50 × 2133 = 106,650 B.t.u.
That is, 106,650 B.t.u. will be required to heat the air for ventilation one day.
In order to express this amount of heat in terms of fuel consumed, it will be assumed that the coal contained 14,000 B.t.u. per pound, this being a fair valuation of good coal. The average house-heating furnace will turn into available heat about 50 per cent. of the fuel burned. This value is taken from house-heating fuel tests made at the Iowa State College. The available heat in each pound of coal then will be 7000 B.t.u.
106,650 ÷ 7000 = 15.2 pounds of coal.
That is, 15.2 pounds of coal per day must be burned in order to furnish 1000 cubic feet of air per person each hour at the desired temperature.
At $10 a ton of 2000 pounds, the fuel costs ½ cent per pound. The cost of ventilation is, therefore, ½ × 15.2 = 7.60 cents a day, not an extravagant amount for good air.
It is evident that with the use of hot-air furnaces which take their entire amount of air from outdoors, the extra amount of heat necessary for this improved quality of atmosphere is very well expended. The use of ventilating devices adds only a relatively small amount to the total cost of heating and provides for the well-being of the occupants of the house—in the form of good air—an amount of healthfulness impossible of calculation.
The best ventilation is attained where a constant supply of fresh air is admitted to the house at points from which the best circulation may be secured and equal quantities of vitiated air are removed from the different apartments.
It is understood that in the process of natural ventilation the desired condition can only be approximated and that the permissible ventilation appliances are so placed as to give results such as to permit the air to follow the natural laws that must prevail.