Which of the following gases is primarily used in make-up air systems?

Natural gas is the primary gas used in make-up air systems due to its properties and widespread availability. Make-up air systems are designed to provide replacement air for exhaust systems, ensuring proper ventilation and maintaining indoor air quality. Natural gas is favored because it burns efficiently, producing a significant amount of heat while minimizing emissions of harmful pollutants.

In terms of energy production, natural gas has a high energy content and is readily accessible in many regions through existing infrastructure, making it cost-effective for various heating applications within make-up air systems. This allows building operators to provide sufficient air flow to replace air that is being exhausted while also providing the necessary heating to maintain comfortable indoor environments.

The other gases listed, while they can be used for heating purposes, do not have the same level of practicality or efficiency in this context. For example, propane and butane are typically used in situations where natural gas is not available, and carbon dioxide is generally not used in heating applications as it is a byproduct of combustion and not a fuel source.