Furnaces  for  residences  are  usually  of  the  portable  type,
Fig.  16,  the  same  being  enclosed  in  an  outer  shell  composed
of  two  metal  casings  having  a  dead  air  space  or  an  asbes-
tos insulation  between  them.  Some  of  the  larger  sized


Fig.   16.
plants,  however,  have  the  furnace  enclosed  in  a  permanent
casement  of  brick  work,  as  in  Fig.  17.  Each  of  the  two
types  of  furnaces  give  good  results.  The  points  usually
governing  the  selection  between  portable  and  permanent
settings  are  price  and  available  floor  space.
The  cylindrical  fire  pot  is  probably  better  than  a  con-
ical or  spherical  one,  there  being  less  danger  of  the  fire
clogging  and  becoming  dirty.  A  lined  fire  pot  is  better
than  an  unlined  one,  because  a  hotter  fire  can  be  maintained
in  it  with  less  detriment  to  the  furnace.  There  is  of  course
a  loss  of  heating  surface  in  the  lined  pot,  and  in  some  forms
of  furnaces  the  fire  pot  is  unlined  to  obtain  this  increased


62


HEATING  AND  VENTILATION


heating  surface.  It  seems  reasonable  to  assume,  however,
that  the  lined  pot  is  longer  lived  and  contaminates  the  air
supply  less.
Pig.   17.
Fig.   18.
FURNACE  HEATING


63


Some  form  of  shaking  or  dumping  grate  should  be  se-
lected, as  a  stationary  grate  is  far  from  satisfactory.  Care
should  be  exercised  also  in  the  selection  of  the  movable
grate,  as  some  forms  not  only  stir  up  the  fire  but  permit
much  of  it  to  fall  through  to  waste  when  being  operated.
