162 GREENHOUSE CONSTRUCTION AND HEATING. 
any of them at will, but it is only really necessary to have 
one valve to each set of pipes, this being placed preferably 
on the flow, all that is necessary for this purpose being 
merely to stop the current. But in order to enable 
the pipes in any one structure to be emptied at will, 
for repairs, or the like, it is necessary to provide tight screw- 
down or steam-valves on both the flow and return connections. 
Unlike the ordinary throttle-valves, these are water-tight, 
but they are of course somewhat more expensive. 
Very long houses, or ranges, are however best heated 
from the centre, or middle, the heating pipes, flow and 
return, branching off right and left from the mains running 
across the structure from the boiler. A 4in. pipe will 
“carry” all right a distance of 200ft. or 250ft., or some- 
what farther still if laid with a free rise to and fall from 
the farther end, but houses of 500ft. or more in length 
are best heated from a boiler or boilers placed at or near 
the middle, the pipes branching off right and left to each 
end. In such a case it is usual to employ 6in. mains to 
and from the boiler. See Fig. 107. 
