What is indicated by a Class IV system?

A Class IV system is characterized by sealed combustion and positive pressure operation. This means that the combustion air is drawn from outside the building and exhaust gases are expelled to the outdoors, which minimizes any impact on indoor air quality. The system's design allows for efficient combustion and greater safety, as it prevents any backdraft or infiltration of exhaust gases into the living space.

Sealed combustion systems are commonly found in modern heating appliances like condensing boilers and certain types of furnaces, which are designed to operate safely and efficiently in residential settings. The positive pressure aspect indicates that the combustion process is maintained at a pressure greater than the surrounding atmosphere within the system, which enhances efficiency and minimizes the risk of exhaust gases leaking back into the home.

The other options do not align with the characteristics of a Class IV system. For instance, a natural draft system with efficiency figures does not relate to the sealed and positive pressure attributes. Similarly, gravity feed systems lack the closed combustion and pressure features, and metal flue systems do not inherently describe the operational classification or efficiency profile needed to define a Class IV system.