Which type of appliance is characterized as having no chimney but still vents inside the building envelope?

The type of appliance characterized as having no chimney and venting inside the building envelope is unvented appliances. These appliances operate without a flue or chimney, meaning they do not discharge combustion gases to the outside. Instead, they provide heat by burning fuel directly within the space where they operate. Because they vent inside the building, the combustion by-products, such as carbon monoxide and water vapor, can contribute to indoor air quality issues if not managed properly.

Unvented appliances are commonly used for their convenience and efficiency, but it is crucial to ensure that the space is adequately ventilated to mitigate any risks associated with the accumulation of harmful gases. This is what differentiates them from vented appliances, which safely exhaust combustion gases outside, or portable heaters and electric appliances, which typically do not involve combustion processes.