HOT WATER INSTALLATION 



185 



Recently various automatic devices using 

 a column of mercury to produce the same re- 

 sult have been placed on the market. One 

 model is designed to be placed in the pipe 

 leading from the return pipe to the expansion 

 tank, the tank in this case being open. The 

 advantage of these devices over the closed 

 tank system lies in the fact that they are 

 less likely to become clogged and stick than 

 are the safety or pop valves. 



In action these so-called 

 "generators" operate as 

 follows: The pressure is 

 determined by the height 

 of the column of mercury- 

 When there is no heat in 

 the boiler the mercury is 

 in the position shown at a, 

 Fig. 98. As soon as the 

 water becomes warm it ex- 

 pands and flows in through 

 the opening x. This forces 

 the mercury down in the 

 cistern and up through the 

 small pipe b. The amount 

 of mercury is so arranged 

 that when it is pushed 

 down to the level of the 



Fig. 98. A type 

 of mercury "gen- 

 erator" 



