The open fire was the earliest method of warning houses, and many persons still consider it by far the best. In early times, it was usual to form the fire upon a solid hearth in the centre of the room or hut, and the smoke was allowed to fill the space, and to find its way out at a hole provided in the roof, either directly over the fire, or at some distance to one side in order that the rain might not extinguish the fire. The obvious objections to this system led gradually to the universal employment of special smoke-flues, and the inconvenience of the solid hearth led to the invention of the grate which could get rid of the ashes. It is interesting to observe, in modern slow-combustion grates, the employment of the solid hearth.

One or two essential points should be borne in mind in considering the subject of open fires. An open fire does not warm the air of the room by direct radiation to any appreciable extent, but the rays of radiant heat strike the solid objects, such as the walls and furniture, and these heat the air by conduction.

A fire of course requires oxygen to keep it burning, and the action of combustion in a fire is of the following nature. Air as it is heated expands, and consequently becomes lighter, volume for volume; a fire therefore causes a column of heated air to rise, and its place is taken by colder air which descends. If the fire be burning out-of-doors in still air, the currents induced will be as shown in Fig. 447. The rising of the heated column of air can easily be seen over a tire, or over a gas-jet or other source of heat. The air - a mixture of nitrogen and oxygen - is drawn into the lower part of the fire; the nitrogen is merely heated and passes away unchanged, but the oxygen unites with the carbon of the incandescent material, and forms carbonic acid gas (CO2). This gas rises through the heated mass, and is changed to carbonic oxide (CO); it then combines with another atom of oxygen, and should pass away as carbonic acid (CO2), if there is perfect combustion. This is the same gas which is produced by human beings and animals in breathing, and should not be allowed to pass into the atmosphere of a room, although this always takes place where coal-gas, oil, or candles are burnt. The presence of the gas is always undesirable, and when present in comparatively large quantities, it is dangerous to life.

The action of an open fire upon the air in a room is represented in Fig. 448, from which it will be seen that there is a constant current of heated air rising op the chimney, and to take its place air is drawn from other places, such as the cracks around the doors and windows; if these be carefully stopped up, the fire will not burn brightly but will gradually die out. There is always a current of cold air passing along the floor towards the fireplace. Part of this air passes directly op the chimney, and part is heated by contact with the fireplace, and rises up the chimney-breast to the ceiling, passes along it, and, as it cools, descends again to the floor level. In a long room, therefore, it is necessary to provide two fires, as the beneficial influence of one will not extend the whole length.

Fig. 447   Currents of Air produced by an Open Fire out of doors.

Fig. 447 - Currents of Air produced by an Open Fire out of doors.

Fig 448   currents of Air produced by an Open Fire la a Soon.

Fig 448 - currents of Air produced by an Open Fire la a Soon.

Open fires are unsatisfactory for the purpose of producing uniform warmth throughout the whole air of a room, and also because cold external air from every crack, and the relatively cold air of the corridors, are drawn into the room, producing draughts. The feet of persons in the room are always subjected to a cold current of air, necessitating the use of stools to raise the feet above the floor. In order to diminish or entirely obviate this current, the external air may be brought in by a special duct discharging the air directly under the grate, as shown in figs. 449 and 450. This would supply the fire with air, and would certainly diminish the draught along the floor. It would, however, be a wasteful plan, as the entry of air at such a relatively low temperature, directly under the fire, would necessarily diminish its efficiency and cause a waste of fuel Many attempts have been made to obviate this objection, by passing the incoming air round a portion of the heated structure, before allowing it to come into contact with the incandescent fuel.

The chief advantages of an open fire are its cheerful appearance, and the assistance it gives to ventilation.

A good open flre-grate will conform to certain well-known principles. It must stand well forward. If a fireplace be set back from the room with a flue directly over the incandescent mass of fuel, a very large proportion of the radiant heat must pass directly up the chimney. The aim, therefore, of inventors is to throw the fire well forward into the room, to take away all parts of the structure of the fireplace which prevent direct radiation from the front of the fire, and to promote radiation from the back and sides of the structure.

In order to retain the heat and not allow it to be readily dissipated, the use of iron at the back of the fireplace should be altogether avoided. The next point is to avoid the use of heavy horizontal bars at the front of the grate, and to supersede them by vertical or curved bars, such as are shown in Fig. 459, p. 75.

Fig 449 Elevation of fire  grate. with Special Air duct

Fig 449 Elevation of fire- grate. with Special Air duct.

Fig 450. Plan of Fire grate, with Special Air duct

Fig 450.-Plan of Fire grate, with Special Air-duct.