Which of the following is not considered a type of furnace?

Furnaces are categorized based on their design and the direction of airflow they facilitate. The up flow, horizontal, and multipoise configurations are standard types of furnace orientations.

An up flow furnace directs air upwards, typically installed in basements or crawl spaces, where heated air rises into the living spaces. A horizontal furnace is designed to lay flat in a utility space, such as an attic or above a ceiling, allowing it to fit in tighter areas. Meanwhile, a multipoise furnace is versatile and can be configured for up flow, down flow, or horizontal installations, making it adaptable to various installation needs and spaces.

On the other hand, a diagonal furnace is not a recognized type of furnace configuration in the HVAC industry. This term does not reflect any standard or commonly used furnace design, which is why this option is the correct answer in this context. Understanding the different types of recognized furnaces is crucial for HVAC professionals in order to select the right equipment for specific applications and installations.