SYSTEMS OF VENTILATION 



335 



g a a 



TEMPERED AIR ROOM, G. In both of these rooms, G and H, 

 the air is under increased pressure due to the force with which 

 the blower forces the air into them. The air in the hot room, 

 H, however has been re-heated as it passed through the hot 

 coils, F. The temperature of this heated air depends upon the 

 outside temperature. It may be 80 or 85F. on a mild day, 

 or 110 to 120F. on a cold day.. The air in the tempered 

 room, G, remains at about the same temperature as in the first 

 tempered air room, D. From the rooms, G or H, or both, 

 the air passes through the flue, F, to the wall register, M, 

 thence into the room to be heated. The foul air escapes from 

 the room through the other wall register, N, into a flue which 

 extends up through the roof. 



368. The Thermostat Control of Temperature. The 

 plenum system is operated on the principle of maintaining a 

 constant temperature. It is necessary, 

 therefore, to control the proportion of 

 heated air from the hot room, H, and of 

 tempered air from the tempered air room, 

 G, which passes through the flue, F, to any 

 room. This control is accomplished auto- 

 matically by a THERMOSTAT SYSTEM. 



The iron cylinder, P, Fig. 237, contains 

 air which is maintained under a constant 

 absolute pressure of 2 atmospheres. The 

 pressure gauge, therefore, reads about 15 

 Ib. to the sq. in. (see Art. 157). A small 

 tube leads from the cylinder to the wall 

 thermostat, T, in the schoolroom. A second similar tube, K, 

 Fig. 238, leads from this thermostat down to the controlling 

 thormostat, S, Fig. 237, in the tempered air room. 



When the schoolroom is below the desired temperature the 

 wall thermostat, T, Fig. 237, closes the tube, thus preventing 

 the compressed air from passing down the tube K ; when the 

 schoolroom is too warm the compressed air passes more freely 

 through the thermostat, T, and down the tube K to the con- 

 trolling thermostat, S. 



FIG. 238. Diagram of 

 thermostat. 



