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No problem with alcohol to fire an open fire or a plain pot boiler from underside, I agree, but with an internally fired boiler a vacuum fan in the chimney is mandatory to start steaming !I also have a lot of excitement when models start to breath...
Gary, I can see the sense in increasing the chimney height but if you you do it by sliding a tube into the chimney you are further constricting the airflow. I'd be inclined to further experiment with the firebox and blower arrangement unless you can lengthen the chimney without reducing its cross section area.