208 GREENHOUSE CONSTRUCTION AND HEATING. 
roomy; (2) to ‘“‘choke” the draught, if anywhere, just 
above or beyond the fire, where it enters the flue (in the 
case of the usual horizontal furnace a “bridge” should 
be put in at this point); (8) to stop the draught, when 
necessary, chiefly below the fire, by means of an ashpit 
door, with draught regulator ; and (4) to provide proper 
soot-doors, or loose bricks, at all the necessary points 
for cleaning the flue easily, these being principally at the 
base of chimney, at the farther end of a double flue, 
and again opposite the end of the ‘return’ flue, in the 
chimney, and also at all corners or elbows. 
Fia. 144. 
A flue may be employed in conjunction with a coil, 
conical, or indeed almost any type of boiler, care being 
taken, of course, that the total length of flues to be 
traversed by the draught is not too great for the height 
of the chimney, or the draught will be very sluggish. 
I have frequently employed a single or double flue to 
afford bottom heat, and obtained the necessary air-warmth 
from a small boiler and pipes. In some small houses, 
where very little heat beyond that provided by a flue is 
necessary, a few feet of piping may be heated without a 
proper boiler at all, by merely carrying a single, double, 
or treble row of pipes from 1}in. to 4in. diameter through 
