BOILERS, FUELS AND FLUES 211 



Fig. 110. Battery of two marine type boilers used for 

 greenhouse heating 



that a steam boiler may be used for hot-water 

 heating, but that a hot-water boiler is rare- 

 ly satisfactory for steam heating. Large 

 steam boilers are quite frequently used in 

 hot-water heating when equipped with the 

 necessary fittings which are described in 

 a succeeding paragraph. 



Boilers for Soft and Hard Coal. Hard coal 

 burns with a "short" flame, and much less 

 fire travel is required to burn the gases than 

 when soft coal, which burns with a "long" 

 flame, is used. More flue way is also re- 

 quired for soft coal and the grates are more 

 open. Most greenhouse boilers which are 



