With the growing trend in restoring to use those fireplaces that have been bricked or boarded up, it is important that these restored fireplaces and chimneys are working efficiently. For wood or coal burning fires to burn well, a good supply of air is required along with a chimney to expel the hot exhaust gases and smoke. Also the fuel should be held in a grate clear of the hearth floor allowing full circulation of the air and waste ash to fall through, so as not to stifle the fire. If the chimney or flue is inadequate or the flow of air insufficient, the fire will not function effectively. In his thesis on the principles of fireplace design published in1799, Count Rumford emphasized the importance of the size of the flue compared with the size of the fireplace opening. He recommended that the cross-sectional area of the flue should be about a tenth of the size of the opening. However, fireplaces tended to be smaller after the mid nineteenth century. Modern flue liner manufacturers favor a ratio of one to seven, and there are sizing charts published that give details of current standards. If your fire smokes or won?t burn properly, see if opening a window improves matters. If it does, you need better ventilation in the room. One solution is to install a window vent, although this may cause a troublesome cross draft. A much more efficient form of ventilation is either a single ducted vent set into the floor in front of the fireplace, or twin ducted vents set into the floor or external walls on each side of the chimney breast. When wood and coal are burned, flammable gases, tarry substances, acids and dust are given off. However, because domestic stoves are relatively inefficient not all of these substances are consumed. Instead, they rise up the chimney and some of them condense on the inside of the flue. Unburned carbon combines with these tars and acids creating soot; which builds up over a period of time thus reducing the size of the flue. It is important to have regularly used chimneys swept at least twice a year, ideally before, during and at the end of the heating season and the local trade directories contain details of fully qualified and suitably insured sweeps. A soot laden flue is a fire hazard, since the unburned elements of the soot can ignite, causing a chimney fire which can reach high temperatures and damage the chimney. If a flue is too large, its size can be reduced to improve its efficiency by the fitting of a liner. A variety of methods and materials are used, including flue liners made of flexible stainless steel, ceramic, lightweight concrete sections, or concrete cast in situ. Other ways of reducing the fireplace opening in order to improve fire efficiency is to raise the level of the hearth or to fit a baffle across the top of the opening. If raising the hearth level or adding a baffle are not practical or unsightly then it may be worth asking a fireplace specialist whether fitting a metal smoke hood or canopy in the opening is the best solution.