The Franklin stove (fig. 19) is a type of modified fireplace. The modified fireplaces of today are of several types, as shown in figures 20 and 21. Both the last two types of modified fireplaces are manufactured as units of heavy metal, designed to be set into place and concealed by the usual brickwork, or other construction, so that no practical change in mantel design is required by their use. The modifications are built-in standard parts of the fireplace--only the grilles show (fig. 22). [Illustration: Figure 20.--In this modified fireplace air enters the inlet, _a_, from outside and is heated as it rises by natural circulation through the back chamber, _c_, and the tubes, _t_, being discharged into the room from the register, _b_. Air for supporting combustion is drawn into the fire at _d_ and passes between the tubes up the flue A damper is also provided to close the air inlet.] 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. [Illustration: 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.]