This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
232. A hot-air furnace should be set at the lowest practicable level, so that even the longest horizontal hot-air pipe may be given a sufficient upward inclination to make it operate well. In some cases the ceiling of the basement is so low that it is necessary to set the furnace in a pit. The extreme top of the furnace casing should never be less than 18 inches below the ceiling or floorbeams, and should be as much more as possible.
The side walls of the pit should be 8 inches thick if made of brick, and 12 inches or more if made of stone. The bottom should be covered with one course of brick on edge, or with concrete at least 4 inches deep, or, better still, with flagstones.