This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
One of the most important features of a British home is the open fire. When properly arranged, this is not only a cheerful and pleasant means of warming a room, but it is also an excellent ventilator, and for this reason a fireplace ought to be provided in every room intended for occupation, especially in every bedroom. On the proper design and construction of the fire-receptacle, and of the shaft for carrying away the products of combustion, the comfort and cleanliness of the room (and indeed of the house) very largely depend. A smoky fireplace, however, leads not only to filth and discomfort; it may also be the cause of colds, and sore throats, besides rendering the room unfit for occupation during a great part of the year.
In the perfect fireplace and chimney there is a continuous, steady, and not too strong up-draught. The attainment of perfection in chimneys, as in everything else, is a difficult matter. Something must be allowed to skill in design, something to careful workmanship, and something also to happy chance. Here are a few hints which may be of service: -
Firsthy, as to the room itself -
1. Provision should be made for the inlet of air otherwise than by the door. If there are no chinks in windows, doors, and floors, the fire cannot draw unless the door or window is open: the chimney is not an air-pump.
2. The fireplace should not be too near the door, or there may be puffs of smoke when the door is quickly closed, - not to mention draughts.
3. The fireplace is not well placed against an external wall, for reasons to be given hereafter.
Secondly, as to the fireplace -
1. Dog-grates and hob-grates are apt to smoke on account of the wide open space between the fire and the flue.
2. The old-fashioned register-grates with the outward splay above the flue-opening seem made for the purpose of allowing the smoke to float into the room; the fault can often be cured by the insertion of a sheet-metal hood, as at a in Fig. 65.
3. Hoods are advantageous in connection with all fire-grates.
4. The more fire-clay that is used in the construction of the fire-box, and the less iron, the better.
5. An open space behind the fire-grate communicating freely with the room and the flue - which is often the case with iron grates - is apt to interfere with the proper draught of the fire.
Thirdly, as to the flue -
1. The gathering from the fireplace opening to the flue should be short: a large space at the foot of the flue may make the draught sluggish for some time after the fire is lit. One of Benison'a fire-clay smoke-receivers may with advantage be used instead of the usual oversailing courses of brickwork or sloping flagstone; the smoke-receiver, as will he seen from Fig. 70, is shaped like a wide, shallow keystone, through which is a hole, large at the bottom, and tapering upwards to the size of the flue above.
2. The flue should not be too large; a flue 14 inches by 9 inches is large enough for all ordinary household fires, kitchen included, and for mast fires smaller flues are better, especially if formed with fire-clay tubes.
3. Fire-clay tubes - which are made circular, square, oblong, and square and oblong with rounded corners1 - increase the draught of a chimney by reducing friction and retaining heat, and also lessen the risk of fire; they should be used wherever the cost can be afforded. The cavity between them and the brickwork should be filled with grout.
4. Where pipes are not used, the flue must be carefully pargeted with mortar and all the angles neatly rounded; this helps to keep the flue warm, reduces friction, and lessens the risk of fire, and of smoke escaping into the rooms or upper fireplaces.
5. Flues against external walls are often chilled by the cold air outside, and draught is stopped in consequence or greatly retarded; hence flues are best placed against intend walls. Where the flue is necessarily against the external wall, the thickness of the wall must not be reduced at the back of the flue, as is so often done, but rather increased, and the flue ought to be lined with fireclay tubes.
Fig. 66. -Hood Inserted In Register-Grate
1 Tubes with two passages (one for smoke and the other for air) are also made, the air-passage commenring in some room which it is intended to ventilate and terminating at an outlet grate in the side of the chimney-stack. It is always better, however, to provide a separate floe for ventilation, as a better outlet can be provided at the top.
6. Slight bends in flues are an advantage; a perfectly straight flue will draw more fiercely than a curved one, but is more liable to sudden gusts of down-draught. On the other hand, long and sinuous bends must be avoided, especially if the curves approach the horizontal, as the friction of the smoke is considerably increased, and there is danger of the pargeting on the upper side of the bend being scamped. Where the flue makes a smaller angle with the horizon than 45 degrees. soot-doors must l»e provided.
7. The flue must be unobstructed throughout its length. It is not an uncommon matter for a flue to be partly blocked with mortar, bricks, etc, dropped into it by careless workmen and left there. A good plan is to draw a bundle of hay or rags up the flue as the work proceeds, so that anything falling into the flue is at once stopped. Or the flue may be "cored", i.e a wire brush or other "core" is passed through it after the chimney has been built.
8. One common cause of obstructions in flues is the bad bonding of the flue divisions with the brickwork in front and behind. In many cases there is absolutely no bond at all, and a clumsy or vicious chimney-sweep may easily displace a brick and so throttle the flue Flue-pipes are advantageous in this respect, as they cannot be easily dislocated; moreover, many flues lined with them never require to be swept.