One great advantage of the indirect system is that

Fig. 173.   Steam Heating System

Fig. 173. - Steam Heating System there is always some ventilation. New air is always entering the rooms, while at the same time the older air must make its escape around windows and doors, or pass out through flues built into the walls of the building for this purpose.

Ideal ventilation is not often secured, however, even by indirect heating, for the air that comes from a cellar is not always pure and fresh. It is more often dusty and odorous from the refuse or decaying matter which frequently lies about a cellar. To overcome this difficulty, the air should be brought in from outside the building by means of an airtight flue or box. The inlet to the box should be carried up high enough outside of the building to avoid drawing in litter and dust and should be covered with a strong wire-mesh screen to keep out rats. In many public school heating systems the outside air, before entering the heater, is purified by being passed through a water-spray curtain.