About as effective as any reasonably efficiently designed wood stove. Usually 60%-80% transfer efficiency at best, depending on the exact design and way it's op

About as effective as any reasonably efficiently designed wood stove. Usually 60%-80% transfer efficiency at best, depending on the exact design and way it's operated. They're a fad because they look cool, not because they're order of magnitude more efficient. You can run them more efficient, but then you have to deal with corrosive condensate, which makes the operational characteristics questionable.