To reduce the cost of hot-water heating and make it also available for this class of small house, the manufacturers produced another type of water heating-plant. In this device the water-heater was installed in one of the rooms of the house, like a stove, but the exterior was designed to serve as a hot-water radiator for the room in which it was placed. From this heater pipes were taken off to distribute heat to other radiators, located in adjoining rooms. The principle remains the same as the former system; the only difference lies in the reduction of cost by eliminating the boiler from the cellar and utilizing it to heat the room in which it was placed. Other attempts to improve the mechanics of heating have been more along the line of perfecting the operation of valves or the utilization of other fuels than coal. Gas-radiators have been tried, but they are so expensive to operate in most parts of the country that they are not always suited to the needs of the small house. Electric heaters, too, are not within the pocketbook of the average person owning the small house. Fuel oil-burners also have been devised to take the place of the coal-grate. Wherever oil is cheap enough to permit their use they are great labor-savers, since they eliminate all the shovelling of coal and handling of ashes. These will be discussed later. Briefly, then, the available systems for the heating of the small house are: _Hot-air.—a._ Furnace with flues. _b._ Furnace without flues. _Steam.—a._ Ordinary gravity system. One-pipe. Two-pipe. _b._ Vapor-vacuum system. _Hot-water.—a._ Ordinary open-tank system. One-pipe. Two-pipe. _b._ Closed-tank system. _c._ Special open-tank system with boiler used as radiator. _d._ Patent system using water in radiators but steam for circulation. _Methods Employed in Calculating the Required Size of Heater_ The basis of calculating the required size of any one of the systems previously mentioned is to assume that a certain temperature of heat is to be maintained when the weather is zero, and then by means of the laws of heat transmission estimate the quantity of heat lost per hour from the house. The amount of heat lost per hour is, of course, the quantity which the heating system must supply. Knowing this, a system is installed which is capable of supplying this heat loss. In such devices as the warm-air furnace the required size can be computed directly to meet the heat loss, but where radiators are used the required sizes of these must first be determined to offset the losses from the rooms in which they are installed, and then the size of the heater must be estimated to supply sufficient heat to the radiators and to make up for the losses of heat through the distributing-pipes.