Thanks Peter, I well remember Bernoulli and his theorem; so would it not be a good idea to put a venturi at the beggining of the burn tunnel to give a good combustion mix?
Tim, I,ve got a very good book (in French) on mass heaters generally and they recommend exactly what you say, the venturi acting as in a simple carburettor will draw air in to the burning area, and with what Peter has said that should give better combustion. I wish I could cross the puddle and come and play with you guys!!