During combustion there may be Carbon Monoxide and
Nitrogen Monoxide present in the fumes. Both gasses are toxic so
any exposed metal pipe inside the building should be thoroughly
sealed to make sure no gas can escape. In the bench above the
whole flue is sealed with cob (sand, clay and straw).
Wall benches like this are comfortable at about
45cm (18 inches) deep (front-to-back) and should be around 40 cm
(16 inches) high. The lower the bench is the less cob (a mixture
of clay, sand and straw) is available to store heat from the
stove. As a seat the bench is most comfortable if it is widest at
the top and undercut by about 10cm (4 inches) so there is space to
tuck your feet when you are sitting and getting up.
The idea of heating a bench or bed is a very old one. In China a
heated living space and bed is called a
kang (PDF 700Kb) and dates back to 5,000 BC.