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My idea of HVAC is that it should be "set and forget", so "smart" is the polar opposite of what I want. The round Honeywell on the wall has been doing its job for the past few decades, and will likely last many more with only an occasional cleaning.

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If you have same schedule every week, don't go on vacation, your kids don't have breaks, don't have seasons, then 'set and forget' dumb thermostat will work for you without any problems. For the rest of us we need some 'smartness' in our thermostats.

You shouldn't need to change the temperature regardless of those things. My house remains at 68-72F basically all the time. Thermal cycling accelerates aging of materials.

Sounds expensive at the equivalent rate of 38 cents a kilowatt hour that I pay during the day.

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.

How about sleep? Ideal temperature is 18C.

That is 64F. Not everyone considers that an ideal temperature.

As in scientifically proven to be biologically ideal, proven to have the best sleep outcomes. You can have different preferences of course, doesn’t change what’s ideal.

Isn't the thermal cycling greater outdoor than indoor because of the differences in the outdoor temperature between day and night? So most ordinary materials indoors should be fine with a delta of even 10 Kelvins/10 Celsius/18 F.

All of the material thermal failures I can think of come from localized and more extreme sources: electronics, lightbulbs, washing/drying clothes, washing dishes in a dishwasher. Water below freezing point too, but that's because the temperature fell below a threshold. None of these are from heating or cooling rooms directly with HVAC.

Yes, it's MUCH worse outdoors, but indoor temperature (and humidity) swings are not good either. I guess most people here just haven't paid enough attention to it as it's a very long-term effect, but definitely noticeable.

So did you have two rooms with the same objects in them but different temperature swings, over a long period of time, and then compared?

What's the baseline and what's the safe delta?

Everyone I know offline keeps their house at a constant temperature year-round, yet apparently it's controversial enough to be a downvoted opinion here? WTF?

For some people they may only opportunistically (perhaps they have excess PV production at that moment) run their AC or temporarily turn it off when they temporarily have other power demands at that moment (eg running dishwasher, washing machine, microwave, kettle, etc) and don’t want to pay for grid energy while doing so.

“HVAC” is essentially not a thing in houses in South Africa likely due to having a mild climate, this also makes insulation in houses optional and mostly not done.

So most people with AC here are probably using it as a luxury and will happily have it be off a little if it meaningfully reduces energy usage while still making their house noticeably more comfortable.

This is exactly the case in my household. Excess solar is pretty much worthless here, so with a smart AC you can dump the excess solar into your house as either heat or cold. This (currently) has a lot better ROI than installing a household battery. (Battery tech and availability might change this.)

I am enamoured by making dumb AC's smart by this method, since you probably have a lot less IT risks than by using the suppliers app. I think the trade off is between IT risk and mechanical / technical risk, since adding your own parts have an effect on warranty (or at least: willingness to repair by the supplier).

Isn’t this is only true with central air though? Most other types of ac are not full climate solutions to leave running all the time and need some manual control.

At least that’s how I use the mini split in my garage. I have to remember to give it 15-20 minutes headstart before I use the garage or will be really hot.

Check out Uncle Pennybags over here with AC in his garage...

All "smart" t-stats can be used in "dumb" mode but the remote control and monitoring is really the killer feature. Scheduling and presence is meh, if you're obsessed with energy savings I guess.

I'd rather know if the system has failed when I'm 500 miles away before the pipes freeze.

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