196 GREENHOUSES 



hot water since with steam heating the tem- 

 perature of the steam must be at least 212 

 degrees, while with hot water the tempera- 

 ture may be varied according to the weather. 

 Automatic air valves are placed at the high- 

 est point of each coil and also in the supply 

 pipes. 



High Pressure Heating. When steam 

 above five pounds pressure is used it is known 

 as high pressure heating. For greenhouse 

 purposes high pressure heating is not satis- 

 factory, as the pipes are too hot. In 

 large establishments, however, a high press- 

 ure is often maintained at the boiler and is 

 passed through a reducing valve before it 

 enters the coils. 



Vacuum and Vapor Systems. Several 

 heating systems are now on the market which 

 endeavor to give to steam heating some of 

 the advantages claimed for hot water, viz., a 

 lower temperature of the heating pipes and 

 less frequent attention to the boiler. They 

 differ from straight steam heating in that a 

 partial vacuum is maintained within the 

 system, thus causing the water in the boiler 

 to give off vapor at a temperature of less 

 than 212 degrees. 



