METHODS OF HEATING 



345 



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 air- 

 tight 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. 



387. Exhaust Steam Heating. Exhaust steam from an 

 engine is often used for heating. The water of condensation 

 from an exhaust steam heating plant is frequently allowed 

 to run to waste, but as its temperature is near boiling, coal is 

 saved if the water is collected in a receiver and pumped back 

 into the boiler. Mill engines that are run with condensers can- 

 not furnish exhaust steam for heating. In such a case, live 

 steam must be taken from a branch opening in the main steam 



FIG. 174. Indirect Heating 

 System. 



