19*2 



GREENHOUSES 



flows back to the boiler through the supply 

 pipe. In this case all pipes have an upward 

 slope from the boiler, with no sags or pock- 

 ets in which the water can collect. This 

 method, sometimes known as the single pipe 

 system, is very commonly used in heating 

 dwellings where the pipes are mostly verti- 



Fig. 100. A mortise coil designed to allow for expan- 

 sion of pipes 



cal, but in greenhouses having long, nearly 

 horizontal coils there is likely to be much 

 hammering in the pipes, caused by the in- 

 terference of the steam with the return 

 water. 



A more satisfactory method for green- 

 house heating is to arrange the pipes much 

 the same as in hot-water heating, pro- 

 portioning the size to the supply and re- 



