I have built a "combo" RMH
unit that will produce steam and hot water
that runs 2 separate coils, 1 around the 4" (insulated)riser, and another looped through the secondary chamber. It works quite well, except it can be a little ticklish to get "kindled up". My stack temperature has yet to exceed 80 degrees F. I have overcome the initial draft problem by the addition of a draft inducer blower, installed in such a way that it can be shut down once the secondary temp is sufficient for sustained drafting. Basically, this works just like a modern 90+% efficiency gas furnace, except it is not necessary to leave the blower in operation during firing. The coils are valved in such a way that I can use them separately or in tandem for water
& steam, steam, or just water.