HEATING BY HOT WATER. 191 
The most inexpensive form of boiler of all those requiring 
to be set in brickwork is the coil, an example of which is 
shown in Fig. 132. This consists merely of a length of 
ordinary wrought iron steam tube bent round four or five 
times into a spiral coil; when heated the water courses 
rapidly through the coil (within which the fire is situated), 
in an upward direction of course. This form of boiler 
is only suitable for heating small quantities of piping of 
from 50ft. to about 200ft., but when properly set a coil 
will do good work for 
several years, though 
sooner or later they be- 
come burnt through at 
some point, usually near 
the top. The best way 
to set a coil boiler is to 
leave a space of about 
1din. between the outside ee 
of the coil and the inside 
of the brickwork. The upper part of the furnace—that above 
the base of the boiler (which should be elevated a course or 
two above the level of the fire-bars)—is most conveniently 
constructed in a circular form, the bricks being laid end on 
to the furnace. This part of the furnace should also be made 
tapering, so as to correspond with the taper in the coil (this 
is given chiefly to prevent the fuel ‘‘ hanging”’ in the coil at 
any time), and it should be brought in rather sharply just 
above the last lap, or turn. If set so that the last lap of 
the coil crosses the exit into the chimney, the pipe will soon 
burn through at this point, but if set as above it will 
withstand the action of the fire for several years. 
