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.
230. These heaters are the same in construction as ordinary hot-air furnaces, except that they are provided with extra parts, which serve to heat water in sufficient quantity to supply a small number of radiators.
In making a selection of a combination heater, particular regard should be given to the construction of the hot-water heater. This is usually of small dimensions and is liable to have small contracted passageways, or small connections, which make it unfit for hot-water service. Any heater which requires that screwed pipe joints or fittings shall be exposed to the direct action of the fire should be avoided.
These water heaters may be divided into two classes: those which are exposed only to hot gases, and those which come into direct contact with the burning fuel. The latter class, when in operation, soon become separated from the live coal by a layer of ashes or dead coal; this hinders the transmission of heat and greatly diminishes the efficiency of the heater. The former class are free from this defect, but, on the other hand, some varieties are liable to have their usefulness impaired by accumulations of ashes and soot.