All air handled by a non-circulating direct gas-fired industrial air heater must be ducted from where?

The correct answer indicates that all air handled by a non-circulating direct gas-fired industrial air heater must be ducted from outdoors. This is crucial for several reasons.

Firstly, non-circulating direct gas-fired heaters operate by using combustion gases to heat the air being supplied to the space. If indoor air were to be used, the heater could deplete the oxygen levels in that space and introduce combustion byproducts back into the indoor environment, which could pose significant health risks and safety hazards.

Using outdoor air for the heating process ensures that the air being brought into the space is fresh and maintains adequate oxygen levels. Additionally, it helps in mitigating the risk of carbon monoxide buildup and other combustion gases that could occur if indoor air were recirculated.

Furthermore, drawing air from outdoors allows for better control of the heating process and can lead to increased energy efficiency since the outside air can be pre-conditioned in some settings before it's heated.

This practice aligns with safety regulations and industry best practices, ensuring that the air quality in industrial settings remains safe for workers and compliant with health standards.