170 



GREENHOUSES 



hill" or the "up hill" system the air which 

 collects in the pipes will eventually reach the 

 highest point when it may be allowed to 

 escape through an automatic air valve. In 

 the "level" system slight sags and raises are 

 likely to occur and the air will collect in the 

 higher parts and cause trouble. 



Fig. 92. Diagram showing "down-hill" and "up- 

 hill" systems of piping. A, boiler; B, flow pipe; 

 C, C', headers; D, radiating pipes or coils; E, re- 

 turn pipe; F, automatic air valve; x indicates 

 height of water column 



The author prefers the "down hill" system 

 when the flow pipes are carried in the upper 

 part of the house and the coils are consider- 

 ably lower. When all the pipes must be in 

 the lower part of the house, or under the 

 benches, he prefers the "up hill" system. The 



