What are they?
They are stoves for burning wood to heat your space, your water, and even to cook on. A basic stove will be of cast iron or steel (lined with fire-brick to retain heat), usually with a door at the front for loading, lighting and ash removal – but sometimes on top in small stoves.
How they work
Open fires are pretty, but most of the heat disappears up the chimney. Also, updraughts from below pass through the logs and draw off volatile gases (which provide most of the heat) and suck them up the chimney unburnt, which wastes heat and causes tar build-up in the chimney. The casing of a stove heats up and radiates heat out into the room. You can have a simple stove for heating, or one with a back boiler to provide hot water / central heating.
What are the benefits?
CO2 is the most important greenhouse gas; wood is a carbon-neutral fuel in that burning releases CO2 (the same amount as if the trees died and rotted) but new trees absorb it. Growing trees absorb more CO2 than mature trees, so as we harvest mature trees, from a climate change perspective, it is essential that we replace them with new ones. Wood is a renewable resource that provides a habitat for wildlife.
It also has a good energy balance, i.e. can be locally produced, requiring very little processing or transport energy (logs more so than pellets, although pellets release less of the pollutants that cause acid rain). Emissions are better than coal, oil or gas as regards NOx and SOx (acid rain) and carbon monoxide but worse for particulates. For space heating, emissions and energy losses from power stations make electricity the worst option environmentally; and wind and photovoltaics are as yet too expensive for heating.
Modern ‘clean burn’ stoves can be used in smokeless zones; they use secondary combustion, baffles or catalyts to maximise combustion of gases and particulates, reducing emissions and increasing efficiency.