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[Preface: as quite a lot of people here know by now, I am doing a large renovation job on a place I bought in a near suburb of Paris earlier in the year. This is the last moment to make big decisions - work (demolition of some walls) should start this week. The place will be very well-insulated (for renovation standards) - 18cm cellulose on most walls, 11cm of cellulose+reflective insulation on the courtyard wall] So, (a) I am finally considering seriously installing a low-temperature heat pump, with low-temperature radiators, rather than simply switch over from a gas boiler to a HT heat pump. If I keep the number of radiators the same (14 radiators, for a place that will be about 120m^2 loi Carrez (= inhabitable area you can stand in, not counting staircases) and 180m^2 non-loi Carrez)), the net cost would be about 5800eur, including 800eur in increased work for the contractor and the architect. (Low-temperature heat-pumps are not just better, they are cheaper; however, low-temperature radiators are expensive - 650 eur a piece); (b) a friend who advises me (N. the Philanthropic Flipper) was telling me: insist on having fewer radiators; the architect finds their number to be right, but N. says architects and contractors always overestimate the number of radiators needed, at least in the Paris area. (c) the architect is a bit bothered by the possibility of a switch to low-temperature simply because low-temperature radiators are large, and we need wall space for bookshelves (I have about 1500 volumes currently, besides a vinyl collection, and the number of books seems to double every ten years) In short: how many low-temperature radiators are likely to be enough? It's a place with high ceilings on the ground floor and first floor (270cm to 290cm). Only the central part of the attic will be above 180cm. Here are the plans (already showing the insulation and the work that will be done): Presumably I want narrow, tall radiators, so as to use space efficiently (putting books right above a radiator is not necessarily a great idea in the long term, is it?). There are currently radiators under the windows, but it is very doubtful that low-temperature radiators will fit in there. For a climate table for Paris, see https://en.wikipedia.org/wiki/Climate_of_Paris Attention: the table there gives the average for 1991-2020. The climate is getting noticeably hotter in summer and milder in winter.

[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 specialty) is insisting that we do need to do work on the conduits, move some radiators, etc. Hence, I am asking myself whether it wouldn't be better to switch over to low temperature. The architect says that the net cost ( = price of switching the network to low temperature, minus a certain amount coming from the fact that low-temperature heat pumps are lower-cost) would be about 5k eur. (I doubt she's taking into account that, if the network is high-temperature, a smaller hot-water heater is needed; for some reason she has put a 300L heater in the plans.) Is it worth it? The architect seems to be taken aback mostly by the fact that it would be more work for her (but it's not as if I am not paying her enough), and that low-temperature heat pumps take a lot of wall space (I've got only about 1500 books currently, but libraries grow). (For further reference: the previous owners had no problem heating the place with their old gas heater set on "medium" (about 50deg?), even though the place had essentially no insulation. We are installing plenty of insulation, obviously, as some very long threads here attest; thanks to everybody.) (Another factor in favor of low-temperature: the city heat network might reach our block some day (it's 100m away, currently) and joining it might then become possible (though the town says that our co-op is too small for that on its own). That network works only with low-temperature systems.)