Which type of gas heaters can be classified as either indirect-fired or direct-fired?

Make-up air gas heaters are classified as either indirect-fired or direct-fired due to their operational designs that accommodate different applications and environments.

Indirect-fired make-up air heaters operate by using a heat exchanger to transfer heat to the air without the combustion gases directly mixing with the air supply. This design is beneficial in environments where maintaining air quality is crucial, such as in commercial kitchens or manufacturing settings where fumes or contaminants from combustion could impact the air quality.

In contrast, direct-fired make-up air heaters directly heat the incoming air by mixing combustion gases with the air stream. This approach is often utilized in spaces where a rapid increase in temperature is necessary and where the quality of the air can be moderated more easily.

This versatility in their design allows make-up air heaters to be effective in a variety of settings, making them suitable for different heating needs while also addressing air quality and efficiency. The other categories, such as unit heaters, radiant heaters, and convection heaters, typically fall under a single heating method without the intermediate flexibility that make-up air heaters provide.