If the limit switch is open, what will happen to the furnace?

When the limit switch is open, it indicates that the furnace has reached its maximum temperature limit, triggering a safety mechanism designed to prevent overheating. In this scenario, the blower fan will typically continue to run to circulate air throughout the space, but the burner is turned off. This ensures that hot air does not continue to be produced, which could lead to dangerous overheating conditions.

The blower running continuously without heat helps maintain airflow and can assist in gradually cooling off the furnace system. This operation is critical because it prevents excess heat build-up, thus protecting the mechanical components of the furnace and maintaining safety within the home environment.

The other options do not accurately describe the operation of a furnace with an open limit switch. The blower's continuous operation with no heat is a designed response, whereas a complete system shutdown or a display error would not align with the typical safety precautions embedded in furnace operations, which specifically allow for continued airflow even in fault conditions.