Växẍ Säbbäth on Nostr: consider a masonry stove or Kachelofen; you need to build it a bit larger and use ...

consider a masonry stove or Kachelofen; you need to build it a bit larger and use masonry for the flue (although metal would be a good thing to enclose it in), but the virtue of that is that it does a good job of complete combustion (so nobody should give you grief over it) and it will trap the heat for a very long time, so it’s very efficient