The quantity of heat necessary to change the temperature of a definite amount of air is easy of calculation. The problem is that of determining the number of heat units required to warm the necessary air to suit the average condition of weather. We will assume that the house is heated to the normal temperature 70°, and that the additional cost of heating the air for ventilation over the amount thus expended is the cost of ventilation. Assuming that the house is so constructed that it is possible to supply air at the rate of 1000 cubic feet per hour to each person of a family of five, this condition will necessitate 5000 cubic feet of air per hour or 120,000 cubic feet of air per day. The house is such that 10 tons of coal are required per year, at a cost of $10 per ton. The period of winter weather will be considered 5 months of 30 days each. This will be 150 days, during which the fuel for heating the house will cost 66-2/3 cents per day. The average temperature of the outdoor air during the entire period will be assumed to be 20°F., thus requiring the air for ventilation to be changed 50° in order to raise it to the normal temperature, 70°. The weight of a cubic foot of air at 70° is practically 0.075 pound. The 120,000 cubic feet of air used per day will, therefore, weigh 0.075 × 120,000 = 9000 pounds which must be raised 50° in temperature. In order to express in B.t.u. the necessary heat required to produce the change of air temperature, the quantity of air is best stated in an equivalent amount of water. The specific heat of air is 0.237; that is, the amount of heat necessary to change a pound of air 1° is 0.237 of the amount used in changing 1 pound of water 1°. The 9000 pounds of air expressed as an equivalent amount of water will then be: 9000 × 0.237 = 2133 pounds of water. This amount of water raised 1° is equivalent to raising 120,000 cubic feet of air 1°. Now the average change in the temperature of the air is 50°, so that 50 × 2133 will be the number of heat units used. 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 1/2 cent per pound. The cost of ventilation is, therefore, 1/2 × 15.2 = 7.60 cents a day, not an extravagant amount for good air.