It is well enough to say just how a fireplace should be built so that it will work satisfactorily, but that does not go far in helping the man who has a fireplace that will not work. Frequently it is possible without any very great expense and trouble to correct a fireplace that has been improperly built. If one has in mind a clear comprehension of the few elementary principles of fireplace construction it will usually be an easy matter to determine the reason why a fireplace smokes or fails to draw.

The cross-section area of the flue is likely to prove the most common difficulty. Usually this cannot be seen from inside the fireplace, because of the narrow throat and the smoke chamber which in some form may be above the shelf. If, therefore, the apparent essentials—such as shape of opening, narrow throat across the whole width, and preferably the slanting back—have been followed out it would be well to determine the area of the flue itself. To do this it will be necessary to reach the top of the chimney and, by lowering a weight on a line, find which flue leads to the fireplace in question. Its area at the top will in all probability be its area throughout. If the flue happens to be the only one in that particular chimney it may sometimes be determined more easily by counting the bricks in its two horizontal directions and in this way estimating what would probably be the inside flue. This conclusion is by no means sure, however, since the chimney may be built with eight-inch walls or it may be simply a four-inch wall with the flue lining. To one with a knowledge of bricklaying, however, the way in which the chimney is laid up will usually indicate the size of the flue.

Having determined the size of the fireplace opening and the cross-section area of the flue itself, it will in many cases be found that the latter is too small for the former. The easiest way to remedy this difficulty naturally would be to decrease the size of the opening in the face of the fireplace. In order to check up the diagnosis, however, it would be well to fit a pair of thin boards to wedge fairly tightly into the opening at the top, one of which boards could be drawn down past the other one so that the fireplace opening may be decreased anywhere from six to twelve inches in height—using two six-inch boards. By testing the fireplace in action in this way it will be readily determined by what amount the opening must be decreased. The boards then being removed, a wrought-iron curtain or decorative projecting hood of wrought iron or copper may be fitted permanently to the front.

It is possible, however, that the opening of the fireplace and the flue area are properly related, in which case it may be found that the trouble is due to the lack of a narrow throat and smoke shelf. This too could be constructed in the fireplace without disturbing anything outside, such as the mantel or chimney breast, unless the fireplace is not large enough to permit the addition of four inches of brick at the back. If it is not, it will be well to examine carefully the thickness of the wall at the back of the fireplace and if this is sufficient, part of it could be taken away where the slope of the back joins the upright wall—about a foot above the hearth surface—and the sloping back built in from there up to form the throat. Or, to make perfectly sure of the result, the mantel itself could be removed—this is usually merely nailed to the plaster—and enough of the chimney breast taken down to permit the introduction of a cast-iron throat damper.