I think BillyBB is a little mistaken in his conversion theory. A back boiler in an open coal fire is a simple (usually copper) tank built into the bottom of the brickwork at the back of the fire. It is sometimes known as a pan loaf boiler because of its shape and size. The bolier tank is connected to water heating pipes and the water in the boiler is simply heated by the fire itself. An oil or gas fired back boiler is also positioned at the back of a fireplace, but not with a live fire. Usually a gas or elecric fire in front.