One pound of coal is capable, if all the heat of combustion is utilized, of raising the temperature of a room, twenty feet square and twelve feet high, to ten degrees above the temperature of the outer air. If the room were not ventilated at all, and the walls were composed of non-conducting materials, the consumption of fuel to maintain this temperature would be very small, but, in proportion as the air of the room was renewed, so would the consumption of fuel necessary to maintain that temperature increase. If the volume of air contained in the room were changed every hour, one pound of coal additional would be required per hour to heat the inflowing air, so that, to maintain the temperature at ten degrees above that of the outer air during twelve hours, would require twelve pounds of coal.
The principle of the ordinary open fireplace is that the coal shall be placed in a grate, by which air is admitted from the bottom and sides to aid in the combustion of coal; and an ordinary fireplace, for a room of twenty feet square and twelve feet high, will contain from about fifteen to twenty pounds at a time, and, if the fire be kept up for twelve hours, probably the consumption will be about one hundred pounds, or the consumption may be assumed at about eight pounds of coal an hour.
One pound of coal may be assumed to require, for its perfect combustion, 150 cubic feet of atmospheric air; 8 lbs. would require 1,200 cubic feet; but, at a very low computation of the velocity of the gases in an ordinary chimney-flue, the air which would pass up the chimney at a rate of from 4 to 6 feet per second, or from 14,000 to 20,000 cubic feet per hour, with the chimneys in ordinary use, and I have often found a velocity of from 10 to 12 feet per second giving an outflow of air of from 35,000 to 40,000 cubic feet per hour—this air comes into the room cold, and when it is beginning to be warmed it is drawn away up the chimney, and its place filled by fresh cold air. A room 20 feet square and 12 feet high contains 4,800 cubic feet of space. In such a room, with a good fire, the air would be removed four or five times an hour with a moderate draught in the chimney, and six or eight times with a blazing fire; the air so removed would be replaced by cold air. The atmosphere of the room is thus being cooled down rapidly by the continued influx of cold air to supply the place of the warmer air drawn up the chimney. The very means adopted to heat the room produces draughts, because the stronger the direct radiation, or rather the brighter the flame in open fireplaces, the stronger must be the draught of the fire and the abstraction of heat. The only way to prevent draughts is to adopt means for providing fresh warmed air to supply the place of that removed.
The most natural way of providing warmed air is to utilize the excess of heat which passes up the chimney, beyond what is required for creating an adequate draught, and to use this heat to warm fresh air; and the warmed air should be admitted into the room in such places
