GREENHOUSE CONSTRUCTION AND HEATING 43 



or cellars which are necessary for the gravity system of 

 steam or hot water; but by means of a trap located above 

 the boiler, the water of condensation is returned to the 

 boiler without causing any trouble in the radiating lines. 

 This system is strongly indorsed by many who are using 

 it for the heating of large establishments. 



(3) High-pressure steam. While this system is some- 

 times used in the heating of greenhouses, it is not satis- 

 factory because of the intensity of the heat. Reducing 

 valves may be used to lower the temperature so that the 

 average temperature in the radiating pipes of the house 

 will be considerably less than in the boiler or mains. In 

 this case it is necessary to use a pump to return the water 

 of condensation to the boiler. The pump may be operated 

 by the high-pressure steam. 



Location of pipes. The pipes should be located where 

 they will not seriously interfere with the work in the 

 houses; nor should they be placed, unless unavoidable, 

 where they will cast shadows on the plants. In practi- 

 cally all vegetable-growing establishments most of the 

 pipes are placed along the walks, with just enough in the 

 central part of the houses to secure the proper circulation 

 of air. Sometimes the central pipes are placed near the 

 ground, but more frequently overhead, and supported by 

 the same iron posts which support the roof. In the 

 Boston district the interior pipes are often 3 or 4 feet 

 above the beds. In narrow houses it is unnecessary to 

 have any central pipe lines, but in houses with a width of 

 20 feet or more central pipes are a great advantage. 



The boiler. Boilers are made either of cast iron or 

 wrought iron. Cast-iron boilers are the more durable, 

 because they do not rust so badly and there are no flues 

 to be burned out as in wrought-iron boilers. On the other 

 hand, fuel consumption is not so economical as in 

 wrought-iron boilers, in which the waterways are thinner. 



A great variety of steam and hot water boilers is avail- 



