This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Furnaces may be divided into two general types known as direct and indirect draft. Fig. 28 shows a common form of direct draft furnace with a brick setting; the better class have a radiator, generally placed at the top, through which the gases pass before reaching the smoke pipe. They have but one damper usually combined with a cold-air check. Many of the cheaper direct draft furnaces have no radiator at all; the gases passing directly into the smoke pipe and carrying away much heat that should be utilized.
Fig. 29 represents another form of direct draft furnace. In this case the radiator is made of sheet steel plates riveted together, and the outer casing is of heavy galvanized iron instead of brick. In the ordinary indirect draft type of furnace (see Fig. 30) the gases pass downward through flues to a radiator located near the base, thence upward, through another flue to the smoke pipe. In addition to the damper in the smoke pipe, a direct draft damper is required to give direct connection with the funnel when coal is first put on, to facilitate the escape of gas to the chimney. When the chimney draft is weak, trouble from gas is more likely to be experienced with furnaces of this type than with those having a direct draft.