Gas Fitter Practice Test

What is the primary function of a fixed draft hood in a gas system?

1. Negative over fire draft

2. Positive over fire draft

3. Zero (neutral) over fire draft

4. Variable over fire draft condition

The correct answer is: Zero (neutral) over fire draft

The primary function of a fixed draft hood in a gas system is to maintain a zero (neutral) over fire draft condition. This means that the draft hood is designed to create a stable environment within the appliance that balances the combustion air entering and the flue gases exiting. A neutral draft helps ensure that the appliance operates efficiently and safely by preventing the escape of flue gases into the living space while allowing adequate air for combustion. By facilitating this balanced condition, the fixed draft hood helps to avoid conditions that could lead to negative or positive pressure imbalances, which can be detrimental to the safe operation of the gas appliance. Negative draft conditions can draw combustion products back into the appliance, leading to possible backdrafting and exposure to harmful gases, while positive draft conditions might cause an inefficient burn or excessive discharge of heat up the flue. Overall, achieving a neutral over fire draft condition is crucial in ensuring the safe and efficient function of gas-fired appliances, contributing to both performance and safety standards.