since the original question was
Does BG have access to Viessmann spare part? do they service
I interpreted this as a question about the big BG distribution and stockholding warehouse keeping parts for Viessmanns; hence the reason somebody would have looked at the records to see how many boilers, of what type, were installed, since the information would be highly relevant to deciding what parts to keep in stock. The person doing this analysis would have no need to know the names and addresses of the customers therefore DPA is not relevant.
It is not a question, or a decision, a local mender in his van would need to, or necessarily be equipped to, handle.