7. The total of all the above gives the number of B. T. U.’s lost each hour from the room when the outside temperature is zero and the inside is 70 degrees Fahrenheit.
Knowing the quantity of heat lost per hour, a radiator must be installed which will supply this amount per hour. As the average steam-radiator supplies about 250 B. T. U.’s per hour from each square foot of its surface, the number of square feet required for a radiator to be installed in the room can be found by dividing 250 into the number of B. T. U.’s which were found to be lost from the room each hour.
A hot-water radiator gives off about 150 B. T. U.’s per hour for each square foot of surface, so that the radiator is generally about one-third larger than the steam-radiator.
Knowing the required number of feet of radiation for the radiator, the proper size can be selected from the manufacturer’s catalogue.
By lumping the total number of square feet of radiation for all the radiators throughout the house together and adding 35 per cent to this to make up for loss through pipes and under-rating of boilers, the size of the boiler can be selected from the catalogue to fit this need.
To estimate the size of a warm-air furnace, the total quantity of heat lost from all the rooms of the house should be calculated in the same way, and then 25 per cent added to allow for cold attics and exposure. This quantity should then be multiplied by 2.4 and divided by 8,000 to find the number of pounds of coal which will be required to be burned per hour. By dividing this amount by 5, the grate area of the required furnace can be found, and the correct size selected from the manufacturer’s catalogue.