How To Build An Efficient Fireplace
( Originally Published 1937 )
BECAUSE it is a long time since fireplaces were generally used as the sole sources of heat, the principles governing their correct construction and operation are not generally understood. I propose to show certain simple principles which, if followed attentively, will get you a fireplace that is not only proof against smoking, but also the most efficient in giving forth its heat to the room.
Two Americans—Benjamin Franklin and Count Rumford—contributed more than any other individuals to the early science of domestic heating. Franklin is remembered for his work with the stove which bears his name, and Rumford for his innovations in fireplace design. The fireplaces with which Rumford dealt differed in one important particular from modern ones: they had no dampers. In most houses nowadays there may be long periods when the fireplace is not used at all. During these periods the flue of a fireplace without a damper would be constantly full of warm air rushing from the room to the cold outdoors. It was in order to prevent this waste of heat that dampers were invented.
The fireplace shown has the correct proportions between its parts, and may be relied upon to give efficient heating and no smoke.
The chimney should be higher by 2 or 3 feet than adjacent trees, roof ridges, dormer windows, or buildings. The walls of the chimney should be at least 9" thick and plastered smoothly with mortar on the in-side. If less thick than this the brickwork should be protected with refractory flue lining. These come in standard brick sizes.
The net cross sectional area of the flue should not be less than one tenth the area of the fireplace opening. Thus a fireplace opening 36" wide and 30" high would require 30 x 36 divided by 10, which equals I08 square inches of flue section. A flue lining 8" x 18" (net flue area 109.7 sq. in.) would do; or, better, a 13" x 13" lining (net flue area 126.6 sq. in.) A round flue is the most efficient of any for its area, and a square flue is more efficient than an oblong one. These facts should be considered when applying the rule given above.
Beams, joists or rafters should always be framed around a chimney: never built into it nor supported by it. Construction of the latter sort is the commonest cause of fires. If you are altering a house and find potential danger spots such as these, be sure to remedy them before the opportunity to do so is past. Wood should not touch a chimney where the walls of it are less than 8" thick, but should be insulated from the brickwork with asbestos.
Necessary changes of direction in flues should be gradual, and the flues should not slope to the side more than 30° from the vertical. They should continue the same diameter all the way up from the top of the smoke chamber.
SMOKE CHAMBER (E)
Above the damper a smoke chamber must be constructed, as wide at the bottom as the fireplace itself, and sloping in at about 30° from the vertical to a point exactly above the center line of the fireplace, before the flue proper is started. Under no circumstances start the flue to one side of the center line.
The floor of the smoke chamber is formed by a smoke-shelf at the back and the damper in front. The smoke-shelf is very important, for it serves the purpose of receiving occasional down-drafts of cold air and deflecting them forward and upward with the smoke and hot gases.
The damper is always placed as near the front as possible. The plate opens with its forward edge, and is the full width of the fireplace. Use a cast iron damper unit made by a reputable firm, supplied with flanges that rest on the brickwork to make a secure, tight joint. In fireplaces without dampers it is unnecessary to have the opening wider than 4" at this point.
A fireplace is more efficient if it is wider than it is high. It should not be too deep. A deep fireplace wastes its heat on the side-walls and chimney. The nearer the fire is to being built right out in the room, the more heat will be radiated directly into the room.
The side walls of the fireplace are at their greatest efficiency when built at an angle of 45 ° to the plane of the front, (A. fig. 29). Where smoking is feared this angle may be increased, but it should never be wider than 60° (B), notwithstanding the fact that many fireplaces are built with an angle of this much of more, the result of either ignorance or indifference.
The back wall should curve forward from a point about 14" above the floor of the hearth, until it meets the bottom of the damper; or, in the case of a fire-place without a damper, until it is just 4" from the breast of the front wall of the chimney. The throat of the chimney—as this narrowest portion is called—(C), should be 6" or 8" above the bottom of the arch (D) (I speak of it as an arch even if it should have no curve in it).
In order to keep the fire well to the front the arch should not be more than 5" thick, and it should be rounded off smoothly on its inner edge (D) to pro-vide a smooth ascent for the smoke and to prevent eddying.
The proportion that gives the most efficient radiation of heat, is where the width of the back is equal to the depth, and the depth equal to 1 the width of the opening in front, the sides being exactly 45° to the plane of the front. Many people will wish to alter these proportions to make it easier to burn longer logs, but in making concessions to this convenience, they should not forget that if they lower the efficiency of the fireplace by making it squarer and deeper in section, they will have to burn more wood to get an equal amount of heat.