It's more like c) the circulating pumps would normally move the hot water both towards the radiators and away from the boiler, to prevent the water from overheating. Without the pumps, convection will do that, but at a much slower rate. The water in the boiler will become much hotter before it can rise up the loop to the radiators. We did find that making the water hotter than recommended did get it to rise through the loop faster, but the manual for the boiler was very explicit that that was not a good idea, certainly not for an extended period of time. There's a section specifically on how to judge how much wood to put into the firebox at each firing to maintain the water at the optimum temperature for convection.