44 THE CONSTRUCTION OF THE FORCING-HOUSE. 
them, should be underneath the bed. The steam or hot 
water is carried into the house by means of one or two 
pipes,—called risers when steam is used, and flow-pipes 
when water is used,— and Is returned to the heater through 
from two to six times as many returns. The risers or flows 
are usually carried into the house overhead, and the returns 
are carried underneath the benches, along the walls, or the 
sides of the walks. Aside from the greater number of pipes 
required in water heating, there is little difference in the 
methods of piping for the two systems. The highest point 
in the steam riser should be directly over the heater, or, 
when this is not practicable, it should be at the near end of 
the house or the system, and the riser should gradually fall 
from this point to the far end of the house. This gives a 
down-hill system. It is generally considered that the 
highest point in the water flow-pipe should be at the far end 
of the house or system. This is an up-hill system. Flow- 
pipes should not be less than 2 inches in diameter for runs 
of 60 ft., while steam may be carried in a 1%-inch riser 
under similar circumstances. In water runs of this length, 
there should be two risers for houses 16 to 20 feet wide, 
particulary if warm plants are to be grown. 
The amount of pipe required for special cases may be 
determined by examining some of the plans and _illustra- 
tions. Fig. 1 illustrates two methods of piping. It may be 
piped by running a water flow-pipe underneath the bench 
on either side and carrying back two returns under each 
flow. This type of heating isnot common. In such cases, 
it would no doubt be necessary to keep the space under 
the benches open next the walk in order to heat the house. 
A commoner method is to carry a riser or flow overhead at 
X, and to bring back six returns from it. In Fig. 2, a 2-inch 
steam riser is carried overhead in the center, and it breaks 
into seven 1%-inch risers underneath the benches. There is 
a similar arrangement in Fig. 3. These two houses would 
grow tomatoes in New York state. In Fig. 4, two 3-inch 
water flows near the peak supply six 114-inch returns under 
