1. A BOILER COMPRISING A BOTTOM WALL, UPWARDLY DISPOSED FRONT, REAR AND SIDE WALLS HAVING THEIR LOWER PORTIONS INWARDLY SPACED PLATES DEFINING A WATER JACKET SURROUNDING A COMBUSTION SPACE, SAID FRONT WALL AND COOPERATING PLATE HAVING AN APERTURE THERETHROUGH FOR MOUNTING A BURNER TO EXTEND FORWARDLY BEYOND THE FRONT WALL, SAID FRONT WALL HAVING AN UPPER PORTION ABOVE SAID COMBUSTION SPACE WHICH IS INCLINED UPWARDLY AND FORWARDLY TO PROJECT BEYOND THE LOWER PORTION, SAID REAR WALL HAVING AN UPPER PORTION WHICH INCLINES FORWARDLY AND UPWARDLY FOR PART OF ITS HEIGHT AND INCLINES REARWARDLY AND UPWARDLY FOR THE REMAINDER OF ITS HEIGHT, A WATER SPACE