Which type of damper is recommended only for solid fuel-burning appliances?

A manually operated damper is specifically recommended for solid fuel-burning appliances because it allows for direct control over the airflow to the appliance. This is particularly important with solid fuels, as they require a consistent airflow to operate efficiently and safely. With a manually operated damper, the user can adjust the damper to regulate combustion air, which can help to optimize efficiency and minimize the production of harmful emissions.

Other types of dampers, such as automatic, electronic, and adjustable dampers, may not provide the same level of control and responsiveness needed for solid fuel applications. While they can be designed to provide convenience and automation, they may not adequately serve the needs of solid fuel-burning systems that depend on manual intervention for effective management of combustion processes. Hence, the preference for manually operated dampers in these scenarios stems from their ability to offer precise control based on the user's judgment and the real-time conditions of the burn.