Which control mechanism shuts off the gas valve if the vent motor fails in a natural gas furnace?

The high limit switch is designed to monitor the temperature within the furnace's heat exchanger. If the vent motor were to fail, it could potentially lead to overheating because the exhaust gases would not be properly expelled. The high limit switch acts as a safety backup by shutting off the gas supply to the burner if it detects that the temperature has exceeded safe operating limits. This mechanism is crucial in preventing potential hazards, such as overheating, which could lead to dangerous conditions like fire or equipment damage.

While other devices like the low limit switch and pressure relief valve play roles in different safety functions within a heating system, they do not specifically address the scenario of vent motor failure as effectively as the high limit switch does.