What does a limit control do if the furnace overheats?

A limit control is a critical safety device in a furnace that is designed to monitor the temperature within the heat exchanger. If the temperature exceeds a predetermined limit, indicating that the furnace is overheating, the limit control responds to prevent potential hazards, such as fire or damage to the furnace components.

When the furnace overheats, the limit control de-energizes the gas valve. This action cuts off the fuel supply to the burner, effectively stopping combustion and allowing the system to cool down. This safety feature is essential to the operation of gas appliances, ensuring that they operate within safe temperature ranges and protecting both the equipment and the premises.

The other choices, while related to furnace operations, do not correctly describe the role of a limit control in the context of overheating. For example, shutting down the blower may be a response to various conditions, but it does not directly address overheating. Activating the pressure switch or opening the vent might relate to other safety checks or operational functions, but they do not serve the primary purpose of preventing the furnace from continuing to operate when unsafe temperatures are detected.