The indirect method of heating (Fig. 174) is the more effective system for large buildings and schools. The heater is generally placed in a cellar or basement. The air is passed over its surface of pipes, and is then directed by a distributing system of sheet iron or tin pipes up into the rooms to be warmed. Heating has the effect of circulating the air in the conducting pipes. The air may, too, be forced into the circulating pipes by a blower or fan located in the cellar. The well-known hot-air furnace is an example of the indirect method of heating.