One advantage claimed for modified fireplace units is that the correctly designed and proportioned firebox, manufactured with throat, damper, smoke shelf, and chamber, provides a form for the masonry, thus reducing the risk of failure and assuring a smokeless fireplace! However, there is no excuse for using incorrect proportions; and the desirability of using a foolproof form, as provided by the modified unit, merely to obtain good proportions should be considered from the standpoint of cost. Even though the unit is well designed, it will not operate properly if the chimney is inadequate; therefore the rules for correct chimney construction must be adhered to with the modified unit as well as with the ordinary fireplace.
Manufacturers claim labor and materials saved tend to offset the purchase price of the unit; also that the saving in fuel justifies any net increase in first cost. A minimum life of 20 years is claimed for the type and thickness of metal commonly used today in these units.
Field tests made by this Bureau have proved that, when properly installed, the better designs of modified-fireplace units circulate heat into the cold corners of rooms and will deliver heated air through ducts to adjoining or upper rooms. For example, heat could be diverted to a bathroom from a living-room fireplace.
Figure 21.—In this fireplace the air is not drawn in directly from outdoors but through the inlet, a, from the room that is being heated. The air is heated by contact with the metal sides and back of the fireplace, rises by natural circulation, and is discharged back into the room from the outlet, b, or to another room on the same floor or in the second story. The inlets and outlets are connected to registers which may be located at the front of the fireplace, as shown in [figure 22]. The registers may be located on the ends of the fireplace or on the wall of an adjacent room.
The quantity and temperature of the heated air discharged from the grilles in figures [20] and [21] were measured to determine the merits of the convection features. These measurements showed that very appreciable amounts of convected heat are produced by the modified unit when properly installed and operated. Discharge-air temperatures in excess of 200° F. were attained from some of the units tested. The heated air delivered from the discharge grilles of some of the medium-sized units represented a heating effect equivalent to that from nearly 40 square feet of cast-iron radiation of the ordinary hot-water heating system, or sufficient to heat a 15- by 18-foot room built with average tightness to 70° F. when the outside temperature is 40° F. Additional convected heat can be produced with some models by the use of forced-circulation fans.
