HEATING BUILDINGS 



105 



into the radiator and also lets the condensed steam or 

 water out of the radiator. Somewhere on the radiator, 

 toward the opposite side or end from the large inlet valve, 

 is a small valve 

 which permits the 

 air to escape when 

 the steam is turned 

 on and also lets in 

 air so that the 

 water can flow out 

 of the radiator 

 when there is no 

 steam in the boiler. 

 The small valve, if 

 in good condition, 

 closes automati- 

 cally as soon as all 

 the air is out of the 

 radiator and the 

 hot steam begins 

 to escape. The 

 valve opens again 

 after it cools. 



In this system the heat is carried from the boiler to 

 the radiator by convection. The radiator is heated 

 on the inside by conduction. The heat passes through 

 the radiator by conduction. The heat passes into the 

 room partly by radiation and partly by conduction. 

 The air which touches the radiator is heated by con- 

 duction and so becomes less dense and is forced toward 

 the ceiling by the cooler air moving toward the radiator. 

 This process causes convection currents to be set up in 

 the room and so the whole room is heated. 



STEAM-HEATING SYSTEM 



