I would love to see an article about hydrogen boilers.  I know adoption in the US is a number of years away, but there is some progress in England on this non-fossil technology.  Where I live in NYS there are a lot of  existing hydronic systems, and I assume this is true for a lot of the country. Air-to-water heat pumps is a parallel technology that needs to be part of the same conversation.  I know it’s now inadequate for old radiators, but capable for radiant systems, panel radiators, and some sorts of hydro-air systems,
Stephen Rose
Cold Spring, NY