It is by ‘design’ of thermostat. I’ve been doing all the development and testing of Master/Thermostat shield on my heating system, and my boiler (ViessMann some condensing unit) does not allow set temp below 20. Also there is no reason going below this temp, better is to shut off boiler and pump completely to save electricity rather than circulating 15-20 degrees water over pipes.
I have not implemented full shutdown for a security reasons. If something goes wrong (MQTT/HomeAssistant hangs, PID calculation fails, thermometer sends incorrect data) boiler would kept at minimum temperature to avoid freezing (for a cases when boiler and thermostat are left in some remote environment where you cant keep an eye on it).
But again, this is more for ‘conventional’ heating system with radiators and floors. If you have some tricky setup that can utilize heat from such a low water temperature - you can easily edit sketch to allow boiler target below 20.
Here’s the code:
