In accordance with the definition¹ the furnace is:
Equipment for heating of the rooms, containing a casing with a system of ducts connecting furnace with chimney.
Mobile metal furnaces, often richly decorated, were used in ancient Rome . Dome kiln made of clay were known in the Middle Ages.
Ceramic stoves, which were a characteristic element of interior design, have been used since the 14th century.
Furnaces with rich architectonic, sculptural decoration built from colourful tiles, sometimes gold-plated, has been present since a 15th and 16th century.
Round furnaces have been constructed since the 17th century, later they were given a shape similar to furniture. In Classicism the furnaces had the form of pyramids, columns, obelisks, etc. In the second half of the twentieth century they had a styleless character.
In folk architecture they had a very different look. In the 20th century the furnaces in Poland, in Masuria and Silesia region, were made of manually painted tiles, while bread ovens had the other form.
Industrial equipment for processes taking place in high temperatures, obtained as a result of combustion of fuel, heating with electrical current or exothermic reactions (exoenergic reaction).
Depending on the source of heat there following furnaces can be distinguished: fired with solid fuel (coal, coke), liquid (e.g. fuel oil) or gas (e.g. generator gas) and electric furnaces.
Electric furnaces, depending on method of conversion of electrical energy to thermal energy are divided into arc furnaces (heated with electric arc), resistance furnaces (heated with heat formed as a result of flow of electric current through resistance conductors) and induction furnaces (heated on the basis of heat formation the action of induced currents).
It follows from the foregoing that it is incorrect to call the boiler a furnace, because of the boiler, contrary to the furnace, is an equipment, which transfers thermal energy to the given medium (in this case to water) and only using it the energy is distributed to the receivers.