HEATING AND VENTILATION. Gen erally speaking, the methods of heating build ings may be divided into two general classes — the direct and the indirect system, or a com bination of the two. Heating by means of an open fire, by a stove and by radiators Placed in the rooms to be warmed are examples of the former method, while furnace-heating and heat ing by means of a current of air warmed by indirect steam or hot-water coils are examples of the latter method. When a direct radiator Is fitted with a connection to the outer air, it is said to be arranged on the direct-indirect principle. Hot water, steam or electricity may be the vehicle used for conveying heat to radiators. Ventilation is only obtained by sup plying air, and in some systems of heating and ventilation the air is made so hot that part of it is available for heating purposes. This is the case in furnace-heating.
It is well known that when two bodies of different temperature exist, heat passes from the warmer to the cooler body until their tem peratures are equal. If a building be of a temperature of 70° F. and the outer air of a lower temperature, heat will be transmitted by the walls, windows and other exposed surfaces, and the temperature of the air in the building will be lowered. It is only by supplying to the building an amount of heat equivalent to that transmitted by the walls and windows that it is possible to maintain the building at constant temperature. If we supply more heat than is transmitted by the walls, the temperature of the room rises.
Heat is measured in units which have as exact a value as a ton of coal or a pound of sugar. British physicists have selected as the unit of heat that quantity which will raise the temperature of one pound of water one degree on the Fahrenheit scale when the water's tem perature is near 39° F. This unit is designated as the British thermal unit. It is known with reasonable accuracy just how many heat-units are transmitted by each square foot of wall, window and other exposed surfaces of the vari ous materials used in building construction, un der such extreme conditions as to building and outside temperature as may exist. With these
data and the plans of a building, calculation will show the heat-lost from a building or a room, and the heating-apparatus should be propor tioned to supply this amount of heat. Allow ances are made for various conditions that may exist, depending upon thejudgment and ex perience of the designer. The heat required can be supplied by radiation from an open fire or from a stove, but this is an unsatisfactory method. Direct radiators supplied with steam or hot water can be placed in a room to furnish the heat necessary, or the heat may be sup plied by hot air from a furnace, or by air heated by indirect radiators supplied with steam or hot water.
Heating by hot air is a slightly more ex pensive method than heating by direct radia tion, for to be effective the air must be taken in from outdoors, sometimes at very low temper ature, and heated above the temperature of the room to be warmed. If cold air at 40° F. is heated to 100° F., and is supplied to a room at this temperature, it is evident that as soon as this air is cooled from 100° to 70° no more heat can pass from the air to the room if the tem perature of the latter remains at 70°. Under these conditions only one-half of the heat that has been supplied to the air is available for heat ing the room. This will tend to show why heating by hot air is more expensive, estimated from the cost of fuel, than the direct system. When the advantages of the air supply that accompanies indirect heating are taken into ac count the increased fuel cost becomes insignifi cant.
Direct heating is usually obtained by steam and hot-water radiators. Although manufac turers have greatly improved the appearance of direct radiators, at best they are unsightly and objectionable from an artistic point of view. This objection may be overcome by concealing the radiators in boxing beneath windows, when the walls of the building are thick enough to permit the boxing to be built in without project ing into the room. A screened opening is pro vided in the front of the boxing near the floor, and one at the top over the radiator, to permit a circulation of air, so that the radiators can be effective.