Outdoor fireplaces 46 Types 47 Obtaining plans 48 Construction 48 Operation 51 Barbecue pits 51 Dutch ovens 51 FIREPLACES AND CHIMNEYS should provide a safe place for an open fire and a flue for draft to expel smoke from the fire passage to the open air. They must be properly designed and constructed (fig. 1) if good performance and protection against fire are to be obtained. [Illustration: Figure 1.--A properly designed and well-built chimney that provides ample draft and protection against fire.] CHIMNEYS DESIGN Solid masonry is the most satisfactory and safest material to use for chimneys and fireplaces. If a chimney fire occurs, the safety of the building may be dependent on the soundness of the flue walls (fig. 2). Cracked and leaky flues not only are inefficient, destroying the draft as well as permitting smoke and gases to pass into adjacent rooms, but are a dangerous fire hazard. The chimney as known today was developed about 600 years ago. Experience has shown that the satisfactory performance of a chimney flue is determined by its size, direction, shape, height, tightness, and smoothness. Draft The draft of a chimney is the current of air created by the difference in pressure resulting from variation in weight between the relatively hot gases in the flue and the cooler outside air. The strength or intensity of the draft depends, for the most part, on the height of the chimney, and the temperature difference between the chimney gases and the outside atmosphere. The draft is not so good in summer as in winter because the difference in temperature between the outside air and the gases in the flue is less. A very common error in chimney design is failure to distinguish between the size of the flue required for free passage of the volume of smoke from a given amount of fuel and that which, with proper height, will produce the required draft. A chimney may be high enough (fig. 3), yet have an area too small to expel the volume of smoke; or the size may be ample (fig. 4) but the height not great enough to produce a strong draft. Either fault or a combination of the two will result in unsatisfactory service. Flue Sites The dimensions of a flue for adequate draft depend principally on the grate area and type of heating plant 1 and on the kind of fuel to be burned, both of which should be determined before construction is begun. If a chimney is found to be inadequate the only method of improving it, short of reconstruction, is to increase its height. This is not always effective and is often impracticable.