The circulation and ventilation of the air in a room is necessary in any method of heating. Warmed air rises to the top of a room and the cooler air settles nearer the floor. A steam radiator warms the air directly in contact with it and this air therefore rises. Cold air takes its place and is in turn warmed. The temperature of the air in the room gradually rises until the air, walls, ceiling, and furniture or machines have all been warmed. A certain amount of heat is lost through the walls, ceiling, and windows, and there is always a leakage of cold air into a room through cracks at windows and doors. This loss of heat outwards and cold air leakage inwards increases as the difference between the temperature of the inside and outside air increases. Double-windows, storm-doors, building-paper under shingles, clapboards, and plastering tend to check these losses.

Fig. 175.   Hot Water Heating System.

Fig. 175. - Hot-Water Heating System.