Hi, the hysteresis setting on my Nest is ridiculous. The room is swinging through 2 degrees C (as measured by an independent temperature sensor) between on and off which is totally unacceptable from a comfort point of view. I have had a Netatmo for many years and it managed to maintain a 0.3C variance. There are many posts about this problem but no solution seems to have been made public. My Nest will be being removed at this rate - its close to useless!!
I think that is what is being debated here - and the fact that Google still has not made the hysteresis ('swing band') adjustable.
But in a general case, you set temp to 20C, furnace would likely run until temp is closer to 21C (by the thermostat) and then not turn on again until it has reached something like closer to 19C.
Too narrow of a band, especially combined with a too large furnace (common in the US), results in short cycling. Furnace turns on and temperature by the thermostat quickly reaches its shut off point. But dwelling hasn't been heated, and just by simple air movement (the warm air quickly cools down, is denser and falls) also drops the temperature at the thermostat, triggering a demand for heat again.
Part from properly dimensioning the furnace, on the small side which is really hard as the environment it operates in varies so much, only way to get around this is to have: 1) adjustable hysteresis, 2) an adjustable timer (like once turned on and off, furnace cannot start until 30 min has elapsed).
Should note that this is also truly much more of an issue for forced air systems than anything. Most of the are 'digital'; On or Off. If you have hydronic heating, as an example, the thermostat does not control the furnace or boiler. Furnace/boiler is controlled by the temperature in the energy storage (water), and the thermostat either controls the distribution of water (circulation pump on/off, or adjusting a shunt valve to regulate the outgoing water temperature). In this setting, furnace/boiler will run until the water has reached a certain temperature, regardless what the inside temperature is. 'Heat' will then be taken from this 'energy storage' (which might only be good for 30 min to 1 hour, or can be good for several days if having a large accumulator tank), but the upside is that when the furnace/boiler runs, it runs. And when it's off, it's really off. For a long time.
Personally, I hate the forced air systems, but they are prevalent in the US because they are cheap, simple, and they do easily provide support for for A/C as well. With the accelerated proliferation of mini-splits, that is finally starting to change, but I'd love also to see more hydronic heating systems. Nothing is more comfortable than large areas that are 'mildly' warm (wall mou8nted radiators or entire floors) versus a very hot small area (heat exchanger in a furnace, a baseboard heater, fan driven or just natural convection). The latter works great to dry out the already dry winter season air and promote all types of health issues (in addition to what the ducting in a forced air system already is responsible for).
Things trailed off here and it's been a while since we discussed the problem you were having. I wanted to check in and see if you were still trying to find a good setting for your setup or to see if you got things figured out. If you're still trying to get things where you want, let me know.
I gave up and just learned to live with poorer functionality than my old Netatmo had. My Nest also failed within first year and was replaced under warranty. Basically Nest is a good looking and expensive package, with fairly poor basic thermostat functionality and reliability. And the app is barely acceptable compared to what the competition have had for about 8 years. But thanks for trying and asking.
I only peak in here once in a blue moon, but will say that Nest has been working OK for me (now at the tail end of the heating season). In my case I think the swing band is still too narrow, but I have not really had any short cycling issues as I keep it cold inside and when furnace comes on, it heats up quickly and stays there long enough for furnace not needing to be run at all in many hours.
But it should be adjustable! That should be such an easy firmware update! Even my cheap Lux had an adjustable swing band back in 2001