42 VEGETABLE FORCING 



this problem that a greenhouse heating specialist should 

 be consulted before a decision upon any given amount of 

 radiation is made, unless the matter has been determined 

 by actual experience or observation. 



Systems of hot water heating. There are three distinct 

 forms of hot water heating, viz., the open tank gravity 

 system, the pressure system and the forced circulation 

 system. The gravity system is the oldest and is still used 

 quite extensively. It provides for open tanks and large 

 pipes. With it there must be ample radiation. Although 

 plants thrive with this system, it is not popular with large 

 commercial growers,- because of the excessive cost of 

 installation, nor is it satisfactory in very long houses. 



The pressure system secured by the use of mercury and 

 sometimes by safety valves is quite popular and satis- 

 factory when properly installed and operated. Of the 

 three methods of heating by hot water, the forced circula- 

 tion system is the most satisfactory for large greenhouses. 

 In this system the velocity of the circulation is increased 

 by means of propellers or pumps operated by motors or 

 engines. With forced circulation the mains and coils 

 need not be so large as with the gravity system, so that 

 the cost of installation is not greatly in excess of steam. 

 This system does not require a large volume of water in 

 the boiler and radiating pipes, so that the temperature of 

 the house is under more perfect control than with the 

 gravity system, and all parts of the house are heated 

 uniformly, a condition not possible in large houses in 

 which the gravity system is installed. 



Systems of steam heating. There are three systems of 

 steam heating. (1) The low-pressure, steam-gravity re- 

 turn. With this system the pipes are laid in the same 

 general positions as with gravity hot water, care being 

 taken to avoid water pockets. 



(2) "Low-pressure steam with steam-return trap. It is 

 often impracticable or undesirable to excavate boiler pits 



