My RMH is a 7" J-tube system, all cob. There is a unique feature in this picture if you look closely. There is an air intake on the bottom left that feeds air from the floor level instead of a couple feet off the ground like most of these kinds of stoves have.
The idea is to draw in the coldest air in the room into the stove instead of air from a couple of feet higher that has already been heated. Doing this creates negative pressure there which pulls the warm air above down to floor level where the floor can be heated quicker. The floor should be considered as mass that needs to be warmed also.
The feed is 16", but the added space for the air intake adds another 4", therefore, I can insert longer pieces of wood if I need to. The pot lid is needed here in order to force the air to be drawn from floor instead of through the feed.
System works great.
