and wood are not the only possible fuels. Also available are various
biomass-based prefab fireplaces (which is to say, varieties of
biomass other than wood: ethanol, gel (composed of isopropyl alcohol)
and recycled saw-dust pellets). Such fuels are the cleanest of all,
both practically and environmentally, and their accompanying units —
the fireplaces themselves — are typically inelaborate and
relatively cheap to install and maintain.
Pellet-burning fireplaces require external ventilation (i.e., a chimney), but ethanol- and gel-based fireplaces are theoretically entirely portable, making them ideal for apartment dwellers and those reluctant to undertake the substantial structural changes that the installation of a wood-, pellet- or vented gas-based fireplace would require. On the other hand, like the pre-chimney fireplaces of antiquity (to which, in a sense, the wholly portable fireplace marks a return), gel- and ethanol-based fireplaces tend to exude less heat on average than structurally integrated gas-, pellet- and wood-burning fireplaces. The former have an average heat output of 4,000 British Thermal Units (BTUs) per hour (which, depending on the amount of surrounding insulation, should serve to heat comfortably a 150-square-foot-space). Gas-burning fireplaces, by comparison, can emit, on average, between 15,000 and 40,000 BTUs per hour; wood- and pellet-burning fireplaces, between 8,000 and 90,000 BTUs per hour. While it's incorrect to call gel- and ethanol-burning fireplaces merely decorative, it is true that they are best considered as supplements — albeit, highly prettified supplements — to a dwelling's larger heating system.