128 THE PRODUCTION AND USE OF HEAT 



vertical pipes filled with hot water, and others filled with 

 cold water. The cold water being the denser and therefore 

 the heavier, and the pipes being connected both at the top and 

 the bottom, the cold water is certain to fall to the bottom of 

 the system and crowd the heated water to the top. This 

 means that the cold water sinks into the heater and that the 

 hot water is forced up into the radiators. The water in the 

 radiators soon becomes cool and the water in the heater soon 

 becomes heated, thus the circulation is maintained. Notice 

 the slope, or PITCH, as it is called, in the case of the horizontal 

 pipes. Why should these pipes slope as they do? 



146. Essential Features of Any Hot Water Heating System. 

 1. In any system of hot water heating, the circulation of 

 the water depends upon the unequal weight of two columns 

 of water, one heated and one cold. The heater, the piping, 

 and the radiators must be placed with this thought in mind. 



2. Since water expands when heated and contracts when 

 cooled, every hot water heating system must be provided 

 with an EXPANSION TANK. This tank is to give the water a 

 chance to expand when heated without bursting the pipes. 

 (See Art. 15, Ex. 11.) 



3. Care must be taken that every portion of the heater, 

 the pipes, and the radiators can be drained completely when 

 not in use; this will prevent freezing and bursting of pipes. 



4. Care must be taken that the water in the expansion tank 

 and in the pipe leading to it does not freeze when the heater 

 is in use; otherwise a severe explosion may occur. Why is 

 this so? 



147. Advantages of Hot Water Heating. Many people 

 regard the hot water system of heating as superior to any other 

 system. When well installed and properly operated, it doubt- 

 less is cheaper to operate and gives a milder, and more even 

 heat than is usually obtained from other systems. It is also 

 possible so to install this system of heating as to give ample 

 ventilation, but in such a case it is doubtful if it is less ex- 

 pensive to operate than a good furnace system providing 



