Which of the following are common types of gas furnaces?

The correct answer highlights the common types of gas furnaces which are single-stage, two-stage, and modulating.

Single-stage furnaces operate at one fixed heat output level, making them straightforward but less efficient in variable temperature conditions. Two-stage furnaces have the ability to run at two different capacities, allowing them to operate more efficiently by adjusting to the heating needs of the space more effectively than a single-stage model. This ability helps in maintaining a more consistent temperature and improving energy efficiency.

Modulating furnaces take efficiency a step further by adjusting their output in small increments based on the heating demand. This means they can run only at the necessary level required to maintain comfort without fully cycling on and off, which makes them the most efficient option among the three.

Understanding these types is crucial for HVAC professionals as it influences factors such as energy efficiency, comfort levels, and operational costs for homeowners. The other choices include combinations that do not accurately describe common gas furnace types recognized in the industry.