Abstract: In an apparatus for heating a heat transfer fluid flowing through concentrically arranged pipe coils in a boiler casing, the casing is supported on top of a furnace in which solid fuel is burned in the combustion chamber. The combustion gases flow directly from the combustion chamber into the boiler casing for passage over the pipe coils. Solid fuel is supplied by a conveyer into a grate below the combustion chamber and air is supplied to a primary combustion air chamber and a secondary combustion air chamber for flow into the combustion chamber. The heat transfer fluid is circulated through the pipe coils by a pump and, if the pump becomes inoperative or flow otherwise stops, the supply of fuel and air is automatically discontinued to avoid overheating of the heat transfer fluid.