Outdoor wood boilers have been a thing for a long time. Back when I had mine, it kept the water at 180F. https://en.wikipedia.org/wiki/Outdoor_wood-fired_boile

Outdoor wood boilers have been a thing for a long time. Back when I had mine, it kept the water at 180F. https://en.wikipedia.org/wiki/Outdoor_wood-fired_boiler https://en.wikipedia.org/wiki/Outdoor_wood-fired_boiler If you have enough sufficiently dry wood, downdraft gasification models burn extremely cleanly (they're effectively rocket stoves), but those models require a fair bit of maintenance, especially if your wood is slightly wet or green, because soot can quickly build up on the water envelope.