Gas Fitter Practice Test

For an appliance without a draft hood and 125,000 Btuh input, what is the minimum size of the combustion air opening from outdoors?

1. 9 square inches

2. 11 square inches

3. 18 square inches

4. 22 square inches

The correct answer is: 9 square inches

The correct answer is determined by understanding the requirements for the proper combustion air supply for appliances, particularly those with high BTU inputs. For appliances without a draft hood, the National Fuel Gas Code provides guidelines for calculating the size of the combustion air openings needed from outdoors to ensure safe and efficient operation. When dealing with appliances that have an input of 125,000 BTUh, the code specifies that the combustion air opening must be sized to allow sufficient air for combustion. Generally, for appliances operating in a confined space, the amount of combustion air required increases with higher BTU inputs. In this case, the minimum size of the combustion air opening must adhere to the guidelines, which typically dictate that a certain square inch of free area is needed per BTU of input. For 125,000 BTUh, it is established that the minimum for combustion air is often around 9 square inches when sized for direct outdoor air. This ensures that the appliance receives an adequate supply of air for proper combustion while avoiding any risks associated with inadequate air supply, such as incomplete combustion or dangerous backdrafting. Thus, the minimum size of the combustion air opening requirement of 9 square inches aligns with these guidelines, ensuring proper operation of the appliance and compliance with safety