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OPEN FIRES

As an open fire burns it draws fresh air in from the room, not just around the burning fuel, but from the whole open 'face' of the fire - the entire width and height of the fireplace opening. Without a chimney tall enough to generate The Chimney Effect with sufficient force, a flue cross-section big enough to carry all this air away, and a sufficient amount of fresh air entering to replace it, the fire will Smoke into the room.

As a general rule the area of the fireplace "face" opening must not be larger than the smallest cross-sectional area of the flue multiplied by the chimney height in metres, and there must be a permanent open air supply into the room equal to at least ½ the cross-sectional area of the flue.

As well as smoke problems, the huge requirement for fresh air can lead to the 'cold back' syndrome, where, away from the direct radiation of the glowing fire, the room is actually made cooler as vast amounts of outside air are drawn in, an effect which was actually used in the 19th Century to provide coal-fired cooling in large buildings.

Large open fires worked well in draughty mediaeval halls with huge chimneys, but are near-impossible to make work in almost any house built since the eighteenth century. Their Efficiency is poor, rarely above 10%. Installing a smoke hood or gather (diagram 1) is unlikely to improve performance as it doesn't significantly reduce the 'face', but building a smaller faced opening inside the existing fireplace (2) can be made to work well.

INSET OPEN FIRES reduce the open face to the very minimum and have a refractory fireback formed with angles to help radiate heat. This type of fireplace is unique to Britain and Ireland and is made to strict dimensions (See: BS1251 Fireplaces) to ensure that a wide range of fires, and even some closed stoves, can fit straight in. Inset fires are moderately efficient (30 to 40%), if fitted correctly, and can become very efficient (over 75%) with certain types of wrap-round back boiler for central heating. Cast-iron grates for BS4834 Open Fires fit into standard inset openings, from the simple loose 'stoolgrate' with its fret front to quite sophisticated air-control open grates which can stay alight all night on certain fuels.

CONVECTOR FIRES, either freestanding or inset into the wall, have a metal casing open to the air at the sides, back, top and even underneath the fire, they can achieve efficiencies of up to 60%

FUELS: Bituminous coal, peat, some types of lignite, soft coke and specially-formulated open fire briquettes are fine, but anthracite, hard coke and the harder briquetted fuels simply won't burn on an open fire. Wood burns best if the white 'cell powder' it produces is prevented from falling away but kept hot and allowed to burn, for this reason wood logs burn best simply laid on a flat hearth with no grate or basket.

SAFETY: A mesh BS3248 Sparkguards is a wise precaution, and essential whenever the fire is unattended or children or the infirm are about, especially as some fuels, notably wood, can occasionally 'spit' sparks.

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