It seams that the RMH i have seen all get combustion air from inside the desired heating area. I have a freestanding wood fireplace that gets it's combustion air from outside the desired heated space. Has anyone designed a RMH that will get combustion air from outside the house, shop, greenhouse that it is trying to heat? it seems that there might be a lot of heated air going up the "chimney" so to speak and maybe outside combustion air might be more efficient? I have not made a RMH but would like to put one in a hoophouse in central N.C. for next winter and was wondering if it would be worthwhile to spend time on the combustion air issue?