Arthur,
Yes I have two devices (well three actually, The thermostat - which I just use as a room temperature sensor, The boiler switch - which switches the boiler off and on AND the generic_thermostat in HA which reads the sensor and instructs the switch.
From what you have described (I can’t be 100% sure as YMMV given differing equipment and implementations) It seems like you are experiencing the classic, “Okay I TRIED to set the temperature value to 20° but it wouldn’t change from 21° so I’ve set my value back to 21°”
If you wait, Im sure someone will come along and help you given an experience set from working with such a device.
Good luck.
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