There’s no doorbell. It would be totally in character with everything else in the house if it was installed in a “handyman” fashion that does technically work just not as well as if it was put in by anyone who knew what they were doing. The unit isn’t really in a draft or the airflow of a vent or the sunlight, the only temperature fluctuation is from the heating itself, and it consistently varies by time of day in a way that doesn’t really make sense if it was trying to (over)compensate for itself at any given point