This is solved in thermostats with hysteresis and even many DIY or close to it solutions like esphome[0] implement it. I know on my Nest when heat and cool mode is on the set heat and cool temperatures have to be two degrees away from one another, which is fine with me because for whatever reasons 67 in the summer feels great but I want close to 69/70/71 in the winter.
The linked esphome docs are actually great whether you care about esphome or not - they explain all of this very well and you can tell from the configuration values and samples what kind of logic goes into all of this.
I really wish I could have my thermostat's set point use "heat index" (a straightforward function of temperature and humidity) instead of just temperature.
I am perfectly comfortable (and therefore want my system to turn off) after my AC has been running long enough to drop the humidity a bit, even if the temp has only dropped a fraction of a degree. As humid air leaks in, I want the AC back on even if the temp has barely increased a degree. Basically the heat index in my house swings enough to change my comfort and therefore could be a useful trigger, even if the temperature doesn't swing nearly enough to be a useful trigger.
The Nest even has a humidity sensor, but it only gets used in very specific (hardly useful) situations.
I considered writing a Works With Nest app for this. Then that platform disappeared in favor of Works With Google, but I never found the time for either...
If you’re willing to spend the money, I’d recommend getting a ZigBee or z wave thermostat(or an EcoBee, as it has local control through HomeKit) and then you can combine the temperature readings with your own humidity/temperature sensors - as mentioned above, ESPHome/Home Assistant may be overkill, but would work great for this kind of setup
> I really wish I could have my thermostat's set point use "heat index" (a straightforward function of temperature and humidity) instead of just temperature.
You could try wrapping your thermostat in a wet sock.
I'm not sure my wife would love that, and I'm not sure how to keep it wet at all times (apply water of a known temperature??) but I am intrigued: how does it work? The temperature of the sock will somehow correlate with the heat index of the ambient air?
Actual web-bulb thermometers would either require manually adding water when you want to make a measurement, or have a large tank of water that gets continuously wicked onto a cloth which is wrapped around the thermometer. The general idea is that, if the surrounding air is dry, the water in the cloth will evaporate and cool the thermometer below the temperature of the air itself (the "dry-bulb temperate"). This is just the same evaporative cooling effect that makes sweating and swamp coolers work. If the surrounding air is very wet (humid), very little evaporate cooling will occur, and the wet-bulb temperature will be very close to the dry-bulb temperature.
I'm not sure how well a thermostat that only used the wet-bulb temperature would work. To actually calculate relative humidity, you would need at least a wet-bulb and dry-bulb measurement. But given that relative humidity in most places probably doesn't swing that wildly, I imagine that just finding a comfortable wet-bulb temperate for your thermostat might work okay in practice.
This is brilliant, thanks. I do see how additional calculations between wet and dry (or just a hygrometer, which the Nest does have already) would be ideal, but as you say, using just a wet-bulb does seem a bit better than using just a dry-bulb in terms of approaching my goal, even if it's not quite all the way there.
Rigging a wet sock (with constant wicking from a water source) onto my actual wall-mounted Nest Thermostat would never fly, as I said, but I have no qualms about doing this experiment with my Nest Temperature Sensor, being a small and relatively cheap thing that I can easily steal for a while...
According to a psychrometric chart [0] it looks like if the dry-bulb temperature stays locked at 70F, while the RH drops from 60% to 40%, the wet-bulb temperature will drop from 62F to 56F. Or if the dry-bulb temperature stays locked at 80F while the RH drops as before, the wet-bulb temperature will drop from 70F to 64F. That's linear enough for my purposes.
Therefore if my conventional (dry-bulb) set point is 75F in the summer, and I'm most comfortable around 50% RH combined with that 75F, then I'll try a set point of 63F with my Nest using the remote Sensor wrapped in wet cloth!
Apparently I also need continuous airflow over this wet-bulb thermometer. I wonder how much.
Humidity definitely plays a part in my preferences but where I live there are 100 degree temperature swings and interacting with the elements outside leads to (for me) more extremes inside.
When I get back from a run in August at near heatstroke levels I will almost never cool down if it's not set on the "chilly" side. Conversely, in winter when heatstroke is replaced with near frostbite I am chilled to the core and need much higher heat to ever warm up.
The linked esphome docs are actually great whether you care about esphome or not - they explain all of this very well and you can tell from the configuration values and samples what kind of logic goes into all of this.
[0] - https://esphome.io/components/climate/thermostat.html