In an earlier project log, I showed the bare bones of integrating a Ventbot with Home Assistant. It's very easy to wire up user interface buttons and other controls to call scripts in the Ventbot's ESPHome firmware. In the time since I started working on this project, I added a heat pump to the house. Along with that came a smart thermostat upgrade. It's an ecobee, but it was chosen by the HVAC contractor as part of a standard bundle. There are lots of smart thermostats on the market.

The important part in the current context is that there is also an ecobee integration with Home Assistant. Besides telling me the current temperature at the thermostat and allowing me to fiddle with a few configuration settings, it also tells me when my HVAC is either cooling or heating. (The ecobee also has a configuration for how many minutes per hour you want the fan to run, regardless of whether it's heating or cooling. I haven't found that in the Home Assistant integration, but I also haven't pursued it.)

To make the Ventbot design usable under various circumstances, it can operate completely standalone. An attached temperature sensor reliably detects when there is cooling or heating going on (though the "run the fan for N minutes per hour" can fool it for when heating or cooling turns off). 

For someone who wants to go to the trouble of integrating with Home Assistant (or any other smart home system), it would be possible to use the smart thermostat's heating or cooling indications to turn the Ventbot on and off. That would be more precise at the cost of a more complex system with more moving parts. The idea is that Home Assistant would receive notifications from the smart thermostat that heating or cooling started or stopped. Home Assistant would then make calls to the appropriate services that communicate with scripts in the Ventbot.

Once you go to the trouble of integrating with a smart home system, there are other possibilities. The temperature sensor in any single Ventbot could act as a master switch to tell the other Ventbots, via the smart home system, to take action. In fact, you could use a temperature sensor that was completely separate from any Ventbot. You could also use relative humidity readings to make some kind of "comfort" calculation in deciding about turning Ventbots on or off, along with time of day, people home or not, phase of the moon, and so on. (The Ventbot BME280 also has a relative humidity sensor, but it's not reliable for this purpose. The relative humidity inside your ductwork is likely quite different from the relative humidity inside the rooms in general.)

I haven't done any of the things described in this project log, except for the basic Home Assistant integration I did for debugging purposes. But it's all pretty standard stuff in the home automation arena, so it shouldn't be too difficult. I might make some tweaks and finesses to the scripts on the Ventbots that are callable from Home Assistant, but OTA updates to the Ventbots make that straightforward at some later point.