I'm just thinking hes got the thing piled full of wood like a stove and there's not enough volume in a 6 inch pipe to exhaust it all while it burns. Sometimes you need a bigger flue pipe to accommodate more btu. We deal with that on natural draft dunkirk boilers all the time the more btu the bigger the flue pipe is other wise it can spill out the front of the boiler and not all go up the chimney.
Same thing with chimney liner sizing they are all sized to btu and height.
