Gas Fitter Practice Test

Which hydronic control de-energizes the main burner if the operating control fails in the closed position?

1. Low limit control

2. Temperature and pressure relief valve

3. High limit control

4. Flow switch

The correct answer is: High limit control

The high limit control plays a crucial role in hydronic heating systems by providing safety and preventing overheating. Specifically, if the operating control fails in the closed position—meaning that the burner continues to operate without the ability to modulate or shut off—the high limit control acts as a safeguard. It senses the temperature of the water in the system and is programmed to shut down the main burner if the temperature exceeds a predetermined limit. This prevents potential damage to the system components and reduces the risk of dangerous conditions. In contrast, while the low limit control is designed to maintain a minimum temperature, it does not deactivate the burner if the system reaches too high of a temperature. The temperature and pressure relief valve, while important for safety, primarily functions as a safety mechanism to release pressure or prevent water from boiling over rather than directly controlling burner operation. A flow switch is used to monitor water flow and may prevent burner operation if flow is insufficient, but it does not directly respond to operating control failure in the manner that the high limit control does. Therefore, the high limit control is the correct answer as it specifically addresses the situation of a failed operating control by shutting down the burner when necessary to maintain safety.