Massachusetts Unrestricted Construction Supervisor License Practice Test

What is the minimum required flue size for a rectangular flue in a chimney serving a fireplace with an opening of 500 square inches and a height of 15 feet?

1. 6 x 10

2. 8 x 10

3. 8 x 12

4. 12 x 12

The correct answer is: 8 x 12

To determine the minimum required flue size for a rectangular flue in a chimney serving a fireplace, it's important to understand how flue sizing is generally calculated based on the size of the fireplace opening. For a fireplace, the flue should be adequate enough to effectively vent the smoke and gases produced during combustion without causing backdrafts or other issues. A common rule of thumb for size is that the flue area should typically be equal to the area of the fireplace opening, calculated to account for necessary adjustments based on chimney height. In this scenario, the fireplace opening is given as 500 square inches, which is crucial for determining the flue size. The opening is calculated using the formula for the area of a rectangle (height multiplied by width). When determining the flue size, code standards often suggest that the flue cross-sectional area should be no less than the area of the fireplace opening, and in some cases, it may require more area based on chimney height and configuration. The answer indicates a flue size of 8 x 12. The area of an 8 x 12 rectangular flue is 96 square inches, which is generally acceptable for a fireplace of this opening size when taking into account the height of the chimney,