[Preface: work is about to start in my major renovation project. I am trying to do everything possible to *bring down* costs at this stage. At the same time, I should consider the following question, because it is now or never.]   As some people here know, I've bought most of a small house from 1930 (Paris area), and I plan to switch over to a high-temperature heat-pump. The rationale was not having to change the current radiators. Now the contractor (who does have heating as his speci