Get rid of radiators!
Oui. Oui!
Even the modern ones are awfully inefficient. ---> "passive-house" is one that requires warming in Winter and stays cool in Summer.
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After having under-floor heating in my first house in the 70s of the previous(!) century, wanted ditto thereafter. But radiators were generally standard.
---> So, we ripped them out, turned 'under-floor' through 90° and slapped it on one wall in each room. Perfect.
Perhaps not the ideal of "passive-house". But water-temperature in the wall pipes is far lower -- and more energy-saving -- than for radiators.