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 varicty of steam and hot water boilers is avail- 
