HEATING BY HOT WATER. 207 
round through the flue, which soon becomes warm. Should 
the fire become low or dull, or the draught become sluggish, 
pull the damper out for a few minutes, and all will go well. 
The chimney, in all cases, should be of sufficient height 
to create a fairly brisk draught; the higher the shaft 
the stronger the draught will be, naturally. The usual rule 
in fixing steam boilers, etc., is to take the total length of all 
horizontal flues, etc., from the fire to the base of chimney, 
when about two-thirds of this distance will be the proper 
height for the chimney, and will afford a good draught. 
Anthracite or ‘‘hard”’ coal of any kind requires a stronger 
draught, however, than coke. For a flue, a chimney whose 
height equals half the total length of the flue, or rather 
more, usually answers well, especially if there is a good 
rise in the flue itself throughout. 
Instead of the ordinary horizontal or ‘copper’ furnace 
usually employed for flues, I have frequently used one 
of a deep, square type, as shown in Fig. 143. These are 
fed from the top, instead of in front, and when charged 
with fuel and the draught almost stopped by means of 
a tight-fitting door in front of the ashpit, the fire remains 
alight a good many hours, and affords a very steady and 
gentle heat. As the furnace has to be built entirely 
outside the house there may be a slight loss of heat at 
this point, but this is counterbalanced by other advantages. 
In constructing a flue of any kind the following points, 
beyond the use of good materials and sound work, must 
be carefully observed :—(1) the avoidance of any damper 
or other obstruction in the base of the chimney, which 
would of course tend to throw the smoke, etc., into the 
house—this part of the flue should be perfectly free and 
