Wood Stove

A wood stove is a closed metal firebox that burns cut firewood indoors and carries its smoke out through a chimney. Closing the fire in separates it from an open hearth: a fireplace leaves only 10 to 20 percent of the wood's heat in the room, a modern stove 80 percent or more. Walls and mass hold the heat it makes but make none.

What decides whether one works well is the fuel, not the weather. Freshly cut wood is around 45 percent water by weight and must be dried, or seasoned, to about 20 percent before it burns properly, because the fire spends its own heat boiling off the rest. Seasoning takes six months at least and one to two years to do properly, so covered storage, split and stacked under a roof, is part of the stove rather than an extra, and the first winter's fuel is cut a year or more ahead. Species matters: a stacked cord of the densest hardwood holds about 8,800 kilowatt-hours (30 million BTU) and soft maple under 5,600 (19 million), so a woodlot is sized to what grows on it.

The danger is creosote, a tarry deposit that collects inside the flue and can catch fire. Three things make it: wet wood, incomplete burning and a cool chimney. The trap is that damping the stove down to stretch a load is what produces it: below about 600 C (1,100 F) the gases coming off the wood stop igniting and go up the pipe to condense. One brisk fire a day with the damper open burns the deposit off while it is thin, and the pipe is checked before the season and monthly through it, since the air control that makes a stove efficient also lets it smoulder. The second failure is silent: a blocked or leaking flue pushes combustion gas into the room, so a carbon monoxide alarm belongs through the house but not beside the stove. Fuel that regrows, such as a coppice cut on rotation, turns the heat from a purchase into a crop. No approved source gives a service life for the appliance.