor air increases evaporator temperature. A higher evaporator temperature means that the system operates with less temperature lift. This means that the system does less work (per BTU pumped) to cool a house that is hot inside, than one that is already conditioned inside.

Equipment longevity also tends to be increased with less-frequent, longer runtimes than with more-frequent, shorter runtimes. There's less temperature cycling, fewer in-rush current events, and the lubricating oil in a running compressor is actively circulated to the parts that need lubricated. Reducing the number of compressor start-ups is a good thing; these machines like to run.

So even if same number of BTUs enter the home and need pumped out whether the aircon is running or not, then: Letting things warm up inside when there's nobody home to care about that constitutes an improvement in a broad number of ways.

Reality is even better: On a hot day, a house that is hot inside takes in fewer BTUs than one that is cool inside, due to the reduced temperature delta between indoors and out. When we have fewer BTUs coming in, then we have fewer BTUs that we must ultimately pay to pump back out.

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