What is the total free area of a combustion air opening that measures 10" x 15" when a grill with 70% free area is used?

To determine the total free area of a combustion air opening when a grill is installed, it is essential to first calculate the overall area of the opening and then account for the percentage of that area that is free for air passage.

In this case, the dimensions of the combustion air opening are 10 inches by 15 inches. The area is calculated by multiplying the width by the height:

Area = Width x Height = 10 in x 15 in = 150 sq in.

Next, since the grill has 70% of its area as free area, you need to calculate 70% of the total area. This is done by multiplying the total area by 0.70:

Free Area = Total Area x Free Area Percentage = 150 sq in x 0.70 = 105 sq in.

Thus, the total free area of the combustion air opening when the specified grill is used is indeed 105 square inches. This calculation demonstrates the importance of considering free area percentage when evaluating combustion air supply for heating systems, as it ultimately affects the efficiency and safety of the equipment.