Most door bell transformers are NOT designed for continuous use such as supplying power to a Nest or similar item.
from https://www.doepke.co.uk/assets/pdfs/Control/Technical-Data-09--RK-3U--0520v1--en.pdf
Nennlast = rated load
from https://www.doepke.co.uk/assets/pdfs/Control/Technical-Data-09--RK-3U--0520v1--en.pdf
Nennlast = rated load