Gas Fitter Test 2025 – 400 Free Practice Questions to Pass the Exam

The correct answer of 58 square inches for the air supply opening needed for the installation of a 400,000 BTU natural draft boiler in a tight enclosure is determined by specific guidelines that govern combustion air requirements. When sizing the air supply for gas appliances, it's necessary to ensure adequate oxygen for combustion and the safety of the installation.

Natural draft boilers generally require a certain amount of combustion air that is usually calculated based on the input BTU rating of the appliance. The typical requirement is to allow either one square inch of combustion air for every 1,000 BTU of input, or the application of specific tables or codes that may provide more precise figures based on unique circumstances.

For a 400,000 BTU natural draft boiler, applying the standard formula, one would calculate the required air opening, which leads to determining the ventilation area needed to facilitate proper airflow and prevent backdrafts or inefficient combustion.

If the choice of 58 square inches aligns with this requirement, it signifies adherence to established safety codes, ensuring adequate air supply to support safe and efficient boiler operation. This prevents issues like incomplete combustion, carbon monoxide production, or overheating. The other options either underestimate or overestimate the air supply needed, not fulfilling the proper clearance and safety